Bad ground?

[Wanderer]

Vehicle: 2004 Chev Tahoe

Mileage: 177,000 +/- a couple

Problem:

Brake lights on the fenders don't want to work unless the truck is wet.

The top brake light works and the rear windshield wiper works.

I've taken off the ground wire behind the front bumper that goes directly to the battery and cleaned it. I've taken off the rear ground wire and gas filler neck ground and cleaned that too. I went so far as pulling the whole fuse/relay box out from under the hood to check connections and for corrosion. No dice.

I've used a test light on the brake/turn signal module tucked inside the right rear fender and have gotten a weak signal on both the old module and new one.

This ground is grinding me down...

Where else should I be looking? The negative to the starter?

Oh yeah, the rear power door locks are weak but the fronts are fine.

There could be something in it for the winner of this one.

[leech~~]

Maybe check the wire harness at the back hatch where it bends when opening and closing it?

[leech~~]

Wow, not a lot of electrifying ideas here Wanderer?

[Wanderer]

Yah, I don't know if it's me or the problem, that's the problem.

I dread electrical problems in vehicles. It takes a lot of time to trace everything out.

[jmd1]

might want to look at the news, i think they are going to recall them for this issue according to the news.

[CAMAN]

might want to look at the news, i think they are going to recall them for this issue according to the news.

That is not for the Tahoe, only Malibus, G6, and Auras

[Macgyver55]

Have you tried running a known good ground to the lights to see if they then work? If not, it's a quick and easy way to verify if that is indeed the problem. Otherwise, since moisture seems to affect the operation of the lights, I would also be looking at the harness connector up near the spare tire. Vehicles that have a tow package often have a harness connector for the trailer lights in that area so open that and inspect and test at that point.

I'm sure you probably remember a similar problem on your Lumina....I wouldn't necessarily rule out the multifunction switch without checking it. (if you don't find a harness problem)

[walt501]

A couple ideas -

Check your T style trailer light connector under the truck. I'm reading these can become corroded and cause problems. I believe you can actually remove this connector and plug the two factory wiring harnesses back together and see if this fixes the problem.

If that doesn't work, check your brake light switch -

[Wanderer]

Bud and Walt -

Thanks for the tips! I'll double check to see if there's a harness connector in the back that I missed. I've checked the visible connectors but there might still be one I missed.

The brake switch check is definitely the type of thing I was hoping to uncover by posting the question. I'll check it out tonight and let you guys know.

Thanks again!

[Wanderer]

Nope and nope.

The brake switch wiring on the Tahoe isn't quite the same but I unhooked it, opened and inspected. All is well.

I re- checked all the connections in the rear of the truck and all were clean. Some had the electric terminal lube I like to use but I sprayed everything with CRC connector cleaner before re-assembling. Also before reassembling, I left all the plugs off the brake/signal module in the right rear fender and tested the brake. The top light still came on.

I found two more ground connections on the frame under the drivers seat. I took off, cleaned and re-assembled. I took off the connectors on whatever brake manifold/bias modulator thing that's tucked in by the frame under the drivers seat. And finally, I took out the starter and sprayed and tightened those connections too.

Blah.... I got nuthin...

[Surface Tension]

How about the turn signals and flashers. If they work then I think your looking at the brake light switch or along those lines.

[Macgyver55]

Come to think about it I worked on an 03 Silverado a while back with a bad flasher unit that gave me fits. Did all kinds of weird stuff....

[Wanderer]

The turn signals and hazards work.

If it was the brake switch, I'm curious to know why the top brake light works. I suppose it's just a different circuit on the harness.

I haven't tested the cruise control yet, as it was mentioned in walt's post. If I get to my next step before taking it to the shop, I was planning on spraying water on the underside in isolated areas, then test after each time to see if I can at least find the area of the problem. The brake lights work fine when the truck is out in the rain.

Thanks for keeping the ideas coming.

Oh, forgot to mention, when I had all the plugs off the module at the rear of the truck, I engaged the brake and tested all the wires for voltage and got 0. So it would seem I have no power going to the rear on the brake circuit.

[Jeremy airjer W]

With your foot on the brake check for power at the VEH STOP fuse in the left instrument panel fuse block. If there is power any time the brake pedal is depreessed than the problem is the in the light blue wire in between the I/P fuse block and the back of the vehicle. That wire runs through the underwood fuse block before it makes it's way to the rear lamps junction block and then finally to the rear lamps.

This does not sound like a ground issue since ground issues usually cause weird things to happen like the rear wiper turns on every time you turn left. Or the radio turns off when you put it in reverse.

[Jeremy airjer W]

The center high mount runs off the same power as the left and right brake. Power from the switch runs to the the I/P fuse fuse block where it splits into three. One goes to the flasher module, one goes to the CHMSL fuse (center high mount stop lamp), and one goes to the VEH STOP fuse. If the high mount is working than the switch and fuse to the switch are fine.

[Wanderer]

I have power to both the underhood 25 amp fuse and I/P 15 amp fuse in the cab. Back under the truck I go.

[Wanderer]

Victory at last!

The problem was the first place I checked but I had to work my way back to it from the front of the truck. Thanks Jeremy for the clear troubleshooting instructions! I just worked my way back, inspecting the run of wires (again), stopping at the tail light block in the back. The whole run is so well shielded I couldn't imagine cutting anything open would be a good idea.

What I missed on my initial inspection was a bent part of the inbound power connector:

With the terminal grease I'd put in there before, I couldn't see that it was bent and apparently didn't get the test probe in the right spot to pick up the power. It's all cleaned out, aligned and the brake lights are working fine now.

Finally done with this so I can move on to the next thing - getting ready for fishing!!!

Thanks again to everyone who offered ideas.

[Neighbor_guy]

Well that took some effort...

You know an engineer would have figured that out on the first try.

Good luck on the water.

[Wanderer]

You know an engineer would have figured that out on the first try.

Not some engineers I've worked with.

[delcecchi]

Sometimes we do and sometimes we don't. There was the time I spent two days chasing a glitch on a prototype System/36 that turned out to be due to the soldering iron being plugged into the outlet next to the tektronix 454 on the cart....

[Wanderer]

Yeah Del, sometimes it doesn't go as easy as you'd expect...

No rub on engineers in general - N_G is drumming up an old inside joke.