Brake light wire short

[9071083A1]

Hello !!

I'll probably get the 3rd degree for this but I've searched all over for a resolve for this problem all over this site and I am all out of ideas ??

So I just purchased a 2001 M1083A1 FMTV .

Its a rescent rebuild done in 2009 , so far everything on this truck is brand new , All of the light assemblies switches its in perfect condition !

When I went to pick it up from the owner and started the truck up , and I put the light switch in service drive mode and the instrument lights on bright , everything in the cab lit up great , My nephew checked all of the headlights , running lights , turn signal left and right and the Hazzards , everything was a go , I pushed the brake pedal and nothing ??? I released the park brake and tried again Nothing ?

Thank god my nephew was there because I drove it 35 miles back to town with him behind me in the driving rain .

So fast forward to today , I've been searching this problem down ever since I got it home and narrowed it down to the brake outputs from the brake switch themselves , I have good power supply from the circuit breaker and relay to the switches , The taillights work great with the signals and hazard flashers , but when the brake pedal is pressed the circuit breaker pops open , I disconnected the outputs from the switches and power passes through to the output side fine on both of them .

I pulled the instrument cluster and the harnesses are perfect , I pulled up the fuse panel and the wiring is in perfect shape , I followed the harness through the cab and outside where it goes down the frame and its in perfect shape , I just cant find where the output wires from the brake switches is shorted to ground ??

I'm thinking where they all intersect the signals , hazards, and the brake lights to go to the taillight housings themselves must have a wire grounded somewhere near , its both outputs from the brake switches and the other circuits that share the same bulbs are un affected ?

I'm really stumped and I am sorry but the schematics I can't really make heads or tails from in order to solve this ?

Any Help would be great

Thanks in advance

 

[NDT]

Maybe reconnect one wire at a time to the brake switches and see which wire causes the short? Maybe one wire engages the exhaust brake and the other the brake lights? Just guessing, sorry, my fmtvs are older.

 

[9071083A1]

I'll try that , At this moment both wires are resting ground .

I think somewhere the main wires that go to the rear of the truck somehow connect with the brake light output , turn signal output, and the Hazzard flasher output .

I'm looking at the schematic trying to locate this area ?

 

[9071083A1]

I've spent the day looking behind every accessible panel to track down this short in the brake wire ?

There is no damage at all to any of the wiring in this truck .

I can get power on the output side of the switches from the brakes , It sounds absurd but could I intercept the left and right wires going out of the cab harness to the bright elements in the rear taillights then wire a pair of relays to interrupt them and run the signal from the brake switches directly to the bulbs ?

This is the only way I can think of to get functional brake lights again in this truck ?

I'm all out of ideas !

 

[NDT]

Unplug the turn signal switch and see if the breaker still trips when you hit the brakes. I think that is where the brake light feed is interrupted so you get directionals when brakes are applied. Could be a short in the switch.

 

[Suprman]

Did you check and see if the short is in the light housing itself?

 

[9071083A1]

I did unplug the turn signal stalk and the brake outputs remain grounded ?

I took apart both taillight assemblies and their brand new , looked them over and their grounded well and the connections are very solid , I ran the hazards for quite a while and never got a circuit breaker to pop, same thing with each turn signal, ran Left then right for a while and didn't see any signs of circuits heating up out breaking?

This is very strange and I can only imagine it's something real stupid that I'm just not seeing !

 

[Ronmar]

What cb # is tripping, CB71 or CB76? If CB71 the issue is probably in the column sw. Since you say the turn signals work fine though(uses the same circuitry), I am going to guess that CB76 is what is popping? Turn signals working also tells me the chassis wiring running aft is also OK as brakes and turn signals same same wiring...

CB76 feeds 12V thru the brake light switches to the main light switch and back to relay K10. If 76 is tripping, the first thing I would do is pull K10. If it had a shorted coil it would cause what you are describing...

 

[NDT]

Maybe the circuit breaker is too small? People love to move relays and CBs around without regard for where they belong.

 

[9071083A1]

Ah ok so it is CB76 that trips.

I pulled K10 relay out then tested the brake switch output wires and they still rest ground ?

Not sure if they should have gone neutral or not ?

One thing I did notice was that all of the relays in the circuit box are all 12 volt ?

 

[NDT]

Possibly the stop light switch output wires will have continuity to ground through the lamp filaments.

 

[Ronmar]

Well with the relay K10 installed, you should see 12V from CB76 on one side of the switch and ground thru the coil(coil resistance) on the other side of the switch.

With the relay pulled I would not expect to see a path to ground on that second leg. Does it still pop CB76 with K10 pulled? If it still pops, the issue most likely lies in the main light switch or down the leg that splits to feed relay K10(read more below).

I tried to upload a screenshot of the A1 schematic the same way I always have and it says my .jpg file is not a valid file type(stupid forum engine)...

Sheet 8 of the A1 schematic(in the schematic section here on SS) has the applicable wiring for this circuit...

NDT, the brake light sw does not feed the brake lights directly, it feeds K10 which feeds the lights from a different CB(and switches from hazard flash to brakes on). But it does feed the trailer light plug and inter-vehicle plug. A short there could also cause CB76 to pop with K10 pulled...

 

[9071083A1]

Hmmmm that's great info Thanks !

When I get out to the truck in the morning I'll check this out.

I want to thank everyone who has helped out with ideas so far.

I've been pulling my hair out lol

 

[9071083A1]

Ok So a Morning update !

With Relay K10 removed and Relay K9 removed which are Brake light and Hazzard the brake switch outputs are still resting ground and will still short the CD76 circuit breaker when the brake pedal is pushed ?

 

[9071083A1]

I had also tried jumping the 12 volt power directly to the stoplight wire with the PX15 plug removed and it still shorts ?
I am not sure if that rules the switch out but I was curious while I had the dash panel loose

 

[NDT]

That seems like it would eliminate any circuits outside the cab. Try unplugging the brake light switches and jumping past them to see if that changes anything.

 

[9071083A1]

So I'm thinking of connecting the 2 brake switch outputs directly and running it to the K10 relay and set if it turns the brake lights on ?

Just bypass the whole circuit from the original harness for testing !

 

[coachgeo]

maybe I missed it but did you swap relays of same type into other places to see if the problem remains or if fixes in one place but becomes a new problem in circuit where a particular relay got swapped too.

 

[coachgeo]

it's been mentioned in the past that there is a series of circuit boards in the Power Distribution Panel (PDP) that had issues...... they were just piss poor. Suprman I believe has pointed this out in the past. Will/Superman what do you think? Are these symptoms that may point to this?

 

[9071083A1]

I am now believing that this truck has had this problem for a long time and that the person I bought it from didn't notice it.

One thing I do see is that they're is a green LED right above each relay and when they are in working order their illuminated.

The brake light LED doors not light in any condition of the light switch.

With K10 removed I can touch where terminal 86 is located and that green LED will light.

I jumped output from the switches on the brake to terminal 86, pressing the brake did illuminate the LEDs but also burned the trace out from terminal 86 so I have to solder a new wire in .

I wonder if I can upgrade this circuit box to a newer type without having to install a new cab harness too