How Does This Stuff Work? - Hot Rod Forum : Hotrodders Bulletin Board -- Hot Rod Forum

Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans Advertise
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Electrical
User Name
lost password?   |   register now

Closed Thread
LinkBack Thread Tools
  #1 (permalink)  
Old 06-18-2006, 02:02 AM
Last wiki edit: Rebuild an alternator
Join Date: Mar 2004
Posts: 7,345
Wiki Edits: 12

Thanks: 0
Thanked 8 Times in 5 Posts
Traditional BASIC systems

Doc here,

From time to time, I will post "How This Stuff" works..

It is a Simple Overview of BASIC TRADITIONAL systems and sub~systems in a Vehicle..

It will contain no Computer ~ ese sections, just basic stuff that has been around for years..

I will try to do one a month..

Hopefully it may help to better understand how some of the magic works! and make it a bit easier to work with and troubleshoot.

As usual, If you see an error or need clarification , PM me so I can make to proper changes! We aren't perfect..!

Contents: June 18, 06
  • How Do I Use My Multi~Meter?
  • How Does My Turnsignals Work?
  • How Does a Relay Work?

Electrical Forum


Last edited by docvette; 06-18-2006 at 02:34 AM. Reason: Updating Contents list
Quick reply to this message
Sponsored Links
  #2 (permalink)  
Old 06-18-2006, 02:15 AM
Last wiki edit: Rebuild an alternator
Join Date: Mar 2004
Posts: 7,345
Wiki Edits: 12

Thanks: 0
Thanked 8 Times in 5 Posts
How Do I Use My Multi Meter?


This is intended to get the novice started with his / her new meter..nothing too technical.
If I missed anything or something is in Error..PM me..with the correction so I can fix it!

Vince 302/Z28


Meters Come in two basic flavors: D'arsonal and Digital.

  • This type of multi~meter has a MECHANICAL meter movement. It's electronic components and Time base is / are designed around Analog circuitry. This type or Meter has no cold climate drawbacks as the LCD meter does..but it is advised to store it warm and dry. Do Not "Jar" your Meter, as it can have detrimental effects on the jeweled meter movement.
  • This type of meter is sufficient to do all basic Voltage, Ohm , And AMP readings. It will not do high speed switching applications accurately, such as rise / Fall times on a semi conductor device, It will show only peak average voltage / Current.

  • This type of meter is basically the same principal of the D'Arsonal Meter, EXCEPT It has a Digital readout, and it's internal circuitry and time base is designed around Digital Electronics. Depending on the cost of the unit, Is proportional to it's time base.
  • Using or Storing an LCD Display Multi meter in server Cold climates is a Bit problematic..ALWAYS Store your LCD meter in a relatively warn area, as the internal media of the display is Liquid..and can / will freeze..Be sure it's warm Prior to use in a cold climate or reading changes may be slow. The LCD is a bit more rugged than the D'Arsonal movement as no jeweled movement can be damaged.

This type of meter is a good choice for all novice applications.
  • Unpack your new meter and install the Battery's according to the instructions provided. If an Input protection Device (usually a 7/10 amp fuse) is provided , Be sure it is installed.
  • Turn your meter ON, and set it for OHMS Scale , R X 1, and touch both probes together. The Meter should read Full scale (on D'Arsonal) or 000 (on a digital) . If lower or higher, find the "Calibrate" knob, and adjust for Exact "0" or "000". Your meter is calibrated.
  • Next unpack and READ the provided instructions for your particular meter.
  • BEFORE any testing , familiarize yourself with the meter probe configurations for a specific test, The Scale settings for your Specific testing, and most importantly , your meter's limitations for all testing to avoid damage to the meter.

This type of testing is used to detect a Closed Or open circuit, without the presents of power in the circuit.

Some Meters have this function built in..You may have a Button that says Continuity. This adds a "Sound Alert" to the Ohms scale R X 1 setting automatically, so that when a complete circuit is detected, it sounds the alert, and the meter scale reads 000.

Say you have a circuit, consisting of a battery, a fuse , a lamp .. and the lamp is not working, Set your meter for Continuity, or R X 1, and remove the battery from circuit.

Place one probe From the + wire from the battery lead, The other probe to the wire at the lamp filament.

Your meter should Beep, and read 000 on the meter readout. If you have this, the wire from the battery to the fuse is good, the Fuse is good, and the wire to the lamp filament or socket is good

Leaving your probe on the lamp filament side of the socket, move the other probe to the ground side of the socket. Now the meter may or may not beep, But the readout should read a few ohms..( the resistance drop across the filament) If you have this, your lamp is good..

Next, leaving the probe on the ground side of the socket, move the other probe to the - hookup where it goes to the battery..It should beep and read 000 on the readout.

If the test is good, the circuit is complete. If it fails to read on any portion of the test, the circuit between the last two points measured has an open..

The measurement between the + side and lamp filament is open, no reading or beep. Most likely the Fuse is open, measure across it, and see if you have a reading..If not replace the fuse.

Some meters have this function built in by the push of a button, but it also is a function of the Ohms Scale , and can be checked on that scale if your meter has no diode function.

Set your meter for Diode check , Or Ohms Scale R X 1, and note the Positions of the Cathode and Anode of the diode..

Place the black probe on the Anode side of the diode.

Next touch the red probe to the Cathode side of the diode.

Your meter should read 000, if the diode is capable of conducting voltage / current in the forward direction.

Now, reverse the probes on the Cathode and anode, the reading should be infinite..(no movement on the meter)

If you have this your diode is basically 99.99% good static, It may break down under current in circuit, but that's another test..

This scale is used to measure Resistance in a circuit, or Component, without the presents of power in the circuit or component, and is taken in Parallel to a component or device being tested.

It may be used as discussed before for continuity checking as well.

The OHMS scale is NEVER used with power on the device or circuit. For device testing of Resistance always remove the device from circuit, or your readings may be inaccurate (residual circuit resistance) and check is performed in Parallel (across the device)

You have an Ignition system Consisting of a Ballast resistor, and coil..and you want to test the ballast resistor for proper value and open.

Remove one side of the resistor from the circuit.

Set your meter to OHMS scale, and the next highest resistance scale you expect the resistor to be or Auto range (if your meter has that function).

Place a probe on each side of the resistor.

Read the scale..and scale the range back to it's lowest scale without pegging the meter out. It should read, in this case, 1.3 OHMS +/- 20 % (usual tolerance value of a resistor)

If it reads 000, It's shorted (or your probes may be touching each other) If it reads infinite (no meter movement) It's open (or your not getting a good connection at the probes)If it reads more or less than +/- 20 % of expected value it is defective..replace it. If it reads 1.3 +/- 20 % It's good.

If you have two resistance sources in a single circuit you wish to measure to find out the total resistance to find a fuse value and wire size for that circuit:

Assume a Theoretical Circuit of a: 1 ohm resistor, and a lamp filament..Place one probe on the non lamp side of the resistor, and the other on the ground of the lamp, read your meter: Say it Say's 2 ohms..We know two things..The lamp filament is 1 ohm and we have a divisor of 2, for finding our fuse and wire value..

12 Volts Divided by: (1 ohm plus 1 ohm) 2 ohms (Rt or total resistance) equals: 6 amp..Our wire size will be 14 gauge , Our fuse will be slightly higher..6.5 OR 7.5 amps.. the wire is good according to the AWG Chart from 0 to 20 amps at 4 feet (usually about the smallest you'll find in a car), and the fuse will protect that wire.

This measurement Can function test an Electrolytic Capacitor for open or short conditions.

Set your meter for OHMS scale, R X 1, calibrated to 000.

Looking at your Capacitor, on the body (Of most ) you will note a + on one side of the body, that lead is the most positive side of the cap..The other is negative.

Put your RED probe on the + lead, and the BLACK probe on the other lead and watch your meter indication.

The readout or movement should jump up rapidly (Capacitor charging from the meter's battery) Then Fall back to zero Slowly..(depending on the meter's internal resistance)

Next reverse the leads / probes, and the same readings should result.

If this happens, your Capacitor is good.

If you get a pegged meter (you can't get to drain back) or No movement at all (under ANY resistance scale) your Capacitor is Bad.

This test is made to determine the voltage of a Given circuit under operation. It is made in Parallel to the Circuit (across Power and ground) and is ALWAYS taken with power applied to the circuit.

Set or configure your meter for The type Voltage mode you plan to measure. ACV or DCV (some meters are the same scale) And the NEXT highest voltage you expect to find in the circuit.

For our purpose say we want to measure a battery from our car..Set your meter for DCV scale and V X 50 Place your RED Probe to POSITIVE on the battery, and the BLACK to NEGATIVE on the battery.

DC volt readings are ALWAYS polarity sensitive..on a DVOM it's no big deal..the readout will show the voltage but flash a big - sign to show your probes are backward.

On a D'Arsonal meter, you MUST be careful..It will force the needle to sub scale, and could damage the movement.

That's just about it for just very basic DC voltage readings..

On a circuit that has a bad ground..and you measure a Ground wire to ground..Don't be surprised if you see 12 VDC!

This is known as a "loaded Ground"..meaning the device Prior to it is not doing useful work, (consuming current) because it has no ground..

You have a dead tail light bulb, You HAVE 12 volts on the power wire and socket contact, but no measure between the socket and ground..and see 12 volts?

Yes, because the filament is not consuming current (it is nothing more than a wire ) and the 12 volts is passing through the bulb to the ground side of the socket.

Now what happens if I ground that 12 volts? Will the Wire heat up and the fuse blow..No!

The reason is because the filament is producing useful work, (the lamp is now lit) and it's resistance dissipates the current to ground so it isn't a short and it all balances out. Now if you read that wire it has 000 volts on it. Remember this if you end up chasing power in places it shouldn't be..It's ground!

This mode of test is rarely used in automotive with the exception of looking at raw alternator voltages..It measures Alternating Current (like a wall plug for example) I'll cover it very briefly here..since we will hardly use it in automotive..

Set your meter for ACV, and (if required ) configure your probes for ACV. Set your meter for the next highest scale than you expect to find in the circuit.

Say you have a dead wall socket, 110 VAC, Set your scale for 250 VAC, ..CAREFULLY put your probes in the Socket, Take your readings. It should read between 100 and 120 VAC. Polarity is NOT an Issue with AC voltage so probe orientation is not required.

Current Measurements (amp-meter)

Measuring current is another quick way to damage your meter. So pay attention while making any current measurements.

When measuring current, the amp-meter must be placed in SERIES with the DEVICE OR CIRCUIT to be measured. Just like a device fuse, the amp-meter must be in-line with the power source.

Battery 0----0 - Meter + 0-----0 Device under test 0---0 Ground of battery.

Selecting the proper CURRENT scale is VERY important. You must have an idea of what current is expected, then you can select a current scale which won't damage your meter, The Next highest or HIGHEST Scale.

Remember, all the current the measured device draws, will pass right through the amp-meter. If the measured device draws 20 amps and your amp-meter is set to a 10-amp scale (usual Max on most meters), then the meter will probably be damaged.

Always ensure that the selected current scale is greater than the current draw required by the device.

Once you have selected the proper current scale, you must be certain to observe polarity of the meter leads. The electron current flow must enter into the negative lead of the amp-meter, and exit the positive lead of the amp-meter.

Do this backward, and the ANALOG meter will probably be damaged.

When you have finished your measurement, remove the meter from the circuit and restore normal circuit connections.

DO NOT leave the meter in current mode.

Place the meter in it's off mode or its voltage mode as soon as the meter is out of the circuit.

If you later try to measure voltage while in current mode, damage to the meter will most certainly take place.

First, and Foremost, Whenever measuring Voltages over 60 VDC or 30 Volts RMS, Be Extra Careful about placement of your hands!! You can get hurt!

  • DO:
  • Store your meter in a warm, dry place .
  • Not leave it sitting in direct sunlight.
  • wrap your probes up neatly when done to keep them from falling apart.
  • ALWAYS! Double check the MODE and SCALE of the meter to be sure it is correct for the test to be performed, BEFORE attempting a Test.
  • Check the Battery's periodically , install fresh ones when needed.
  • ALWAYS return the meter to the "Off" position, reconfigure the probes OR set to the highest Volts scale after a resistance test or AMP test..Or the next time you use it you may damage it

  • DON'T:
  • DO NOT use a meter that has been exposed to water ..It can bite you!
  • Do NOT allow an LCD to Freeze.
  • DO Not use any probes that are fray or Ratty.
  • DO NOT Assume that the meter is set properly..DOUBLE CHECK!
  • DO NOT get so involved in readings as to forget about safety!! Be sure your hands are clear of probes, voltage sources, and moving parts (Like Fans!)
  • DO NOT wear jewelry DURING any Electrical work..You can suffer Burns and lose extremities..

For the Novice..Follow those Basic Steps and you and your meter will last a long time and provide good service. There are more aspects to testing , that are device specific, but what is here, you should be able to troubleshoot just about anything on a vehicle Electrical system with little or no effort!



Vince 302/Z28
Quick reply to this message
  #3 (permalink)  
Old 06-18-2006, 02:29 AM
Last wiki edit: Rebuild an alternator
Join Date: Mar 2004
Posts: 7,345
Wiki Edits: 12

Thanks: 0
Thanked 8 Times in 5 Posts
How Do My TurnSignals Work?

**************HOW DO MY TURN SIGNALS WORK? *****************

Turn-signals are one of the most misunderstood sub systems in most Vehicles today. They really are simple when you stop and think about HOW the system performs its job, and why it is wired the way it is. Most commonly, the system is miss-wired and interacts with the brake lamp system causing non functioning turn-signals under braking Conditions. And 4 way flashing action when not in braking mode. Hopefully we can walk you through the "Pitfalls" of Turn-signals and make it a bit easier to install and troubleshoot...


The Turn-signal system is a System that is isolated from other lamp circuits, but appears to function with the Brake lamp system, which, in part is true...

The Turn signal on a traditional system is a shared element with the brake lamp function at the bulb to the rear of the vehicle. The most common misconception is "I can wire my Turn-signals and brake lamps together at the light housing and it will be fine...and that's where the trouble begins. As you read on, hopefully you will see how this system functions and how the isolation is important to make it function properly.


Power on a traditional flash unit is delivered to the system VIA the Flash unit. A FUSED, SWITCHED lead is run from the Fuse buss to the flasher, so that it only receives power in the "Hot in Run" position.

FROM the Flasher, a wire is run To the Center wiper (or Movable contact) of the Turn signal switch inside the steering column.

The flasher operates from internal heat on a tungsten bi~metallic"strip" that makes or breaks the contact at the switch side of the contact, Or power output side.

As the system Lamp load is imposed on the flasher unit, the bimetallic strip heats up, and curls up... breaking contact to the switch... It then cools down very rapidly, and makes contact again, the lamp load again heats the strip and it opens up. THUS: "Flash"

When the load is under matched to the Flash unit (Bulb(s) burned out), the flash cycle is slowed or stuck "On”. This is what we perceive as a lamp outage and when this occurs. We pull over and have a new bulb put in...

The reason this occurs. Is because the Lamp load on the bimetallic Strip inside the flasher unit has been reduced to a point where the time allotted for heat is extended (Slowing the flash rate) OR never reaches the heat point of the strip (turning on and staying on)..

When these flash units fail. Usually the load on the strip has exceeded it's capability to recover and the strip just melts, Leaving the switch contact side "Open” from power... This is when we normally put a new flasher in. Causes of this type of failure can range from Age of the flash unit, to overtaxing of the lamp load... (As in Trailer lamps) Or a short within the electrical system.

The other side of the Coin is too many bulbs within the lamp load circuit. Taxing the Flasher unit. This causes the bimetallic tungsten strip to heat and cool at a Very rapid rate...

The more current draw THROUGH the flasher unit the faster it opens and closes the contact...

The Root causes of this type of failure are when you EXCEED the lamp count, Exceed the recommended WATTAGE of the bulbs, Install the WRONG bulb in the system (I.E. a single element bulb, where a duel should live), Shorting of the lamp system (common Culprit, Trailer plugs), And the addition of EXTRA lamps (such as a trailer) in the circuit without upgrading the flash unit.

ALWAYS install the recommended WATTAGE or Bulb Number when replacing the bulbs or this malady can occur...

The net result usually is a very rapid flash rate. And in some cases, failure of the flasher unit...Or premature failures on subsequent units...


This little switch has been responsible for more "Hair pulling” Incidents than most other parts of the Vehicle... When you understand HOW it works, and WHY it works the way it does. It’s really quite simple.

At the bottom of the steering column on traditional Turn signal systems, there is a Plug. From that plug all the system signals (power) goes in and comes out. And an added system, a wire from the brake lamp switch.

At this plug you will find Flash unit power in to the switch Handel, TWO sets of wires for the Left and right Turn signal lamp socket power, Those will be front left, rear left, front right, and rear right. These wires bring power to the lamps for turn signal AND brake lamps to the rear. Also at that plug you will find a wire for the brake lamp system from the Brake lamp switch.

The switch itself, Located below the horn ring on most traditional Systems, is like any other, except...It is two switches in one, the Left side and Right side. It also provides the isolation required between the Brake lamp system and the turn signal system.

To understand how the switch works, we can view one side of it for an example. If you remove the Steering wheel, and usually the horn assembly you can see the turn signal switches, usually on a plastic base plate, driven by a "plastic Cam" set off by two "spring steel" contacts that shift contact points when the base plate is activated with turn signal arm.

Let's look at the left side only for now...

As you activate the left turn signal, the following takes place. The cam rotates and the spring steel contact "Flexes" removing contact from the normally closed contact to the normally open contact.

This switch is wired traditionally as such:
  • The Center wiper (movable contact or cam contact) is Lamp output.
  • The Normally Closed Contact , Is Brake lamp power From The Brake lamp switch.
  • The Normally open Contact is Power from the flash unit.
When No activation is present at the turn signal, the brake lamps are configured to light when the pedal is depressed through the Normally closed contact of the switch and the Center wiper (lamp output).

Now , assuming you are at a stop light, turning left, The contact switches power when activated by the cam assembly, and opens (or turns OFF) the left brake lamp from the normally CLOSED contact And It now has contact Between the center wiper (lamp output) and the normally OPEN contact (the Flash unit).

Were the brake lamp and the Turn signal to both be on together, The result would be you would never see the Flashing of the signal. Since the same filament is shared by both systems, Even though both would be operable, when the flash unit was off, the brake power would still be on, making no noticeable change in the bulb.

If it were wired this way and both brake lamps were common to each other on the power leg. AND no brake pedal was pushed (brake power off) BOTH rear lamps would flash together.

In the above PROPER configuration, both systems would Gain Isolation from each other through the switch...

The EXACT same wiring is true for the right side, with the exception of the brake lamp power is picked up from the Left contact (daisy chained).


Let's Review. The system works in the following manner.

When NO turn signal is selected, BOTH brake lamps will receive power when the pedal is pushed...

When the Left Turn signal is selected, The Brake lamp for the left side is disabled by the turn signal switch and the bulb now receives power from the flash unit. At the same time NO change has been made to the right side Brake lamp. The Result:

Left lamp flashing, Right lamp steady with brakes on. Right lamp is out when brakes are off.

Now, Switch to the right side. The reverse becomes true...

The right side Brake lamp is disabled, and power to the lamp is now provided by the flash unit. Brakes on or off...

The left side remains on (no Change to the left side) steady with brake power, and out without it.

This is how the two systems are Isolated...and the most problematic for a Novice to understand, primarily because nobody ever took the time to explain it to them in any detail.

And the most common Failures:


This is one of the most common failures found on vehicle lamp systems. The first thing to do is check the bulb. If the bulb is good, check for power at the socket. To Do that:

Get your DVOM out, set it for Volts scale, V X 50 or higher, and turn the Key on then the offending signal. CAREFULLY insert the probe of your meter INTO the Socket, to the contact pad (S) on the bottom of the Socket, and the other to a GOOD Ground...

If you read 0, then 12 volts then zero... (Flash signal)The Power to the lamps AND the system are working fine. You have a bonding issue...

Shut off power to the system, and key, and return to the offending socket.

New Cars today have Plastic Bumper’s and Anti Collision Systems. Proper Ground Bonding is becoming Scarce Compared to Bygone Days...and Lighting systems have had to relay on "Wired" or "Remote Location" Grounds. It is quite common to lose ground to a single socket and disable it.

Set your Meter for OHMS Scale, R X 1, Calibrated 000, and measure between the socket and a KNOWN good ground. Your reading here must be 000, if infinite, or random, you need to perform good bonding repairs to the socket...

Using a Good Contact cleaner, carefully CLEAN the socket, Pads and lamp Brass base and its pads. Next recheck the socket for bonding, if your readings are still infinite or random, replace the ground wire to a known good ground area. Use a star washer and burnish all the paint and dirt from the bonding area.

That will get a Single lamp back to operation.


If none of the turn signal functions work AT ALL...

The first stop is the fuse. pull it and with your DVOM, measure it on OHMS Scale, R X 1 , Calibrated 000, If it reads infinite, it is open Replace the fuse and test the signal system again. If it reads 000, it is good, put it back in the buss box.

Next set your meter to DCV, V X 50 or auto range, Turn the Key On and remove the flasher. Place the black probe on ground, the other into either of the two flasher terminals, One will read infinite, the other should read 12 volts. If you have that, power is good to the flash unit.

Next, set your meter to OHMS, RX1 scale and measure across the flasher, it should read 000, if it does not, it is open, replace the flasher. If it does read 000, it is good (at least for one flash) if that is what happens, Replace it with a known good or new one.

NEXT, Replace the flasher, Turn either signal on, turn on the key, and isolate the wire from the flasher to the steering column PLUG. Measure it, With your DVOM set for DCV, V X 50 or auto range, and ground, It should read an intermittent 12 volts OR a steady 12 volts if nothing in the system is working. If you have neither, check the wire between the flasher and the column plug for cut condition, or a burned socket. If you have either:

Check the Other side of the column plug. It should read the same as the first reading. If you don't have that, pull the plug, inspect the pins for clean mating surfaces, and burned or bent pins or wires. Repair or replace as required.

If not, and still dead, check any lead front, or rear of the selected side. It should have 12 volts, either steady or intermittent on it. If not, your trouble is in the column harness between the plug and the switch. OR the switch itself is bad. Repair or replace as required.

Or Visa Versa:

Isolate the two wires the effected end, from the column plug. Using your voltmeter set to the above parameters, selects a side and measures the wire from the plug. It should show 12 volts (solid or intermittent), If not, you have a switch / harness failure. Inspect, and repair or replace as required.

If you have 12 volts, trace the harness rearward (or forward) until it comes to a plug. Inspect the harness for cut or burned condition as you go. Repair or replace as needed. If good inspect the plug (s) for clean proper mating pins, and burned wires. If good,

Go to the lamp buckets, and Measure between the socket ground and a known good ground, if it shows 12 volts, you may have an open ground wire. Replace or install a ground from each socket to a hard frame ground, following proper bonding procedures. (Star washers and burnishing)

EITHER SIDE DEAD (front & Rear):

Follow the same procedure as outlined for Front or rear dead above, EXCEPT apply it to the affected side.


Inspect the turn signal switch for a short between the brake lamp circuit and the signal circuit. Check the Column Plug / Harness for shorts. Ohm the harness out with NO Power on, But either side selected. You should have no reading from the selected side to the brake lamp circuit, and continuity in the "OFF" position to the brake lamps. If not, inspect the switch / harness and repair or replace as needed.

This will cover MOST of the BASIC & common Problems found in the signal system...

Tools needed: DVOM, Jumper wire set, A Good diagram, Contact cleaner, Acid Brush, Standard hand tools, Dental Pick, Emory Cloth, Miss Hardware.

Quick reply to this message
  #4 (permalink)  
Old 06-18-2006, 03:00 AM
Last wiki edit: Rebuild an alternator
Join Date: Mar 2004
Posts: 7,345
Wiki Edits: 12

Thanks: 0
Thanked 8 Times in 5 Posts
How Does a Relay Work? Why use one?

**********************How Does a Relay Work? ****************


A Relay is nothing more than an Automatic Switch. It consists or two related but isolated parts, The Magnetic Coil, And the Contact Sets. (Or Poles)

When Power and ground applicable to the COIL are applied, it energizes an Electromagnetic Coil that is wound on a round core.

As the core is magnetized, it pulls the movable spring loaded arm assembly from the contact side of the relay toward it. At the end of the movable contact arm (also known as the Center-wiper contact) are a set or sets of contacts on a spring steel extension arm, that mate with another set of stationary contacts.

If you have a clear cover relay, you can look inside and see the center-wiper assembly, and that the contacts are hardwired to the Pins on the bottom of the relay, different from the stationary contacts, which is passing through to the bottom of the relay.

The stationary Contacts are mating contacts; depending on the status of the coil energize line. At rest, Coil de~Energized, The stationary contact the center-wiper rests upon is known as the NORMALLY CLOSED CONTACT. This means if power were applied to the center-wiper, it could be read with a meter on the normally closed contact without coil energy.

When power is applied to the coil. The center-wiper contact is pulled toward another set of contact (s). These are known as NORMALLY OPEN CONTACT (S)

The following occurs: Power is now removed from the N.C. Contact and applied to the N.O. Contact.

  • If you had a relay, wired for switched 12 volts to the center-wiper contact.
  • A RED lamp to the normally open contact.
  • A GREEN light to the normally closed contact.
  • One side of the coil to 12 volts power at all times.
  • The ground to an engine temp switch / sender,

The result would be:

When the Ignition was turned on, the GREEN lamp would be lit.

As the engine warmed up to the sender predetermined temperature, the ground would be present at the Sender, completing the coil circuit...the relay fires, and the GREEN light goes out, and the RED lamp goes on.

This will remain so until the temperature declines, the sender shuts down (removing coil ground) and the relay de~energizes, power goes off the RED lamp and Reverts back to the GREEN.


Relays can come as Single pole, Single throw (one movement between 2 single poles) to multiple Poles, and multiple throws, depending upon selection.

The Electrical Ratings USUALLY are printed right on the Relay Body. For Instance: Coil: 12 VDC/VAC 1 AMP. / Contact Rating: 30 AMP, 250 VAC.

The latter is an Arc over rating, or melt point of the contactor the Voltage at which the contact COULD arc over the both contacts (not dropping the circuit) OR at which point the Contact COULD be welded shut. It does not mean you must run 250 VAC at the terminals.

What is Important to note, is the AMP rating of the contact. If you have a 25 amp fan, you want a relay with 30 amp contact ratings. And the Coil voltage must match the system your using it in (6 volt, 12 volt, 24 volt, Etc) and the current draw of the coil.


Relays are one of the most misunderstood devices we use in Vehicles today. The reason is because Most Novices get "Lost" in the "Magic" going on inside the Relay. Especially in interlock circuits and motor reverse circuits.

It's not all that bad if you always keep in mind that it is a switch! Nothing more. You are doing nothing more than switching power or signal from the center wiper to one of two states (N.O. or N.C. depending on the state of the coil)

Power or signal to the Center-wiper continuously, and power to the N.C. wiper without coil. Is "On" doing nothing...

Power on to the coil. And The "ON" changes to the Normally Open contact. The "OFF" changes to the Normally Closed Contact. Simple.

Relays can be very useful in their applications. The can Handle LARGE amounts of device current (the Contacts) with a Small amount of Control circuit current (Coil draw).


A 30 amp Fan running through a relay with 10 gauge wire, controlled by a 1 amp coil and 5 amp switch and 20 gauge control wire.

By doing this, you are delivering MORE Current (source) to the device (load) making it run better, doing more useful work, with less heat loss. using a control circuit that , directly wired , would not stand up to the demands of the device or load (most off the rack aftermarket switches are about 5 amp contact rated) And eliminating the risk of harness burnout or fire trying to run under rated wire or control devices.


They can be used to isolate circuits but control them with the same function of the coil.


You want to turn off a Steady lit bulb and momentarily imply a flashed signal to the same bulb (which wouldn't work if the lamp was lit full time...)

The bulb would go to the center-wiper,

The flash to the normally open contact

The Lamp steady power to the normally closed contact.

The coil would be switched by a switch that supplied power to the flash unit Steady selected on power).When the flash is selected the coil is energized, the steady lit lamp power goes out and the flash is put on the bulb.

We have isolated two Circuits to the same Device (the bulb) that normally wouldn't play together... (Steady on condition)


A Relay can be used to reverse a DC motor. (Like power seat or windows)


In the case of a motor, the control switch (Console switch) has forward and reverse selections, Center off.

12 volts and ground goes into the center of this switch, and when pushed forward brings power and ground to the Normally Closed Contacts (Double pole single throw relay) of the RELAY.

The motor is wired to the relay center-wipers. The relay does NOT fire, the motor moves forward. (Switch power to motor power, Switch ground to motor ground)

To reverse the RELAY, COIL power is attached to the "Reverse" normally open pole of the CONSOLE control switch, and gets power from the switch contact when in Reverse. The relay closes...

The Normally open RELAY contacts (power and ground) are cross wired to the normally Closed RELAY contacts in short jumpers, in an "X" pattern.

Power and ground from the normally open side goes to the normally closed side but reversed from the forward selection in relation to the center-wiper (motor Power to Switch Ground, Motor Ground to switch power)

This reverses the power and ground to the motor, allowing it to run backward.


Relays can be used as "Interlock" Systems... Or "Mechanical "AND" Gates. To get a desired result... you must have Signal at relay 1 AND signal to relay 2 in order for the result.


Switch 1 and switch 2 must be on to light a lamp. You have 2 relays acting in concert as an interlock system. It would be wired thus:

Coil for relay 1 has 12 volts constant, and the ground is run to switch 1, though the "ON" side of the switch to ground...

12 volts are wired to the normally open contact of relay 1,

The center-wiper is wired to the normally open contact of relay 2

Relay 2 Coil is wired as relay one EXCEPT to Switch 2...

The Normally open contact is already wired to relay 1 center-wiper contact.

The center wiper of relay 2 goes to the lamp power...

In this configuration, the system will only light the lamp if relay 1 AND relay 2 are energized...

If you have switch one on, but not switch two. Nothing happens. Power can't get past relay two...

If you have switch 2 on AND switch 1 off, nothing happens because relay contacts of relay 2 are not getting power...

Switch 1 "AND" Switch 2 must be "ON" to light the lamp, thus creating an "Interlock"

To transfer this to the real world. Substrate Switch 1, for say a fuel pump pressure sender, and switch 2 for example an oil pressure sender. The output (being the lamp) substitute for ignition coil power...

In order to start this engine, it would require cranking to bring oil and fuel pressure up before it actually started... (No dry starts) AND if for instance the engine lost oil pressure, it would shut down the coil... (No dry bearing damage) Less important is the loss of fuel pressure... but this is Only a Theoretical Circuit anyway.

This system is also the "MECHANICAL" version of a TTL (transistor to transistor logic) "AND" gate used in computers over the years. Instead of a relay, they use a chip about the size of your fingernail...

The reason it is called an "AND" gate is simple. In order for it to function you must have the following...1 AND 2=output

Signal at input 1 + Signal at input 2 = output high. Or yes.

No signal at input 1 + Signal at input 2= output low or No.

Signal At input 1 + no signal at input 2=output low or no.

This relates to what they say, Digital Electronics is just high speed switching. Your first lesson in TTL...and it didn't even hurt...


A Relay is a non polarity device in its basic Package (with few exceptions) so it doesn’t care where power and ground is applied to the coil. You can hook to either terminal.

You may if desirable, assign polarity to the relay coil by means of diode strapping across the coil terminals or an electrolytic Capacitor. Then you must observe polarity.


Another Feature that can be used with a relay is Time Delay. This is achieved with the employment of a resistor / Capacitor combination on the coil terminals (depending on terminal configuration, = Time delay on, or time delay off.)

A relay can be given time delay or contact hold in time depending on the values of a resistor Capacitor network on the ground terminal..


Simple English is AFTER power has been removed from the relay coil. The network of cap and resistor must drain off (through the coil) before the coil can drop out...


You want your headlights to stay on 30 seconds after you shut off power...

The relay contacts are wired normally. Headlights to the normally open contact and the center-wiper to power...

The Coil has power to the coil and the ground side has the RC Network.

After you shut power off to the coil it takes time for the 12 volts to drain off the coil before it can dropout because the Cap holds the storage of power. And drains at a constant rate (depending on values selected)

The reverse is true for "ON" delay. By switching the coil power to the RC network. Now it must CHARGE the cap before the coil fires and the device turns on...

For more on this ... and the math, look at any Basic Electronics guide under RC networks or Capacitors.

In conclusion, Relays can give you the ability to deliver more current from the source to the load, than over a straight wire and through underrated switches, adding harness safety and less heat...

The ability's to reverse motors, and control a bunch of circuits that otherwise would make no sense to direct wire.

They are easy to understand, and actually fun to "Invent" with.

Quick reply to this message
Closed Thread

Recent Electrical posts with photos

Quick Reply

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
I am never going to let anybody else work on my car. onebadmerc Hotrodders' Lounge 19 12-22-2004 07:52 PM
.035 wire for body work? Meathead Body - Exterior 9 09-02-2004 09:00 PM
Learned behavior. coldknock Hotrodders' Lounge 19 08-25-2004 02:50 PM
Stuff I work on at work :) esears Hotrodders' Lounge 6 09-29-2003 01:08 PM
Ever work on the impossible? johnsongrass1 Hotrodders' Lounge 16 08-20-2003 01:59 PM

All times are GMT -6. The time now is 05:58 PM.

Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.