Third Brake Light - NOW WITH FIXEDNESS

I'm getting the 'new' Cruiser ready for inspection. Like most wagons I get, the third brake light doesn't work. Usually this is a simple fix, scuff the plunger connections on the gate and the contacts on the light assembly and all is good.

This one is giving me problems though. When I first put a test light on the gate plungers I got a light, no issues there. So I cleaned up and rebuilt the light assembly, it now lights up every time when I put current though it.

Now though, when I put a test light on the plungers I get a VERY dim glow or none at all. Now I've heard of the plunger assembly itself going bad so that is certainly suspect. However, when I put the test light on the plug connectors inside the gate, I'm not getting anything either. The tail lights light up no problem.

Before I start tracing the wiring out, any pointers on where to look for failures?

check the battery, or do your electrical testing with the engine running...

buickestate wrote:

check the battery, or do your electrical testing with the engine running...

Thanks, I will confirm but I think the battery is good. I did some work with the engine running but will re-test. Or just stick a charger on it just in case.

Okay, this one has me stumped.

Battery seems fine though I haven't done a load test on it. Good voltage.

Here's what I've determined:
- When I put a tester on the yellow wire (power to light) I get 12 volts when I press the brake pedal (and tail lights come on.)
- I close the glass with the pedal depressed.
- As soon as the third brake lamp housing touches the plunger contacts, the voltage drops to zero, no light of course.
- I pop the glass and I get partial voltage back but it won't go back to 12 again until I release and reapply the brake pedal.

Here's what I've done:
- Tried three different light assemblies. All will light up if power is applied directly to the contacts on the base. One was a known working unit from my other 92 Cruiser.
- The FSM shows two grounds for this light. One the ground at the base of the D pillar and the other at the base of the A pillar. I checked both, they are clean. Went ahead and scuffed them to bare metal anyway and reattached.
- Inspected as much of the wiring as I could see without removing most of the interior. No noticeable breaks or pinches.
- Cleaned all contacts

I have not replaced the plunger connectors on the gate. I know they can go bad sometimes but I've put a test light on them and pushed them in and released with the brake pedal depressed and they give continuous voltage.

Ideas guys??

C'mon guys, no one's dealt with this?

Mike:

Check the ground on the tailgate itself. Sounds to me that the circuit is finding a ground through something else, like a lock solinoid or door switch. When you just have a meter on it it will show 12V but put any load on it and the circuit can't handle it. You should find a common ground somewhere on the sheet metal of the gate itself. That may not be making a good connection.

Bill

convert2diesel wrote:

Mike:

Check the ground on the tailgate itself. .

Bill

X2,.
but does sound like the plungers are nfg,

clip the light to the 12v supply,. then use it to check ground,.

Nick

Thanks, I wasn't aware there was a ground on the gate itself. I'll look for one.

I'm certainly not ruling out the plungers either. It's just that they seem to have consistent voltage when I put a tester on them and push them down. I think I have a spare somewhere.

Such a pain that the plungers are riveted in. Everything else is screws. Why rivets on this one component? I know the answer, I'm just venting. Yes, I have a bad plunger. Bill and I spent a sunny afternoon taking apart CHMSLs one day trying to get my cruise control to work. The cruise got fixed. The CHMSL did not.

I can't find a ground in the gate itself, but I'm tired will look more tomorrow.

I hate drilling those rivets out too, and finding screws that not just fit but don't have too high a head so as to interfere with the glass closing. I pulled out my spare plunger though, I may just suck it up and do that tomorrow, see if it fixes the problem.

Yes, I have a spare plunger, too. It is currently riveted in my 92 RMW that is about to deal with winter.

The problem with the one that is on my 94 is that the spring isn't springing. I can stick my finger on the back, and bring the contacts up, but they won't stay in place. Maybe I'll drill it out tomorrow and see if I can get the spring working again... so many projects... not enough beer.

mike you,ve proved that you have power,. to 1 plunger isn't the 2nd one for ground??

mike you,ve proved that you have power,. to 1 plunger isn't the 2nd one for ground??

DBeaSSt wrote:

Okay, this one has me stumped.

Battery seems fine though I haven't done a load test on it. Good voltage.

Here's what I've determined:
- When I put a tester on the yellow wire (power to light) I get 12 volts when I press the brake pedal (and tail lights come on.)
- I close the glass with the pedal depressed.
- As soon as the third brake lamp housing touches the plunger contacts, the voltage drops to zero, no light of course.

This probably only proves that the current draw of the CHMSL is quite a bit more than the test light and is sufficient to exceed the available current.

When the light is energized it drags the voltage down to zero. Since you didn't mention blowing any fuses, it doesn't seem to be shorting to ground.

The logical deduction from the above is a high resistance somewhere in the circuit. If we rule out the tailgate pins and wiring harness from your clean & inspect work, it must be something else.

DBeaSSt wrote:

- I pop the glass and I get partial voltage back but it won't go back to 12 again until I release and reapply the brake pedal.??

This seems to point to the brake pedal switch itself - i.e. that the contacts are either corroded and/or the switch is starting the fail mechanically.

- J

DBeaSSt wrote:

- I have not replaced the plunger connectors on the gate. I know they can go bad sometimes but I've put a test light on them and pushed them in and released with the brake pedal depressed and they give continuous voltage.

Ideas guys??

phantom 309 wrote:

mike you,ve proved that you have power,. to 1 plunger isn't the 2nd one for ground??

Yes, yellow wire goes to one pin and black to the other. The yellow comes from all the way up front, brake pedal switch. Black, goes from the other pin back into the quarter panel area and uses the shared ground there.

I'm going to go ahead and replace the plungers. It's the only thing I haven't replaced or tested/cleaned at this point. I'll certainly let you know the results!

Thanks for all the tips!

Mike:

If that doesn't work, check where the brake light bolts onto the glass. It is possible, you may have a leak or stray connection through to the heating grid. That would explain the heavy draw/resistance. Just a thought.

Bill

And the saga continues.

Not the plungers. Turns out I had two spares, tried both, same results.

Just to rule out the defroster grid as some extra load, took a good brake light assembly and touched it to the plunger contacts, same results.

So I guess that leaves either the brake pedal switch as suggested previously or faulty wiring somewhere in there.

I guess I"ll pick up a brake pedal switch first as tearing the interior apart doesn't really appeal to me right now.

It's a good thing I really like this car or I might be angry by now. On the plus side, oil changed, fuel filter changed and accessory belt changed. They all work! I think I'll tackle the power antenna now as I'm apparently masochistic.

mike use a jumper wire across the brake light switch connections,.doubt thats the problem tho,..
what i do in this sort of case is to use jumpers right from the battery,. alternate your ground and power,. see which one is pooched,. i.e run a power wire right from the battery to the yellow connection,. now you know you have strong 12v,.if the light is bright, its power,. use the same wire and run it from battery ground to the other connection press the brake ,. if its all good its bad ground,.
Nick
'sadist and a masochist moved in together,.first night in bed the masochist begged,. "beat me beat me"
sadist just smiled and said..."no",..

so mike,. how'd you make out on this problem??

Nick

I picked up a replacement brake switch Wed. Haven't had a chance to test it yet. It's (one of) my weekend project. I'll certainly post once I figure it out!

If the switch doesn't work, I'll be making a LONG test wire to do as you suggest.

GM Part # 25523463, brake light switch $20 from my local GM dealer.

Replaced, works great. First time I've had one of these go bad on me. I bought a spare for later use.

Thanks for all the ideas and pointers guys!!

If you can spare one of those plunger contact sets, I have a wagon that needs it...

DBeaSSt wrote:

GM Part # 25523463, brake light switch $20 from my local GM dealer.

Replaced, works great. First time I've had one of these go bad on me. I bought a spare for later use.

Thanks for all the ideas and pointers guys!!

You realize of course that you just protected yourself from ever experiencing another brake switch failure .

Glad to hear you got it working, I have had some third brake light / cruise control issues and was following this thread closely.

i cant get my 3rd brake light to work either. i cleaned all the contacts as well. ill try that brake light switch. its down by the pedal right?

The third brake light uses a different set of contacts in the brake switch than the other brake lights. You might have a bad brake light switch. My reasoning is that the voltage will return to 12 after pressing and releasing the brake pedal.

I never tackle electrical problems without the proper tools

http://www.powerprobe.com/powerprobe/PP3.html

To hell with jumper wires and hacking into stuff

94Woody wrote:

I never tackle electrical problems without the proper tools

http://www.powerprobe.com/powerprobe/PP3.html

To hell with jumper wires and hacking into stuff

I never have enough money to buy the right tools for fixing stuff,.
guess my $5.00 test light and some used jumper wires'll have to do,.
Nick