Can somebody please help me with a electrical fault?

[Paul Y]

Good evening Gentlemen,

For those of you who have read my build thread you will know full well that I am not keen on electrics.

Might be because I feed the damn Mogwai after dark or some other cardinal sin but pretty close to the match and petrol resolution, well if it was warm enough for the petrol to ignite....

Anyway lets talk brake lights...



Those on my passenger side fail to operate. Changed the bulb from the drivers side and still not lighty.

Chased the cable back to the junction on the drivers side and tested it there.



From what I can make out there are 3 wires into the junction which then split it into left and right tail light, left brake and indicator and right brake and indicator (and of course the number plate light)

I have power to the tail light on the passenger side.

I have power to the indicator on the passenger side.

What I don't have is a brake light.

How can that be possible? The indicator 'flashes' the brake light, and the brake light wire from the cab is shared with the drivers side. only splitting at the junction.

So, could someone far more intelligent than I make a suggestion? Or lend me a lighter....

P.

[64shortbed]

Most likely bad ground. Had a customer i bought a harness for and it was acting all funny. They finally ran a ground form battery to both light housing and they worked great. But i'm sure you dont want a wire hanging under there for ever. but try that and see if it helps.

[SS317]

I agree with the ground. connect a small wire from the base of the light housing to the frame, it won't need to be more than a few feet long. Grounds cause all sorts of strange problems on these trucks.

[Paul Y]

I also thought it was an earth problem so took the whole housing out and used a test light with a known good earth. Everything works apart from the brake light.

So...are you suggesting I put it all back together and run a cable from the housing to a shared earth? But is the test light not doing the same thing?

P.

[cooperhw]

It's an easy thing to try. My electrical guy drills a small hole into the housing and puts a small #10 (or so) bolt with the head only inside the housing. The nut and lock washer stick out the back side where he runs a dedicated ground all the way from the wiring harness. Just branch off wherever you need a ground. Other than that I would say you either have power where you want it or you don't. If you don't backtrack and see where you do have it. Somewhere between do have and don't have is the problem.
I would bet money on grounding.

[66_c10_tpi]

Does the right turn signal work? If that works I wouldn't think it is aground problem since it is the same filament in the bulb.

[Paul Y]

Originally Posted by 66_c10_tpi

Does the right turn signal work? If that works I wouldn't think it is aground problem since it is the same filament in the bulb.

Thank you.

I have actually removed the housing and am using a test light straight onto the connectors. In that way I have eliminated any earth fault as it is the same across all three functions. Light, indicator and brake.

To answer your question fully the indicator does work, as do the lights. It is only the brake light that does not work on the passenger side.

P.

[rickywideglide]

im thinking turn signal switch inside housing those little copper pieces are dirty or worn out, i thought brake signal comes up through there,if turn signal works on side that brake light doesnt the wiring to back should be ok since its the same wire.. if that makes since

[66_c10_tpi]

Like you said you have three wires coming back. 1 wire feeds the tail lights and the license plate light. 2nd wire feeds the left turn signal/brake light and the 3rd wire feeds the right turn signal/brake light. Both turn signal/brake light wires are feed from the turn signal switch. I would check the wires at the steering column junction block to make sure you have power there with your test light.

[Paul Y]

OK, so my evening shower has given me a little thinking time...

I believe I am making this more complicated than it needs to be so can somebody please sanity check my approach?

1. The wiring is good from the cab back as all light function as they should - apart from the errant brake light.

2. The indicator uses the same wiring from the junction at the rear bumper as the brake light so the wiring is good.

3. Therefore the Gremlins are somewhere in the junction at the rear bumper intent on giving me a stroke...

Mmmmmm... think that covers it.....

P.

[Paul Y]

Originally Posted by 66_c10_tpi

Like you said you have three wires coming back. 1 wire feeds the tail lights and the license plate light. 2nd wire feeds the left turn signal/brake light and the 3rd wire feeds the right turn signal/brake light. Both turn signal/brake light wires are feed from the turn signal switch. I would check the wires at the steering column junction block to make sure you have power there with your test light.

Too late this evening but will check tomorrow and report back. Thanks for the reply, it is appreciated.

P.

[66_c10_tpi]

Maybe I am a little confused also. When you talk about the indicator. I am not for sure what you are calling the indicator.

[Paul Y]

Originally Posted by 66_c10_tpi

Maybe I am a little confused also. When you talk about the indicator. I am not for sure what you are calling the indicator.

A common language separated by 3000miles, bit like when I asked the concierge at the Beverly Hilton for a rubber...

Indicator = Turn signal.

P.

[66_c10_tpi]

One last comment. The three wires you have.
Brown wire is for tail lights and license plate light.
Yellow is left turn/brake light
Dark green is right turn/brake light
As you stated everything works but right brake light. I would agree with rickywideglide it sounds like a problem with turn signal switch.

[luvbowties]

For what it is worth, my bet is on the turn-signal-brake wires inside the steering column. Being a bit positive, W H E N you convert to tilt steering column, your problem will prolly(American short for "probably")be solved!
Luck, & enjoy working on this marvel as the rest of us do ours!
Sam

PS:I recall a guy once who actually finished his project--went into deep depression until he could find another one with problems. Moral: with these 60-66's, never give up, quit, or finish!!!

[donthekawguy]

My problems were all in the dimmer switch. I changed the switch and still had problems. A buddy came over with his fancy electrical dodad. All the connections were loose.

[joedoh]

Originally Posted by Paul Y

I have actually removed the housing and am using a test light straight onto the connectors. In that way I have eliminated any earth fault as it is the same across all three functions. Light, indicator and brake.

well, I think you have only proven that the lights should work with a proper ground.

please. take the advice of everyone who has said it. connect a wire directly to the light housing and connect the other side directly to the frame. it fixes taillight problems 99.99% of the time. It fixed my problem, and I had this same problem with a 1998 OEM GM harness in my 1965 truck. Without the little additional ground wire, no light. with the extra wire, brake turn everything.

currently (pun intended) the lights get their 12v+ from the wires and 12v- from their contact with the bed metal. if the light bucket has a poor connection to the bed or bed has a poor connection to the frame or the battery has a poor connection to the frame or any of a hundred places you could have a ground problem, it wont work.

By the way, love your project thread already!

[fleetsidelarry]

Originally Posted by Paul Y

...the brake light wire from the cab is shared with the drivers side. only splitting at the junction.
P.

if you haven't already noticed, this is incorrect

Originally Posted by 66_c10_tpi

One last comment. The three wires you have.
Brown wire is for tail lights and license plate light.
Yellow is left turn/brake light
Dark green is right turn/brake light

this correct.

the wire that splits is for tail lights

power all the way back to the passenger-side turn signal indicates that that circuit (which includes the brake) is fine from the firewall plug. a working turn signal bulb suggests to me that the ground (earth) is ok.

power for the brake/turn signals comes from the headlight switch to the brake light switch then to a connector (see http://67-72chevytrucks.com/vboard/s...61&postcount=1) and, finally up to the turn signal switch and back.

using a test light, check the connector. the dark green wire should show a blinking test light when key is on and turn signal lever is in the passenger side mode.

To test the brake circuit, the white wire from the brake light switch should light up your test light when the brake is applied. the dark green wire should also light when the brake is applied. so should the yellow wire. if the fault is in the turn signal switch, the dark green wire won't light the test light

[OldGMJon]

I had the exact same issue on my 65' GMC and after all the frustration I took it to a local shop that told me it was either the turn signal mechanism on the column or the light switch itself. They wanted to replace them but those repairs could end up being an expensive can of worms at a shop with no guarantee of it being fixed so figured I could replace those myself...I haven't actually tried the repair yet because my rear main seal went out but thought it may give you another direction to try. jon

[wayno]

I'm sorry didn't he say that they work on the drivers but not the passenger - doesn't that take all the switch stuff out of the mix... doesn't he have power at least to the junction to light the driver's side or am I missing something - again electrical isn't my thing either - so I should probably sit in silence and take notes

[wildearp]

Originally Posted by OldGMJon

I had the exact same issue on my 65' GMC and after all the frustration I took it to a local shop that told me it was either the turn signal mechanism on the column or the light switch itself. They wanted to replace them but those repairs could end up being an expensive can of worms at a shop with no guarantee of it being fixed so figured I could replace those myself...I haven't actually tried the repair yet because my rear main seal went out but thought it may give you another direction to try. jon

The turn signal and flasher can interrupts the same filament used for the brake light. Either the brake light is on solid, or it is flashing when the turn signal switch is activated. This should be checked out. If you have a column with a lock plate, a tool is sold on Amazon for $8. I finally bought one after years of using a socket and pliers.

Your light socket inserts can go bad too, and it is hard to see with an older set of eyeballs.

Being from the electronics field, it is absolutely laughable just how many people say everything is a bad ground and honestly think that is helpful.

My dead shorted starter was a "bad ground" to the guy that thought he was helping me. I am sure he would have fought to the death over his troubleshooting.

[SS317]

[QUOTEBeing from the electronics field, it is absolutely laughable just how many people say everything is a bad ground and honestly think that is helpful.

My dead shorted starter was a "bad ground" to the guy that thought he was helping me. I am sure he would have fought to the death over his troubleshooting. [/QUOTE]

Well, being I have over 20 years of experience doing complex avionics systems in jets I used to think the "bad ground" thing was laughable as well, but then I took the advise of a few people and started checking and installing new grounds. I'm not saying that it is the exact cause of your problem, I ended up installing entire new wiring harnesses into my truck it was so messed up wiring wise, but be careful not to discount the knowledge here. After a while people will stop answering your questions if you do.

[Paul Y]

Originally Posted by fleetsidelarry

if you haven't already noticed, this is incorrect



this correct.

the wire that splits is for tail lights

power all the way back to the passenger-side turn signal indicates that that circuit (which includes the brake) is fine from the firewall plug. a working turn signal bulb suggests to me that the ground (earth) is ok.

power for the brake/turn signals comes from the headlight switch to the brake light switch then to a connector (see http://67-72chevytrucks.com/vboard/s...61&postcount=1) and, finally up to the turn signal switch and back.

using a test light, check the connector. the dark green wire should show a blinking test light when key is on and turn signal lever is in the passenger side mode.

To test the brake circuit, the white wire from the brake light switch should light up your test light when the brake is applied. the dark green wire should also light when the brake is applied. so should the yellow wire. if the fault is in the turn signal switch, the dark green wire won't light the test light

Larry,

Thank you for such an informative and well written trouble shooting piece. Exactly the voice of experience I hoped would contribute.

Good Lady's birthday tomorrow so might be a day or two before I report back.

Thanks again,

P.

[joedoh]

Originally Posted by wildearp

Being from the electronics field, it is absolutely laughable just how many people say everything is a bad ground and honestly think that is helpful.

My dead shorted starter was a "bad ground" to the guy that thought he was helping me. I am sure he would have fought to the death over his troubleshooting.

I am from the electronics field too, I am a master degreed electrical engineer who integrates avionics and designs electrical equipment for aerospace. your cavalier attitude about the importance of grounding has given me plenty to laugh about too, so thanks.

I have seen first hand how improperly or otherwise ungrounded equipment goes hunting for a ground, sometimes with catastrophic results. catastrophic is not a word used to embellish or weight my argument, it is a damage event class identifier used in designing equipment to 10^-9 reliability.

in the case of 60-66 taillights, this may not be catastrophic, just annoying, but it is well documented. Search is disabled but when it isnt you should look for the number of threads where it is insisted that it CANT be the ground, and eventually it is.

[fleetsidelarry]

good grounding is vital and should be verified, I agree completely. just, in this case, since the same wire and bulb filament works while flashing but not during braking, it didn't seem likely to be a grounding problem.