Turn signal and brake lights.

[Murdock]

The 87 GMC we have, has a electrical issue. When you apply the brakes the brake lights won't work, but the turn signal indicators in the truck will light up. Also the turn signal switch is hard to click in place, near impossible to.

So am I correct in assuming that its the switch that went bad and causing these issues?

 

[Boone83K10]

Let me see if I read what you are saying correctly...

The brake lights do not light up when you press the pedal, but the indicator arrows on the dash light up??

Those are completely different circuits and I don't see how that is happening except there is a short somewhere under the dash, particularly the place where all those wires come into the column.

Under the column, look for a connector that has the following wire colors.

Yellow is the left rear wire, Dark Green is the right rear wire. Light blue is the left dash indicator light as well as the left front signal light. Dark blue is the right dash indicator light as well as the right front signal light. Those are getting crossed up somewhere, and it may very well be the multi-function switch you referenced.

 

[chengny]

Listen to Boone. He has recently been through some intensive training in dealing with rear light issues. It is all fresh in his mind.

Read this thread for help - it covers a lot of possible answers as to why the lights don't work they way they should:

http://www.gmsquarebody.com/forum/showthread.php?t=7823

First check that all the sockets are clean and all the lamps are the correct type.

 

[Murdock]

Alright I will have to be sure to check this, but as I said earlier, the Switch doesn't click into place. You have to physically hold it in place for it to work for right...even then it doesn't work right, and left you have to force it down for it to click. So just wondering if that is the solo issue.

 

[Boone83K10]

read that thread with caution..make your head spin!

I wouldn't have fixed mine if chengny hadn't helped me through it.

but now I am like, pssh electrical work, no prob boxed and now I want to fix all the other jerry rigged wiring in the engine bay and my broken, held on with zip tie selector valve connector.


PS - chengny, remember I said that my truck never has converter lockup. I guess checking every wire made a connection somewhere because now it works...

 

[Murdock]

I just read all that, amazing all that and it was just melted at the truck frame in the back it seems. It reminds me of trying to trace down the electrical gremlins in my Volkswagen Beetle. Everything is body grounded, but if one little thing is not perfectly connected....then you have to go through and check it all.

So defiantly checking the area you said and the rear part as well as the bed is off now.

Just a note, it was all working before, but we stopped using the truck during the winter and I noticed the issues during the summer.

 

[chengny]

When you apply the brakes the brake lights won't work, but the turn signal indicators in the truck will light up.

Someone will correct me if I'm wrong but I believe the brake light is the directional light and vice-versa. The same filament within the dual filament lamp is used to indicate braking or turning.

There are only 4 wires that serve the rear lighting:

1. Brown wire - supplies parking (one of the filaments in the dual lamp), marker and license plate lights. Energized through the headlight switch by pulling at least halfway out. These lights stay on at all times that the H/L switch is in the parking light position (or headlights).

2. Light Green wire - supplies power to the reversing/back up lamps. It is energized when the gear selector is in "R" and the back up contacts within the neutral safety switch close.

3 & 4. Dark Green & Yellow wires - these wires supply the combination brake light/directional lights which are the other filaments within the dual filament lamp. Dk Grn is for the RH side and Y is for the LH side. The power can come from either the brake switch or the directional switch. It gets involved because the splitting of the signal happens in the MFS (multi function switch).


So, long story short - by design, stepping on the brake should illuminate the directional filament (because it is also the brake filament.

If you have a turn signal on and then step on the brakes - the side that is indicating the direction of your turn will act as a blinker. The other non-turn side will go to a steady light and act as a brake light.


Having said all that - I have to ask what exactly you mean by this:

When you apply the brakes the brake lights won't work, but the turn signal indicators in the truck will light up.

#1 - Do you mean that you never - at any time - get any lights at all when you step on the brakes?

#2 - Or do you mean that when you press the brakes the brake lights don't light up but the directionals do?

If you mean problem # 1- begin by checking the brake light switch


If however, you mean #2, did the explanation above clear things up. Or did I only confuse the issue even more?

If the brake light switch is in place (and switching the power to the white wire off and on as designed), you have a problem in the brake/blinker common area of the MFS.

Since you are having a big problem just even changing the position of the MFS stalk - I would not be surprised if the brake light switch proves good and the MFS is found to be shorted.

 

[chengny]

I had a 67 Beetle. Like all Bugs, the battery was located under the rear seat.

One night we went to pick up a keg. Carried it out and threw it into the back - on the seat.

On the drive back to the party, things went bad.

Smelled smoke, lost all electrical power and then as we scratched our heads - the back seat burst into flames.

Investigation revealed that the weight of the keg had forced the seat's coil springs down to the point that they bridged the battery terminals. The coils then became like giant toaster elements and heated up cherry red - hence the black out and seat fire.

Luckily, the keg was unharmed. Later that evening it helped to ease the pain of losing my back seat. The whole incident also made for a good beer drinking story.

 

[Murdock]

I mean as you apply the brakes, the turn signal indicators in the dash will light up. I never recall this happening before, and before the bed was off the turn signals didn't work at all on any side.

Anyways I did find a black wire that was not hooked up in the wiring harness that connects to the rear lights, and that the tag lamps are not wired either.

However, now that the bed is off and the rear lights are not hooked up to the harness, the turn signals work....until I turn on the lights. When the lights are on, the passenger side will work, but not the driver side. Also, when the lights are off the signals work for both sides but the driver side is very dim.

So Chengny I don't think that helped me a whole lot. I fail to see how with the tail lights how if I step on the brakes the indicators in the dash light up but not the outside lights.

Also I have a 1960 Beetle and that must have been one heavy keg or one broken seat to sag that much. But funny story none the less.

 

[chengny]

I mean as you apply the brakes, the turn signal indicators in the dash will light up. I never recall this happening before, and before the bed was off the turn signals didn't work at all on any side.

However, now that the bed is off and the rear lights are not hooked up to the harness, the turn signals work....until I turn on the lights. When the lights are on, the passenger side will work, but not the driver side. Also, when the lights are off the signals work for both sides but the driver side is very dim.


When I responded previously - I did not have any of the above information available.

This is what I had to work with:

When you apply the brakes the brake lights won't work, but the turn signal indicators in the truck will light up.

First thing is to locate the ground wires for the rear harness and be sure that the connections to the frame are clean of rust and tight. They will be black and have ring terminals at the ends.

There may be more than one, but the main ground wire will be bolted to the cross-frame brace - over to the drivers side:

You must be registered for see images attach

 

[Murdock]

Yeah, I haven't seen that and I been under there a few times this week. Would this affect the other stuff though, the parking lamps, and front signals?

 

[chengny]

Lack of a good ground path (back to the negative terminal of the battery by way of the frame) will certainly wreak havoc with all parts of the lighting system.

Any service manual will always list a faulty ground connection as the second suggestion when dealing with any electrical issue (check the fuses is first).

 

[Boone83K10]

my truck doesn't have that ground, or at least I don't see it

 

[Murdock]

Also boone, I did check the connector and it wasn't fully plugged all the way. However the problem still is there.

I guess the only way to rule out the switch is to check all the connections then.

Joy...

 

[chengny]

In some cases the ground connection is in the cavities that the rear light assemblies sit in.

Pull your lenses, set them aside and then release the socket housing screws. There is enough slack in the harness to pull the casings out while you work behind them (no need to remove the actual sockets/lamp assemblies - just let them hang).

If there isn't a common ground on the cross-brace, there should be one behind at least one of the lighting assemblies.