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Old 05-09-2005, 07:56 AM
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Elec. fuel pump relay needed?

Well I got my electric fuel pump all wired up , turned the key to "on" and touched the cable to the positive terminal on me 70 Monte carlo wich has the battery mounted in the trunk.The Mallery pump breifly ran , then the relay "popped" and had an electrical smell.I could not get the pump to work again.I have over 12 volts coming from the battery to the relay and from the ignition switch at the relay.My question is this , if there is over 12 volts coming from the on/off switch( ignition) do I even need to use the relay?

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Old 05-09-2005, 04:19 PM
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Quote:
Originally Posted by upstand2
Well I got my electric fuel pump all wired up , turned the key to "on" and touched the cable to the positive terminal on me 70 Monte carlo wich has the battery mounted in the trunk.The Mallery pump breifly ran , then the relay "popped" and had an electrical smell.I could not get the pump to work again.I have over 12 volts coming from the battery to the relay and from the ignition switch at the relay.My question is this , if there is over 12 volts coming from the on/off switch( ignition) do I even need to use the relay?
No, those pumps draw relatively little current so relays aren't necessary. Just another thing to go wrong as you just found out!
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Old 05-09-2005, 06:39 PM
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Voltage Vs Current: Relays Nomenclature

Doc here,

Voltage is not the issue Here..Current IS.

A Good rule of thumb for safety Is: Relay Anything That is high current or motor controlled and draws over 15 amps continueos...

Most Vehicle wiring is Capable of handling up to 20 amps on it's own perfectly well, Motors are a variable equation...

If a motor suffers a "Rotor lock" (jammed bearing or something stuck on the motor shaft locking it up) it can draw tons of current ... and the weakest link will burn out first, usually the fuse..but sometimes the wire will act like a fuse.

Some Folks will say, not so bad replace one wire and it's all good again, but consider the possibility that that wire is bundled in a harness with 10 other wires...those wires now have suffered melted and burned insulation, and are all shorting together..NOW you have fuses and links poping all over the place, and possibly, the harness going up in flames!

The proper use of a relay, Is to wire the relay coil, (also known as the Control circuit) With The smaller gauge wire (usually stock wiring on an aftermarket addition, as in your case..)To the coil power, and ground the other side of the coil..Relay coils generally ARE not polarity Senisitive..(unless you strap a diode across the coil terminals to GIVE it polarity or an electrolytic Cap to give it polarity AND time delay, so you can use any Coil terminal for ground or power, it won't care..).

Find the draw of the relay coil, (usually written right on the relay) It should be around: 12 volts, at 1 to 3 amps, and fuse the coil for that amount, maybe just a tiny bit higher..This will protect the Control Circuit, and the wire in-between in case of short or coil failure (Coil insulation breaking down, and making a dead short through the coil internally)

Next find the CONTACT rating..and CONTACT arangement..This is referred to as poles:
  • N/C (Normally Closed when the relay is static, at rest ..or Off, in relationship to the Center wiper or movable contact pole)
  • N/O (normally open,when the relay is static, or at rest or off, in relationship to the center wiper, or movable contact pole..)
  • C/W or center wiper, is the movable contact that completes or breaks a the circuit, or both, as the magnetic coil (control circuit) Energizes or de~energizes the Center wiper arm.

The Contact rating will usually be written on the relay, and read something like this: 30 A / 250 volts ac/dc...The Current Rating, (30 A ) is the amount of current the CONTACTS can safely handle, without burning or destroying the contact points internal the relay...

The VOLTAGE rating is simply the "ARC~OVER" rating, or the Voltage at which the contacts will suffer an arc over opening or closing, or weld point..where the contacts become permanently Normally Closed..

If you want to run a motor, Such as a fan, that draws 25 amp controlled by something else, say like a thermostat as the control circuit (Coil), get a 30 amp relay, Your Contact arrangement will be:
  • N/O (Normally open) 12 Volts, This wire will be of the proper gauge (say for grins, 10 gauge) to a FUSED source of 25-30 amps. (Keyed or Battery Direct, it doesn't really matter)
  • C/W (center wiper) Will Be the wire say, 10 gauge, to your fan motor.

The normally closed contact (if one is present) will not be used, and is the reason for putting the 12 volts on the N/O contact..so at rest. or off, you don't have the possibility or something shorting the normally closed (and unused) pole out..If you were to wire 12 volts to the center wiper (and most do) and had an unused N/C pole..the N/C pole would receive 12 volts when "Off" via the C/W. (one more thing to possibly go wrong)

Now lets assume The thermostat is an engine mounted "Ground type " of bi metallic Switch, as the heat rises, the contacts close and ground out..Your CONTROL CIRCUIT will be arranged thusly:
  • The Coil either terminal, will be wired to a 12 volt FUSED (1 to 3 amps) say, 18 gauge wire, Ignition Source, so that it receives CONSTANT voltage anytime the key is on..
  • The Ground side, instead of hard grounding the wire, will run To the Thermostat switch on the engine..assume 18 gauge also.

Your circuit will operate this way: When the key is on, and the thermostat is at or below threshold point of the closed contact temp, The Fan will do nothing..BECAUSE the control circuit has no ground to complete the circuit..as the temp rises at the thermostat, the switch internal the sender closes and the control circuit is complete to ground (ground provided through the sender), fires Control circuit (coil) the contacts (or C/W , movable contact) and the N/O contacts make a closed circuit, result: the fan runs..as the temp decreases , The ground is removed, from the opening of the sender, and the control circuit goes open, the contacts open, and the fan stops running..

What have we Done here? :

We have controlled a 30 amp motor, with 18 gauge wire, and a 3 amp fuse Safely, and managed to get the control contacts (the sender, which won't handle more than about 5 amps ) To turn on and off a 30 amp motor..

AND (by using the 10 gauge wire on the secondary circuit or contacts) We are delivering More current to the motor than we could trying to control it directly with smaller gauge wire, and eliminated the possibly of Harness burnout..

Yes, It does add more to the mix as far as possible things to go wrong..2 fuses and a relay, but it IS the only way to control motors and high current applications safely.

Sorry for the dissertation, But I'm asked this a lot..when to use one or not, what are N/O, N/C, C/W, How does it protect the harness, and the like, so I thought I'd Take the time to put this out there,...Most you guys already know this stuff.

As Willys Said, If the pump is a low current draw, and/or IS NOT controlled by another outside source (such as a "LOW OIL signal, high engine temp, or rollover switch) You Don't need a relay, unless you want to install one for peace of mind...OR install other control functions.

Doc
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Old 05-10-2005, 05:26 AM
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Thanks for the replys.Well your right , the first relay "popped" because my electric pump went bad.I put another pump in and a new relay and everything is good now.I'll have to read that last post again and sleep on it to soak all the info in.Thanks again.
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Old 05-10-2005, 06:21 AM
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Very Good Information...
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