The Stovebolt Site Forums The Shop Area The Electrical Bay Taillights not working !!

When I turn on my headlights,my turnsignals dont work on my taillights.work up front.Changed the headlight switch but didnt solve the problem.58 GMC stepside truck.Any ADVICE???????
Thanks,Danny

If they work with the headlights turned off, you've got a bad ground between the bed and the frame. The turn signal is backing up through the tail light wiring to find a ground until voltage goes onto the tail light wires. Since current can't flow both ways through a wire, the turn signals quit working with the tail lights on.
Jerry

Danny,

Welcome to the Stovebolt..I think half of Texas is on board.

This has always a tough concept for me to grasp. A picture of a '58 Stepside shows two single light assemblies in back. I think this means each housing has a single dual element lamp in it. The lamp would consist of a large stop filament and a smaller tail filament.

Wouldn't the base of each bulb be the common ground for both filament to housing/bed/frame. If this is true, then having the taillights function would mean the ground path is good. Does this sound reasonable.

If there were two housings in back of the truck, that would mean they would both have to have identical ground problems.

Finding ground through a nearby filament is known to happen, but I can't see how it would work in this scenario.

I would pull the lens off the lights and see exactly what the bulbs were doing under each condition. This might give you some direction.

Stuart

Stuart, here's what's probably happening: The lack of a ground between the bed and the frame is preventing either one of the filaments from grounding through the brass bulb base as they should. When the voltage cannot find its normal ground, it backs up through the other filament, and finds its ground by running backwards through what should be a hot wire. When the lights are turned on, both wires have voltage, preventing any path to ground. It's a very common problem, one that few mechanics, even ones with extensive electrical experience, really understand. The simple way to troubleshoot it is to run a long wire from a known good ground and touch it to the brass base of the bulb. If the lights work normally, you've found the problem. The permanent fix is to run a heavy ground wire from the bed to the frame, and make sure the taillight assemblies are well-grounded to the bed.
Jerry

I'd say something is wired wrong - the rear turn lights are actually the brake lights, and having the headlights on should have no effect on them - unless somethings shorted somewhere

the circuit is: power [orange wire from S on light sw] to the brake switch, then to the turn sig sw assy, then on to the rear brake light, one wire to each light assy ... the tail lights [black wire] get power from the T terminal on light sw, and that wire is common to both lights

when you signal, the seperate power from the signal assy is sent to the relevent rear light brake element - if you are applying the brakes at the same time, the flasher interrupts the power applied from the pedal sw ... make sure you have the connectors underneath hooked up correctly [info for Chevy, but I'd expect to find the same on GMC] - if you have aftermarket rather than factory signals, should have a similar routing setup

Bill

Bil,Jerry,Stuart--

Thnx 4 the advice.Will try grounding bed to frame . I'll let you'll know how it works.
Danny

The path to ground from ungrounded tail lights would be as follows: Right blinker filament to tail light filament, through the common wiring up to the front parking lights and down through the front parking light filaments to ground. Usually with a problem like this it is most practical to renew the ground before trying anything else.

I had almost exactly the same problem as you when I bout my truck and was told this very same thing, grounding to the box was my problem.

Ground the bed to the frame using #12 wire.Didnt fix my problem.Oh yeah, I have led taillites back there-shouldn't matter though right?

call an electrician,,, just joking.

I use to tap on mine sometimes to get them to work.

Did you run a ground wire from the lamp to the frame, as suggested by HRL? Then you can be sure of a good ground all the way to the bulb (from lamp back to battery, to be completely sure).

LED's?? I don't know what the answer it but if you get it figured out please post o.k.? My buddy is having the same problem on his street rod. Everything works fine with regular bulbs but the LED package he bought won't work right.

The first place to verify a good ground is always under the tailight housing where the assembly screws into the taillight opening in the bed, clean off the metal surface of the bed and taillight housing where there is contact. Wire brush the screws and use some new star washers. And rusty surfaces do not count as bare metal, and neither does paint. Scratch the paint off with a screw driver where it makes contact. Then add a ground wire from the frame to the box frame. If your bed boards are rotten, and the bed is loose because of it, most likely the bolts are all rusty also giving you a poor ground to the box. It won't matter if you have regular bulbs or LEDs. If you don't get a good ground, both styles will give problems.

a good ground system is a must!! I also paint all my grounds points w/ an anti-corrosive paste.
it does matter about combining regular bulbs with LED's.

LED's are polority sensitive
so
truck's hot +12v must go to hot +12v, gnd to gnd, of the light.

Flasher units work because the spring inside gets hot, they are heat sensitive and the spring breaks the contact w/in the flasher plug-in unit's contact.
http://auto.howstuffworks.com/turn-signal2.htm
In order for an LED to work as a blinker you must either
1. change the flasher unit to an LED type
or
2. have some resistance in the same circuit as the LED so the stock flasher unit will activate the circuit. this resistance can be a normal bulb located in the front of the truck (incandesant blinker lamp). this also must have a good ground.

http://www.howstuffworks.com/framed.htm?parent=turn-signal.htm&url=http://knology.net/~rkmason/flashers.htm

The brass sockets corrode inside over the years and make poor contact, and you can lose the ground. I use a dremel tool, with a small sanding drum to polish the inside of the sockets.

When I turn on my headlights,my turnsignals dont work on my taillights.work up front.Changed the headlight switch but didnt solve the problem. ............ Oh yeah, I have led taillites back there-shouldn't matter though right? '''''
danny could use better diction but he'll learn about clarity soon enough.


if he has LED's he probably has no sockets.

key work here is '''''probably''''' as the ones that fit into sockets require a small amount
of room inside the housing and the way bolts sockets are, do not have that much .

You can buy LED bulbs that fit the original sockets. I have Model A f##d that has them.

Got the tail lights working!!
I sand-papered everywhere the lights touched metal till shiney.Also sanded where they bolted to bed.
YAHOO !!!

I went all the way here-
I sanded as mentioned plus new ground cable to (sanded)frame.New ground cable from engine to (sanded)frame.New ground from bed to (sanded)frame.

THANKS for all of the advice,Danny

now coat them for protection
http://s67.photobucket.com/albums/h298/1951otter/?action=view&current=2650080-R2-028-12A.jpg
http://s67.photobucket.com/albums/h298/1951otter/?action=view&current=2650080-R2-008-2A.jpg

Coat the terminals with corrosion preventative spray, You can also coat the connectors with petroleum jelly or battery anticorrosion paste.

dtwbcs, If you have LED and have a stock Flasher. This is where the problem for you may be. Or at least start there.

There is a special Flasher unit for use with LED lights.

NOTE: if he only has LED's on the back and incandesant bulbs on the front all should work fine as long as all the lights are grounded well.

HEHEH Told yuh.

Personally I'd rather shave a bobcats butt in a phone booth with a butter knife than try and find an electrical short. AS most of the replies you've had tell you, it almost (ALMOST)always is a bad ground. Good luck and I'll let you borrow my butter knife if you want.

Where would I find the anti-corrosive spray?