97 k1500 extended cab CHMSL and brake lights intermittent

[crowfather]

I have been searching the forum, reading the electrical diagrams, and testing voltages this morning. I cannot seem to figure out what is going on.

Symptoms: I had no cargo or 3rd brake light. Regular brake lights seemed to be working. I pulled off the cover to change bulbs. Old bulbs were black/grey so seemed to be an easy fix. Bulbs replaced and cargo lights are working. However, third brake light isn't.

I will define the connector in the following way since I cannot find the actual connector in the diagram. Diagram references a 6 pin connector C411, but I have a 4 pin connector.
Upper left is white 1, upper right is black and blue, lower left is white 2, lower right is black.
With cargo switch on and door open I had the following voltages:
13.7V from black and blue to black
13.2V from black and blue to lower left white 2
These voltages go to 0V when the cargo switch is turned off or the door is closed.
However, there is always a 0.5V difference between black and white 2

This diagram for he cargo lights seems confusing since it never references ground, and the fuse 3 says always hot on the black and blue wire, that I interpreted to be the ground wire.

For the 3rd light I never saw any voltage from white 1 with the brake pressed or not. However, I also noticed intermittent brake lights on the back of the truck, which I hadn't seen before. The diagram is a bit confusing there, so that's when I decided to ask my question. I wonder if every time I was testing the 3rd light I was also not getting rear brake lights. Seems strange. Any help is appreciated.

P.S. I have this random extra switch above the rearview mirror that I cannot figure out what it does. Anybody have something like this?

 

[AK49BWL]

That switch, AND the console it's mounted to, ARE NOT FACTORY. So no idea what it could be for. You have the right idea as far as checking the lights. There's a difference between white 2 and black because you're not referencing ground to ground - unless the dome lights are on, as they're ground-switched. And even then it may not always be the same. I would chase down white 1, which should run from the brake switch directly to the 3rd brake light via the driver side door sill, which is where wires like to chafe over time and develop breaks. Now as for the intermittency from the rear, that may be pointing more toward either a bad ground back there, or a faulty brake switch unit, the latter of which seems a little more likely given the 3rd brake issue.

 

[crowfather]

That switch, AND the console it's mounted to, ARE NOT FACTORY. So no idea what it could be for. You have the right idea as far as checking the lights. There's a difference between white 2 and black because you're not referencing ground to ground - unless the dome lights are on, as they're ground-switched. And even then it may not always be the same. I would chase down white 1, which should run from the brake switch directly to the 3rd brake light via the driver side door sill, which is where wires like to chafe over time and develop breaks. Now as for the intermittency from the rear, that may be pointing more toward either a bad ground back there, or a faulty brake switch unit, the latter of which seems a little more likely given the 3rd brake issue.

Yeah I know they aren't factory. The one I have has some custom interior job.

Thanks for the direction. I will look in those places.

 

[GoToGuy]

Also are you using the factory service manual or a generic Haynes, clymer , etc. there wire diagrams are a guess at best. Those are great for cleaning coffee spills or targets at 300 yards.
You want the real deal , in a sticky above in the eng thread are links for the factory OE service manuals in PDF. Download your year, everything is in the OE service manual, wire locations, , ground locations, black W/ blue stripe, systems operation, everything from bumper to bumper.
Anything stock or OE the knowledge is here, aftermarket add ons someone probably knows.
Are your turn signals intermittent also?

 

[crowfather]

Also are you using the factory service manual or a generic Haynes, clymer , etc. there wire diagrams are a guess at best. Those are great for cleaning coffee spills or targets at 300 yards.
You want the real deal , in a sticky above in the eng thread are links for the factory OE service manuals in PDF. Download your year, everything is in the OE service manual, wire locations, , ground locations, black W/ blue stripe, systems operation, everything from bumper to bumper.
Anything stock or OE the knowledge is here, aftermarket add ons someone probably knows.
Are your turn signals intermittent also?

I am using the factory manuals from here. Turn signals are consistent.

There was one point where the brake lights didn't work until I closed the door. I wonder if there is something odd going on with the wiring in that area, seems to indicate toward what AK49BWL said. However, while the rear brakes were somewhat intermittent, I haven't gotten any light from 3rd light even after changing bulbs.

 

[AK49BWL]

Well if the wire in the door sill is busted, it's possible it completely disconnected the CHMSL and is grounding out, but not enough to blow the fuse...yet.

 

[crowfather]

Well if the wire in the door sill is busted, it's possible it completely disconnected the CHMSL and is grounding out, but not enough to blow the fuse...yet.

Interesting. That could be the case. All fuses checked good, which I didn't mention in my original post. I'll try to look at it after dinner today. Otherwise it will be a next weekend problem.

 

[crowfather]

Let me see if I have my wiring understood properly. Following white 1 back from manual page 8A-110-7 (D) to (D) on page 8A-110-1. The wire follows to the left hand continuation, which looks like it should pair up to the right hand side of page 8A-110-0. S220, on 8A-110-0, looks like it sends power and signal to the 3rd brake light, and the brake light relay. Either the Stoplamp switch or the hazard/turn switch can send power through s220.

 

[AK49BWL]

Let me see if I have my wiring understood properly. Following white 1 back from manual page 8A-110-7 (D) to (D) on page 8A-110-1. The wire follows to the left hand continuation, which looks like it should pair up to the right hand side of page 8A-110-0. S220, on 8A-110-0, looks like it sends power and signal to the 3rd brake light, and the brake light relay. Either the Stoplamp switch or the hazard/turn switch can send power through s220.

All but one thing. The stoplamp switch is the only power provider - the input to the hazard/turn switch simply joins the brake signal with the rear blinker output for your brake lights in the back.

 

[crowfather]

Here is a fun one. I checked the power pins the relay connects to. This would be the always hot stoplamp relay. The power socket is always on, as its supposed to be (E7 I think on the diagram). Also, the D7 socket is on when I initially put the brake on, indicating power from the stoplamp switch. I removed my 2x4 to see if power went away, which it did. However, when I replaced the 2x4 there was no power from the stoplamp switch. I did this with the vehicle off, so the pedal was harder after a few pumps, perhaps its related. But I wonder if this is indicative of a stoplamp switch that is going.

Also, for fun, you'll notice a lead coming off one of the ECM 20A fuses. It leads to another relay behind the box. So that will be fun to figure out later.