LMTV - Brake lights stopped working (Troubleshooing) Follow-Along

[Overdrive]

OK, I have been piddling around with not fixing my brake lights that stopped working when I picked up an M1082 from Ft. Leonard Wood for a friend. So, I am determined to troubleshoot and fix the problem and will keep updating this thread with the adventure...

1. Issue - brake lights were working then while on a trip at some point stopped working.

2. Status of other lights - All other service drive lights work, headlights, turn signals, running lights, and hazards. Need to verify blackout drive lights next and see if the BO brake lights also are not working (needed for section of TM troubleshooting). During the trip in question trailer lights were not functioning either. Missing relay so need to plug in a 24V relay and test to see if trailer lights are still not working.

3. TM used and page numbers - Troubleshooting electrical system in TM 9-2320-365-20-1. Going to look at two different fault numbers e59 One or Both Stoplights Do No Illuminate (2-710) and e61 Stoplights and BO Stoplights do not operate (2-740).

Some things already done:

• Verified bulbs are good.
• I have a multi-meter but no STE/ICE test box
• Brake light switch no power - turned on stop light on light switch then checked for voltage at one of the two terminals on brake switch - no power - have to go redo this test as it looks like there are two switches off the brake pedal valve.
• I checked a couple of the relays. One had no power to it at all. I need to go back and document which relay and its label on the PDP chart

Tips:

• Remember, the main light switch on the dash ALWAYS has power to it even if the master power switch is off

OK, this is where I am so far. As I troubleshoot and figure things out I will keep updating this post. I am switching to following the TM so we will see where that takes me.

Hope this helps someone.

OD

 

[Suprman]

Tm had troubleshooting flow chart but there may be a relay used for brake lights. To the left of the steer column under the dash is a black break out box it has lighting connections in there not sure if all though. The light switches are known to go bad maybe the extra load of lighting the trailer lights did it in.

 

[Overdrive]

ah.... I didn't know that about break out box.

 

[Suprman]

Me neither till I was curious and took one apart. There is a voltage regulator next to it I have yet to figure out what it does. Unplugging it dosnt seem to have much effect though. Maybe for the dash dimmer or something.

 

[Overdrive]

Went thru the TM 20-1 troubleshooting and figured out that there is no continuity from the brake switches on the pedal to the relay that energizes the brake lights. As Suprman suggested to me originally I ran a new wire from the brake pedal switches to the relay and now the brake lights work. Not my first choice to have a wire flopping around I would rather fix the issue but after inspecting as much of the suspect wire at the switch and relay ends under the PDP I could not see any shorts or breaks so this fix is going to have to do for now. So mark this issue as FIXED but with an * for not finding the break in the original wire.

 

[GoldenStateFireSupport]

Went thru the TM 20-1 troubleshooting and figured out that there is no continuity from the brake switches on the pedal to the relay that energizes the brake lights. As Suprman suggested to me originally I ran a new wire from the brake pedal switches to the relay and now the brake lights work. Not my first choice to have a wire flopping around I would rather fix the issue but after inspecting as much of the suspect wire at the switch and relay ends


under the PDP I could not see any shorts or breaks so this fix is going to have to do for now. So mark this issue as FIXED but with an * for not finding the break in the original wire.

How difficult was it to add the wires? Can you upload a photo or description on how you did it?

Thank you very much.!!!!

 

[Suprman]

Wires between the dash panel and power panel run behind the heater box. It's not hard to do. What are you looking to run?

 

[GoldenStateFireSupport]

Wires between the dash panel and power panel run behind the heater box. It's not hard to do. What are you looking to run?

Did you have to add wire from all four terminals on the brake pedal to the relay?

1997 M1083

 

[1088logtruck]

How difficult was it to add the wires? Can you upload a photo or description on how you did it?

Thank you very much.!!!!

I am having the exact same issue with my 1088...
-have all lights but brakes
- hazards for some reason only light up in the front
-no power at brake switch(all 4 wires connecting to the two switches at the foot valve are grounded.(probe power to any of the 4 on or off the switches and it pops power probe breaker.)

Just curious which relay you ran power to? and what pin? Also which brake switch did you use? Thankyou.

 

[Ronmar]

Brake lights get prime 12v power from the main switch. It has to be on to power K6.

with the main sw on you should see 12v on one terminal of the brake light switches. It flows from switch, past K2, thru CB76, thru the coil of K6 to the brake switches.

 

[1088logtruck]

Ya definitely no power at brake switches its a short to ground on all 4 wires. Going to look at the outputs on the main light switch and see if I have power coming out of what wire number??? This is my first military truck and God damn the wiring is complete bullshit...

What is k6??? Or what is cb76?? I have the wiring section of the manual printed and even so I have 4 5 hrs into reading it and labeling and going page over page over page and still have not been able to trace this whole brake circuit or even half of it.

 

[1088logtruck]

Also seems there is something different about the 1088 brake wiring from one page I found in the manual.

 

[Ronmar]

K6 is the stop relay in the power panel…. CB76 is a circuit breaker….

If you are measuring ground on all 4 brake switch terminals(only 2 connect to ground), you probably have a short in your dash wiring between the switches and relay K6. That path to ground is keeping K6 energized and the brake lights on, the same way the brake switches are supposed to power K6, by providing a path to ground thru the switches when you step on the pedal.

 

[Ronmar]

If you pull relay K6 out of the panel do your brake lights go out?

 

[1088logtruck]

Yes they do and now that all makes sense Thankyou for that. If I run a new wire from the switches to cb76 that should fix it then?

 

[1088logtruck]

I have 2 brake switches...so both are in parallel ran to k6?

 

[Ronmar]

I have 2 brake switches...so both are in parallel ran to k6?

That is correct. Of the 4 connections on the switches, 2 should go to ground and the other 2 should go to K6 pin 85.

Since they are spliced together and only one wire for each wire actually makes the journey across the dash to the ground terminal buss and the relay, I would be looking for a wire that is up against dash metal and shorted between switches and K6. You can lift the power panel out by removing 3 philips head screws up under where the cover latches are and 3 more right across the front of the dash(right in front of your knees when seated). Then the power panel lifts right out and you can tip it right out onto your lap to get to the backside of yge relays. Disconnect the 12ans24 at tge battery first…

 

[Ronmar]

If I run a new wire from the switches to cb76 that should fix it then?

No

there are 2 ways to control a relay. Connect one side of the coil to ground and switch power on and off to the other side of the coil to control current flow, or connect one side to power and switch the path to ground on and off to control current flow thru the coil.

K6 is a ground switch controlled relay. CB76 feeds power to K6-86 whenever the main sw is on and the brake switches switch pin 85 to ground to control the relay…

 

[1088logtruck]

Thankyou so much for the explanation!! I think I got it I'll post back, hopefully with success this weekend when i get to the shop to work on the truck.

 

[1088logtruck]

I've had the whole dash out and the whole relay panel a few times now(the yellow locks for relay terminals were missing and dislodged), that and getting 12/24 relays to correct spot fixed 99% of electrical issues I had.
But I didn't see any shorted wires....I'd guess it's behind the heater?? Or up behind the grille or something.