V1 No rear running lights, cluster, or license light.

[Robf]

Looking for some help trying to diagnose an issue I'm having. Last night I went on a drive and noticed my rear running lights, license plate light and instrument cluster arent lighting up.

I read through some stuff online. It seems like most of the time it's blown the prk lamp 15amp fuse. I checked every single fuse in the bcm and the main fuse panel with my multi-meter not one fuse is blown. I ran a test with my obd2 reader and noticed the ambient light circuit malfunction. The ambient light sensor hasn't worked since becoming a goblin. I've never had that code pop up in all of the full system scans I've performed in the past. Initially when I got my wiring harness from df I was installing the rear taillights and I noticed the passenger side running light would work but not the drivers side or the license plate light. I ended up adding those lights to the rear ground point under the fuse box and everything was working as it should... until last night.

Today I unwrapped the tail light cables to make sure nothing was disconnected, which there weren't. I checked continuity on the ground and I'm grounded. I checked the main grounds and they are clean. And secure. I don't have any volts at the prk lamp fuse in the fuse box. Unfortunately it sounds like I might have an issue buried in my harness somewhere. Does anyone have any other suggestions?

 

[Gtstorey]

Parking light fuse is after the relay. Need to check the ground signal from the bcm that closes the relay is there. It could be a bad relay inside the fuse box (non-replaceable on the circuit board). I don’t have the diagram for the older model to give you pin #s

 

[Robf]

Parking light fuse is after the relay. Need to check the ground signal from the bcm that closes the relay is there. It could be a bad relay inside the fuse box (non-replaceable on the circuit board). I don’t have the diagram for the older model to give you pin #s

I also want to note that I don't have any power at the bcm A7 which is power for park lamp. I removed the red plug from the bcm and fed a jumper wire from the battery to A7 pin and I was able to get my lights to turn on like they should. I should also note that I was having that lack of power in 1st and second gear due to brake torque management. So maybe it's something going on with the bcm itself.

 

[Gtstorey]

With the caveat that I've got the wiring diagrams for the late model.

I don't remeber if we are using the circuit for "tail lights" or "parking lights" but my memory says that we are only using one of those. Parking lights are powered off the main fuse box. Tail lights are powered off the BCM. If it's the tail lights off the BCM and you aren't getting power on BCM X3 A7 then it's either a BCM problem or a problem with the ground signal for the switch. You can ground BCM X2 Pin 5 to check the ground signal for the switch.

 

[Gtstorey]

This is from 2010 service manual your is probably similar but I don’t know for sure.

 

[Robf]

With the caveat that I've got the wiring diagrams for the late model.

I don't remeber if we are using the circuit for "tail lights" or "parking lights" but my memory says that we are only using one of those. Parking lights are powered off the main fuse box. Tail lights are powered off the BCM. If it's the tail lights off the BCM and you aren't getting power on BCM X3 A7 then it's either a BCM problem or a problem with the ground signal for the switch. You can ground BCM X2 Pin 5 to check the ground signal for the switch.

Here is what I've came up with so far. I can back feed off the battery to the bcm A7 (prk light supply) to get the lights to come on. I'm not exactly sure what you're meaning by the bcm x2 pin 5

 

[Robf]

This is from 2010 service manual your is probably similar but I don’t know for sure.

Nevermind I see what you're saying. So just check pin 5 with ground continuity? If I dont have continuity it's a bad ground If I do it's a bad pcm?

 

[Gtstorey]

It looks like your diagram isn't consistent on park lamp/tail light designation (GM put out some crappy manuals in the mid 2000's). The front lights are park lamps powered from the relay on the fuse box. The rear lights are park lamps on the one page but tail lights on the other. I was reading these diagrams wrong. Both are powered by the relay on the fuse box. The "Park Lamp Sply Volt" (X3 A7 on my diagram) is a check by the BCM to see if the lights are working, not a supply. This is probably why you are getting a code. So you need to see if the "Park Lamp Rly Ctrl" (white) is grounding when you turn the lights on. It could still be a bad relay on the fuse box PCB.

You can ground with a jumper wire "Park Lamp SW Sig" to eliminate the switch as a problem.

 

[Robf]

It looks like your diagram isn't consistent on park lamp/tail light designation (GM put out some crappy manuals in the mid 2000's). The front lights are park lamps powered from the relay on the fuse box. The rear lights are park lamps on the one page but tail lights on the other. I was reading these diagrams wrong. Both are powered by the relay on the fuse box. The "Park Lamp Sply Volt" (X3 A7 on my diagram) is a check by the BCM to see if the lights are working, not a supply. This is probably why you are getting a code. So you need to see if the "Park Lamp Rly Ctrl" (white) is grounding when you turn the lights on. It could still be a bad relay on the fuse box PCB.

You can ground with a jumper wire "Park Lamp SW Sig" to eliminate the switch as a problem.

Ok I think I'm tracking.
On BCM x1 pin 8 white wire labeled "park lamp relay control" I should have continuity with ground. If I do the relay isn't the issue.
If it doesn't ground the relay is bad.

If I add a jumper from bcm x2 pin 5 BN/WH wire labeled "park lamp switch on signal". To ground it will bypass the relay in the fuse box and my lights should work? Again pointing out the non serviceable relay is bad in the fuse box. If that doesn't work then I have a wire that's been damaged somewhere in my loom?

 

[Gtstorey]

Ok I think I'm tracking.
On BCM x1 pin 8 white wire labeled "park lamp relay control" I should have continuity with ground. If I do the relay isn't the issue.
If it doesn't ground the relay is bad.

If I add a jumper from bcm x2 pin 5 BN/WH wire labeled "park lamp switch on signal". To ground it will bypass the relay in the fuse box and my lights should work? Again pointing out the non serviceable relay is bad in the fuse box. If that doesn't work then I have a wire that's been damaged somewhere in my loom?

First paragraph is backward. If the relay control is grounding, the BCM is working correctly and the problem is after the BCM, either wiring on relay. If the not grounding the problem is before that point.

Second paragraph. Grounding X2-5 will take the light switch out of the equation. It will not mean anything for the relay. It is a signal for the BCM, not an actual Ground for the relay. The light switch tells the BCM that you want to turn on the lights and then the processor/computer closes a different internal switch to ground the fuse box PCB relay. You could ground the X1-8 to check the fuse box relay.

 

[Robf]

First paragraph is backward. If the relay control is grounding, the BCM is working correctly and the problem is after the BCM, either wiring on relay. If the not grounding the problem is before that point.

Second paragraph. Grounding X2-5 will take the light switch out of the equation. It will not mean anything for the relay. It is a signal for the BCM, not an actual Ground for the relay. The light switch tells the BCM that you want to turn on the lights and then the processor/computer closes a different internal switch to ground the fuse box PCB relay. You could ground the X1-8 to check the fuse box relay.

I have continuity grounding x2-5 as well as x1-8. This is with everything plugged into the bcm, negative terminal on battery, with the running lights turned on at the turn signal switch. Do I need to test it differently? Or is that acceptable?

 

[Gtstorey]

Please stop using the word continuity in this case. If you want to use that word see if you have continuity from x1-8 along the white wire to the fuse box (make sure the white wire is continuous). Or make sure you have ground at the white wire at the fuse box.

You are looking for a break in the chain. In this case 3 chains.

The 1st a grounding chain starting at ground. The light switch closes and this ground is transferred to the BCM.

The BCM then starts another grounding chain. The BCM grounds the Fuse Box relay which closes the switch in the relay and start the 3rd chain

The 3rd chain is a + voltage chain. Relay switch closes, passing voltage to the fuse. Fuse then passes voltage to the Brown wires.

 

[Robf]

Please stop using the word continuity in this case. If you want to use that word see if you have continuity from x1-8 along the white wire to the fuse box (make sure the white wire is continuous). Or make sure you have ground at the white wire at the fuse box.

You are looking for a break in the chain. In this case 3 chains.

The 1st a grounding chain starting at ground. The light switch closes and this ground is transferred to the BCM.
View attachment 40361

The BCM then starts another grounding chain. The BCM grounds the Fuse Box relay which closes the switch in the relay and start the 3rd chain
View attachment 40362

The 3rd chain is a + voltage chain. Relay switch closes, passing voltage to the fuse. Fuse then passes voltage to the Brown wires.
View attachment 40363

I'm beyond frustrated at this point, obviously electrical work isn't my strong suit. Here is what I've done.
My car is in the on position with my turn signal switch turned to lights on position.

Do these findings point to a bad park pcb relay?

I also had a lot of issues with brake torque being active when driving for no reason. I was able to deactivate it on hp tuners. The green line goes from the electronic brake control module to the fuse box.

It seems like things are adding up to fuse box issues?

 

[Gtstorey]

Do you get the beep from the volt meter in the first two pictures?

The brake torque thing might be unrelated. It will take a lot more digging that I have time for right now. Focus on one problem at a time.

 

[Robf]

Do you get the beep from the volt meter in the first two pictures?

The brake torque thing might be unrelated. It will take a lot more digging that I have time for right now. Focus on one problem at a time.

Yes it beeps in every spot. The only reason I mention the brake torque is because it all comes back to that fuse box. When it was sitting on the shelf a little bit of brake fluid spilled on it. I took the whole thing apart with lonnys help looking for issues on a no start condition and cleaned it all off. So what I'm saying is maybe the issues are all connected. I'll stop by the junk yard tomorrow and see if I can find one. I assume I need to find an 07 cobalt ls for them to be compatible.

 

[Gtstorey]

Maybe they are connected, maybe not. You know need to check and see if the ground signal from the BCM is making it to the fuse box. You will need to check for ground at the other end of the white wire, or check continuity from one end to the other. You may just have a loose connection.

Don't be a parts changer to solve problems.

 

[Desert Sasqwatch]

When you are saying brake torque, what exactly are you referring to? Torque limiting or ??? If so, the electrical issues you are chasing now don't have any relevant connection to what the ECM does to limit how much power your engine will output under load/acceleration. This issue is not causing your electrical issues and vice versa.

As @Gtstorey has already stated, focus on one issue at a time. Easier to drink from a cup than from a fire hose - don't do it all at once, especially if electrical issues are not your strong suit. Recommend you follow what he is asking you to check and you should be able to get through this. Electrical gremlins take time and checking things in sequence to isolate and resolve them.

 

[Robf]

Maybe they are connected, maybe not. You know need to check and see if the ground signal from the BCM is making it to the fuse box. You will need to check for ground at the other end of the white wire, or check continuity from one end to the other. You may just have a loose connection.

Don't be a parts changer to solve problems.

I don't have a pinout of what each wire goes to in the fuse block but by process of elimination I've narrowed it down to the large tan plug. With the white wire in the corner. I don't have continuity when jumping a wire and I don't have ground. Am I in the right spot?

 

[Robf]

When you are saying brake torque, what exactly are you referring to? Torque limiting or ??? If so, the electrical issues you are chasing now don't have any relevant connection to what the ECM does to limit how much power your engine will output under load/acceleration. This issue is not causing your electrical issues and vice versa.

As @Gtstorey has already stated, focus on one issue at a time. Easier to drink from a cup than from a fire hose - don't do it all at once, especially if electrical issues are not your strong suit. Recommend you follow what he is asking you to check and you should be able to get through this. Electrical gremlins take time and checking things in sequence to isolate and resolve them.

What I was referring to is another post I had done where I didn't have power in 1st and second gear as my throttle body wasn't opening. It ended up being brake torque limits were being applied for no reason. Which I turned off with hp tuners and band-aid that issue. I'm not all that worried about it the only reason I mentioned it was because it was shown on the same schematic. I'm only working on this problem right now. Electrical problems are irritating to me!

 

[Gtstorey]

I don't have a pinout of what each wire goes to in the fuse block but by process of elimination I've narrowed it down to the large tan plug. With the white wire in the corner. I don't have continuity when jumping a wire and I don't have ground. Am I in the right spot?

If the earlier model matches the later model, it should be pin B7 which is the other white wire on that connector. You are on F1 which is probably the rear defogger.