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Old 12-20-2010, 08:13 PM
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Taillights/turn signals/running lights Not Working

Okay, I have searched the forums and haven't found anyone with a similar problem so here goes. I have a 1973 3/4 ton 4X4, 350/350th Chevrolet fleetside with a wood bed.

My taillights do not turn on.

With my headlights off, my blinkers flash, but when I turn on the headlights, the blinkers flash very slowly and are very dim. The turn signal indicators on the dash stay on.

My hazard lights work, but when I turn my left turn
signal on, both rear signals flash.

My brake lights do work.

Reverse lights do not work.

I have made my own harness using the original connectors from the firewall back to the taillight harness, and also made my own taillight harness. The original wires were cracked and cut up in several areas, I figured that was the problem, but the issues persist.

I thought maybe these were ground issues, so to test this, I removed the ground wires from taillights to bed sheetmetal and nothing changes.

I changed the headlight switch and brake switch, still the same. I have not changed the turn signal switch, and the turn signal lever does feel very sticky. I have not seen a bed to frame ground. I would like to put a 4 gauge ground wire from the bed to the frame and will do that soon. Fuses all "look" okay. All sockets and bulbs are new.

I really have no other ideas on what else to look for. Auto-electric is one of my weaknesses, so I'd appreciate any input from you guys. Thanks.
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Old 12-20-2010, 09:42 PM
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Quote:
Originally Posted by PancakesandSausage
With my headlights off, my blinkers flash, but when I turn on the headlights, the blinkers flash very slowly and are very dim. The turn signal indicators on the dash stay on.
Could be a bad ground in the FRONT.

Quote:
Originally Posted by PancakesandSausage
....I have not seen a bed to frame ground. I would like to put a 4 gauge ground wire from the bed to the frame and will do that soon.
That sounds like a logical place to start. 4 gauge is definitely overkill, 10 gauge would be more than adequate.

Hope this helps...
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Old 12-20-2010, 09:53 PM
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Quote:
Originally Posted by Joe G
Could be a bad ground in the FRONT.


That sounds like a logical place to start. 4 gauge is definitely overkill, 10 gauge would be more than adequate.

Hope this helps...

As far as the front goes, I have a ground from the (-) post on the battery to the radiator support, from the same bolt I have a ground to the frame. I also have a 4 gauge ground from the firewall to the engine (right head at rear). My right front blinker is not working (possibly a bad bulb--I haven't checked).

I will ground the bed directly to the frame tomorrow. As far as gauges for ground wires go, would there be any difference in using a 4 gauge vs a 10 gauge wire? I'd much rather use a 10 gauge for cost effectiveness, of course.

I appreciate the input. I know I haven't gone through everything yet, but I wanted some ideas on what to look for. I always appreciate the expertise here.

Mando
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Old 12-20-2010, 10:39 PM
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Would the fact that I removed the grounds at each taillamp and there was no change in anything confirm this is a ground issue?
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Old 12-20-2010, 11:08 PM
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Pancakeandsauage,

Turn switch is where i would look.
The lack of grounds will cause problems like the slow actions of the turn signals.
I think you may have connected wires incorrectly someplace in your making of the new harness.

Find a wiring diagram and trace each and every wire.

Have fun

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Old 12-20-2010, 11:57 PM
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I would make sure the grounds are all good. Not just the bed to the frame, but all the way back to the negative post of the battery.
Make sure the taillights ground to the bed.
Make sure the bed grounds to the frame.
Make sure the frame grounds to the cab.
Make sure the engine grounds to the frame, and the cab.
Make sure the battery grounds to the engine.

If you do not have all these grounds in place, you can have all sorts of wacky electrical problems. Things like the cab trying to ground through the clutch, or gas pedal linkage.
Once you know all the grounds are good, then look for other issues.

10 ga wire is good for 30 amps of current. It should be sufficient for almost everything, except starter wiring, and alternator output wiring.
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Old 12-21-2010, 04:07 AM
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I'd run a temp ground, 10 gauge should be OK, directly from the neg batt post back to the taillights and make sure it was in fact a ground issue.... if they work... clamp the temp grd wire to where you have the bed grounded.... it they still work.... move progressively back to the neg battery post etc.....
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Old 12-21-2010, 05:38 AM
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Do you have a tilt colum????
If so, you could have some broken wires inside the column (turn switch wiring)
Yep---a bad ground in the front can cause issues in the rear, or even side to side.

Those bulb sockets in the front can be troublesome, even those in the rear for that matter.

Taillights not working------do you have dash lights??? The pin for the dash lights (on the headlight switch) is powered by the taillight pin, could have one issue here as well.

Flasher cans-------the turn and the 4-way should be different cans.
If you are not using the truck for towing, and have no extra rear brake and signal lights, a heavy duty flasher in the turns may not work at all. Larger than stock wiring can affect this as well. The stock wiring was about 18ga wire, and that helps to create resistance and heat that trips the flasher.
Going to larger wiring for the lamps reduces the resistance and the flasher may not trip.

Had an instance on a 69 Olds where the left front turn would not flash.
The guy had a heavy duty flasher in the car, but there was not enough amperage draw to trip the breaker only on the left side. The left side of the car wiring is about a total of 10ft shorter than the right-----just enough to reduce the draw on the flasher.

Brake lights run thru the turn switch-----should be an orange wire feeding the brake switch and a white wire going to the turn switch.
A brown wire feeds the 4-way flashers & a purple wire feeds the turn signals.

Check each lamps ground to the frame or body with a meter-----should get a "000" reading if the grounds ar good. A loose ground at any lamp can create issues at another lamp.
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Old 12-21-2010, 10:27 AM
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If you are using an Ohmmeter to check the grounds at the taillight, you really should get a long piece of wire, and use the negative battery post as your "ground reference".

The old style of turn signal flasher has a bimetal strip in it, and current flowing through it heats it up, and in bends away from a contact inside the flasher, and makes the turn signals flash. The more resistance there is in the entire circuit for the turn signal, the smaller the total current flow, and the longer it takes for the flasher to get warm, and it flashes slower, if at all.
Using too small of a wire increases the total resistance. The current flow drops. The same thing happens if you replace the "normal" turn signal bulbs with LED lights. LED light do not draw much current, the thermal flasher quits flashing. If you hook up a trailer to a car, with this type of flasher, the turn signal now will flash faster, because the increase in current heats the flasher up really fast.

On many cars the rear turn signals have some of the brake circuitry in them. This is so the brake light can override the turn signal. Likewise, the four way flasher can override the turn signal, and maybe the brake light. In some cases, this may just be an extra switch that is "on" when the four way flasher switch is off, allowing the turn signal electricity to pass through it.

When dealing with signal light problems I get the four way flashers working first. Then get the brake lights working, and finally the turn signals. That helps eliminate a problem with the turn signals, that is actually in the four way flasher wiring. When you get everything working, then you need to go back and try to combine things, and confirm they still work.

By combining things, I mean do the turn signals still flash, when the taillights are on, and you step on the brake pedal, with the car in reverse, to also turn on the back up lamps.
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Old 12-21-2010, 07:31 PM
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Thanks for the suggestions, folks. I'll get back out there tomorrow and give it another shot.

Quote:
Originally Posted by EOD Guy
I'd run a temp ground, 10 gauge should be OK, directly from the neg batt post back to the taillights and make sure it was in fact a ground issue.... if they work... clamp the temp grd wire to where you have the bed grounded.... it they still work.... move progressively back to the neg battery post etc.....

By doing this, is this a way to find the area that is not grounded properly? One thing I did notice when I got this truck--the firewall to engine ground strap was not connected to the engine. It was just hanging there. I have since replaced that strap.
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Old 12-21-2010, 07:36 PM
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Quote:
Originally Posted by DanielC
If you are using an Ohmmeter to check the grounds at the taillight, you really should get a long piece of wire, and use the negative battery post as your "ground reference".

The old style of turn signal flasher has a bimetal strip in it, and current flowing through it heats it up, and in bends away from a contact inside the flasher, and makes the turn signals flash. The more resistance there is in the entire circuit for the turn signal, the smaller the total current flow, and the longer it takes for the flasher to get warm, and it flashes slower, if at all.
Using too small of a wire increases the total resistance.
When dealing with signal light problems I get the four way flashers working first. Then get the brake lights working, and finally the turn signals. That helps eliminate a problem with the turn signals, that is actually in the four way flasher wiring. When you get everything working, then you need to go back and try to combine things, and confirm they still work.
.
If I'm not mistaken, the factory used 18 gauge wire for these harnesses. I remade them with 16 gauge wire, but I don't think this will make a difference?
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Old 12-21-2010, 07:39 PM
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Quote:
Originally Posted by Bryan59EC
Do you have a tilt colum????
If so, you could have some broken wires inside the column (turn switch wiring)
Yep---a bad ground in the front can cause issues in the rear, or even side to side.

Those bulb sockets in the front can be troublesome, even those in the rear for that matter.

Taillights not working------do you have dash lights??? The pin for the dash lights (on the headlight switch) is powered by the taillight pin, could have one issue here as well.

Flasher cans-------the turn and the 4-way should be different cans.
If you are not using the truck for towing, and have no extra rear brake and signal lights, a heavy duty flasher in the turns may not work at all. Larger than stock wiring can affect this as well. The stock wiring was about 18ga wire, and that helps to create resistance and heat that trips the flasher.
Going to larger wiring for the lamps reduces the resistance and the flasher may not trip.

Had an instance on a 69 Olds where the left front turn would not flash.
The guy had a heavy duty flasher in the car, but there was not enough amperage draw to trip the breaker only on the left side. The left side of the car wiring is about a total of 10ft shorter than the right-----just enough to reduce the draw on the flasher.

Brake lights run thru the turn switch-----should be an orange wire feeding the brake switch and a white wire going to the turn switch.
A brown wire feeds the 4-way flashers & a purple wire feeds the turn signals.

Check each lamps ground to the frame or body with a meter-----should get a "000" reading if the grounds ar good. A loose ground at any lamp can create issues at another lamp.
I appreciate this info. I passed on the heavy duty/towing flashers. Excuse my ignorance, but how should I test the ground locations? I have a test light. I'm assuming I would put the key in the "on" position, ground the test light on metal, and touch the ground wire?

I do not have a tilt column, and I will be replacing the turn signal switch also.
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Old 12-21-2010, 08:44 PM
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This is one of the best pages I have found about explaining how to troubleshoot electrical problems.
http://www.vernco.com/sparks/

If you do have a bad ground, the information in that will help you isolate the problem.
Replacing the 18 ga wire with 16 ga wire is fine, in fact, it is better.
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Old 12-22-2010, 04:21 AM
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The correct way is to use an ohm meter and measure the resistance from the neg post of the batt to the eng block, body sheet metal, frame etc....... The fastest way is using a temp wire run from the neg post of the batt directly to the grounding point on your tail lights (might be the case/body of the tail light or a seperate wire)....

if the tail light works when you turn them on, move the temp ground wire to the sheet metal that the tail light is mounted on/in....

if the light doesn't work you need to check the connection from the light to the mount or the internal wiring for a dirty or corroded connection......

If the light did work, move the temp ground to the frame...... if the light doesn't work check the bots bolting the bed to the frame or install a ground wire from the bed to the frame (I don't believe you'll find a fault here)

keep working the temp ground wire back to where the neg post on the battery connects to the body and the eng block (this is the point I think you might find your issue) sometimes the pig tail from the neg battery post that connects to the body at some post becomes damaged or isn't connected or the ground cable is just connected to the engine and a seperate ground wire is connected from the eng (somewhere other than the the original battery cable) over time has become damaged or disconnected.
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Old 12-22-2010, 08:40 AM
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"The correct way is to use an ohm meter and measure the resistance from the neg post of the batt to the eng block, body sheet metal, frame etc."

I would kindly suggest you read the Vernco article I posted above. If you have a corroded connection, or a case where a damaged wire has only one strand making contact, the ohmmeter will still read very close to zero, because it does not run any appreciable current through the circuit being tested.

Run the wire like above, directly to the negative battery terminal, and then go and turn on the lights. Check the voltage between the wire, and the outside of the socket holding the lamp. Like the Vernco article said, .2 volts, or less is good. .5 volts is ok, more than that, and you need to do some work, finding out why.

If the taillights are still dim, After checking the ground side, you can move the wire directly to the positive post, and again turn on the lights. Checking the voltage between the wire going directly to the battery, and the wire feeding the lamp that is on will again show a voltage drop.

Here is the beauty of the voltage drop method. It works with any circuit, from a single taillight, to the cranking circuit on an engine.


If in the case of the taillights, if you find excessive voltage, leave the volt meter connected to the wire going directly to the battery. Start moving away from the taillight. If you are checking the grounds, check the voltage at the bed, then the frame, and back to the battery, until the voltage drop is at an acceptable level. When it is, you passed the bad connection. Then you can confirm by checking the voltage drop between the bed and the frame, for example, if that is where the voltage drop seemed to go away..
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