M1008 Brake Light Problem

bigburb11

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I lost power to my brake lights. All other lights are OK.

After looking at the wiring manual, I checked the fuse box (12V at the terminal is OK, fuse is good) and the brake switch @ the pedal (brake switch functioning fine, 12V @ the switch is OK).

Then I checked the sockets @ the lamp. They were a bit corroded and will be getting replaced, but aren't the problem. Bulbs are OK. There's 0V @ the sockets when brake pedal depressed (should be 12V).

I check @ the weatherpack connector (lt green wire for tail light power) on the rear crossmember - 0V there when the brake pedal is depressed.

After this I had some trouble reading the wiring diagrams. It appears as if the lighting control switch is part of this brake light circuit, but I'm not sure why.

I'm looking for suggestions if you have them - should I order a (dash) light control switch? Other ideas?

Thank you in advance. I love this truck and wish I could be driving it on the road...
 

doghead

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Where were you getting your ground from when you tested for voltage?
 

bigburb11

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I used a multimeter to check for voltage. I used a PowerProbe to test the socket's (PowerProbe hooks up to the battery to power devices). With the multimeter I tried ground on the body at the tail lamp housing AND hooked a lead up to the negative post on the battery just to be sure. No voltage at the light green wire (stop lamp power wire) both times. Also, the body is grounded well - I checked for continuity to the negative battery post in several locations.

Finally, The wire loom looks good along the fram rail (no pinches or chaffing). Thanks for the responses.
 

Warthog

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Is your Blackout Service Light switch turned on? I 'm assume yes but just want to make sure.

Have you checked the brake switch itself? You said you had power at the orange wire. What about the White/Black wire with the pedal not push and with it pushed?

The headlight switch is not part of the circuit (just shares a wire) but the Hazard Flasher switch is. It the hazard switch working correctly? Many stick or break.
 
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bigburb11

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I have checked the brake switch - functioning properly (open when pedal is "up", closed when pedal is "down"). I will check that hazard flasher switch on the steering column next. Thanks again.

Service light switch is turned on, all other lights functioning properly.
 

cucvrus

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That brake light has its own fuse. It is on the same leg in the fuse box as the service lights. Did you cut into the fuse box to bypass that troublesome 30 amp light fuse? If you did you may have cut the power to the brake lights. You will need to stick an inline fuse into that circuit also. I have been there and done that. I was confused at first but relived it when my M1009 failed inspection. The dealer wanted to tear into it. At $99.50 an hour. I went out in the shop and powered up the circuit just enough for the brake lights to work. They worked at that moment and he passed it for inspection. I went home and corrected the issue I already thought I had corrected. I hope that helps you out.
 

bigburb11

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I haven't cut into the fuse box. Did check the 10amp brake light fuse - it is ok and there is power at the box. Next step check hazard switch. If that's ok I think I have an "open" wire somewhere... Fun
 

cucvrus

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The turn signal travels through the same wire to the rear as the brake lamp signal. When I put a high mount 3rd brake lamp on an older civi/CUCV type vehicle I pick - up my brake signal at the brake lamp switch right under the dash at the brake pedal. Do you have current going in there and out when the switch is triggered?
 

bigburb11

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The turn signal travels through the same wire to the rear as the brake lamp signal. When I put a high mount 3rd brake lamp on an older civi/CUCV type vehicle I pick - up my brake signal at the brake lamp switch right under the dash at the brake pedal. Do you have current going in there and out when the switch is triggered?
Yes. 12 volts at the switch and the switch is good.
 

T.Barney

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I have good fuses, tail lights work,front turn signals work,non in back. no power to brake light switch on orange wire.
 

HeisenBob

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the the orange wire goes to the turn sw?
I don't believe so. I was replying to the original poster. I believe the orange wire going to the brake light switch comes directly from the fuse panel. Check that 10amp brake light fuse right above the 30amp light fuse. I believe it's number 6. If you don't have power there, check the 30amp headlight fuse below. There might be an issue with that jumper bar. I believe that 10amp fuse gets power from the jumper bar.

Also, best advice I can offer is to go to the CUCV Helpful Threads section at the top of the CUCV forum. Tons of good threads with helpful info. There's a thread on headlights with great help. There's a step by step instruction of where the power is routed from the batteries to the fuse panel, to switches...etc. Lots of pictures and diagrams from the manuals. Print all of that off and put it in order. My printer is black and white so I had to take sharpies and trace the lines the correct colors to the corresponding wires. It makes troubleshooting so much easier. Every time I'm working on a system in the truck, I go to that page first and see if the issue has been covered before and what the possible resolutions could be. I print these diagrams found in the TMs from those threads and put them together in a folder for future reference. Has been a godsend. Good luck!


 

cucvrus

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My quick tip. May be accurate may be BS. But if the 30 amp light fuse holder has heat damage and has been altered in any way, Check that out. The same leg (power source) is serves the light fuse and the brake light fuse. I done an in line replacement of the light fuse and then discovered I had no brake lights when inspection time came around. I was perplexed and went and looked at a scrap wiring harness and I seen the error of my repair. I must have been driving 6 months with no brake lights on my M1009 Crown of Thorns. DSCF6472.JPGDSCF6473.JPGDSCF6474.JPGDSCF6475.JPGI also had the high mount 3rd brake light mounted on the roof with LED license plate light and mid ship turn signals. Good Luck. Be Safe.
 

Warthog

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I have good fuses, tail lights work,front turn signals work,non in back. no power to brake light switch on orange wire.
Have you split and cleaned the electrical block on the firewall? That block is actually three wiring harnesses, the rear lights is one of them. The "grease" GM used turns into a black goo that sometimes a causes issues
 

MarcusOReallyus

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The "grease" GM used turns into a black goo that sometimes a causes issues
Yep. Use silicone dielectric grease instead. That's what it's made for. Don't use one of the so-called "conductive" greases. They are not conductive, and they are made for AC power connections with aluminum connectors, not for low voltage DC work.
 

HeisenBob

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Yep. Use silicone dielectric grease instead. That's what it's made for. Don't use one of the so-called "conductive" greases. They are not conductive, and they are made for AC power connections with aluminum connectors, not for low voltage DC work.
What's the best way to clean that black goo out? When I was in cleaning mode, I took apart the main wiring harness from the fuse panel and noticed that black grease. I sprayed it with some electrical connection aerosol cleaner. I don't think it did much good but I didn't really have a good way to clean those plugs. I just sprayed it a few times and put it back together.
 

MarcusOReallyus

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Try some turpentine, or isopropyl alcohol, if you can find any. The same idiots that bought up all the toilet paper also went bonkers on that stuff. Tried to find some for a brake system flush the other day and found out the mindless wonders had cleaned it out.

(And yes, to any reader who is wondering, if you are one of those people who went out and bought all you could, I am calling you an idiot.)


Here's a good discussion on solvents and plastics.

 
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