Do you have a moment for a CHMSL / third brake light issue? My 3rd brake light doesn't work, but all other lights do. I've reviewed your instructions on: http://www.justanswer.com/gm/4cwgi-gmc-sonoma-4x4-01-sonoma-no-brake-lights-no-hazards-fuses.html The brake light relay was clicking, the fuses were all OK. When I removed the brake light relay and jumped pins 30 to 87, the brake fuse blew. That doesn't make sense to me, I double checked the numbers, and I was jumped the correct ones. Not sure what next steps are. Any suggestions?
Hello and welcome to JustAnswer!
The third brake light is powered from a dedicated circuit from the brake switch rather than through a relay. Replace the fuse that blew and reinstall the relay and verify that all other lights do still work.
Start by checking the orange wire at the brake switch and make sure it has power, this comes from the stop lamp fuse in the power distribution center under the hood.
If that power feed is ok then press the brake pedal with the switch plugged in and make sure there is power on the white wire leaving the switch. If there is no power then you have a bad brake switch.
If power there is ok then go to the fuse labeled VECHMSL in the box under the hood and make sure it isn't blown, if ok make sure it has power on it with the brake pressed. If there is no power but there was power on the white wire at the brake switch then the wire is open in between.
If you do find power at the fuse then the next step would be to check for power and ground at the third brake lamp. Power leaves this fuse and goes to the lamp, if you're missing power and it was ok at the fuse then the circuit is open in between.
If you have power at the lamp but no ground then you have an open ground circuit or the ground is ground itself is corroded, off or loose.
The brake switch wires are difficult to get to, so I started at the fuse. I measured 12v with no press on the brake, then 0 volts when brake pressed. (The fuse was jumpered with a paperclip.)
Since that seems wrong, I moved down to under the dash.
Is the white wire in A1 (http://ww2.justanswer.com/uploads/Joecamel90/2010-12-20_042246_wire.pdf) the same as the white wire you mention?
On A1 I measured 12v when brake pressed and 0 with no pressure.
This doesn't seem definitive to me, am I missing something?
I tested at the correct CHMSL fuse this time. Good call :-)
But I get the same results, voltage with no pressure on the peddle.Drops to near zero when brake is pressed.
It may be because of a "bias voltage" (https://www.youtube.com/watch?v=McnXLcJNVfI see 5mins through 12 mins)
But I'm not 100% certain. I only have a multimeter, not a test light. I'm not sure if the bias voltage applies to this.
Continuity... 100ohms... Yes, but seems high?
I checked both sides of the fuse to the electrical connections in the third brake light. One side of the fuse+light gave .8-1 ohm. Pretty good. The other side of the fuse+light was about 80 ohms.
I wonder if it's a bad ground?
I tried to pry out the light fixture to examine the back but it didn't want to come. It seems like it will break before it comes out, unless I'm missing something.
What I meant is that the fixture (the part the bulbs are mounted in) won't come out easily.
The cover came off just fine, and that's how i was able to get the test results above, after removing the bulbs and testing from the bulb connections to the fuse.
so .. there is actually 12v at the outlet. :/
I don't have a test light, but i did jumper up a rear turn light by hand. I couldn't get it to light up any more than the existing fuse-style lights are. :(
2 ohms to ground on one side. 90 ohms to ground on the other side... thoughts?
Yes. The 90 ohms to ground is on the yellow (left) side. The 2 ohms to ground on the black side.
If you have power and ground at the lamp connector then the problem is the lamp, bulb or connection. Make sure the terminals are ok.
Is it an LED unit or an incandescent bulb? Either way the problem is connection at the lamp connector or it's the bulb(s).
Ok, so bad assumption on my part to start with: I didn't think BOTH bulbs could have gone bad, and they still "looked" OK with a filament running through them. But they were both bad.
Thanks for your help.