Brake light wire short

Ok, this forum engine wont let me upload ipad screenshots anymore

CB76 goes to the brake switches then to S4, the main light switch. The output of S4 goes 3 places.

1. Pin 86 on relay K10.
2. Inter-vehicle connector
3. Trailer connector

A short along any of those 3 lines will cause CB76 to trip.

You jumped power directly to the K10 socket pin 86 and it burnt the trace to pin 86? That tells me you have a short back down that pathway toward the S4 output or the path to the inter vehicle and trailer plugs...

If you message me an Email address I can send you a marked up screenshot of the schematic that shows the circuit...

Both the trailer plugs and Intervehicle plug look great , I am going to have to see if I can get a good look at the back of them and their harness routes .

This truck had rescently been reconditioned by Stewart and Stevenson in 2009 , Everything looks beautiful in condition.

Could I have a bad Circuit board ?

Yes, or a bad lightswitch S4, or a short along wire 22... pic sent via email...

I did find something kind of interesting just a minute ago .

I unplugged the PX15 plug from the lightswitch .

I found a pinout diagram and used a jumper wire to test the individual circuits , I could get the headlights to illuminate , the clearence lights , reverse lights , dash lights , tailights but when I jumped the pin H which was supposed to be marker lights both brake lights illuminate ?

Of course when I jum the brake lights it arcs still indicating a short .

Why would the brake lights come on with the marker light circuit ?

Could the pin be in the wrong location ?

Sure it was the brakes and not the running lights? There are two lights in the assembly. If you plug K10 back in and turn on the hazard flashers, are they the same brightness as the lights that came on when you jumped the marker lights?

Yup thats what made me test it again , the lights were too bright to be marker lights .

I switched back to jumping Pin C to get the running lights back on .

Now since the K10 relay is removed jumping pin H does not illuminate brake lights anymore !

I found an interesting condition this morning .

So I have the PX15 plug disconnected from the light switch , I also have both legs disconnected from the brake pedal switches .

I traced the wires comming from the brake switches up through the grommett into the dash one of the wires from the output of one of the brake switches runs up to Pin K on the PX15 plug .

With both the wire hanging loose at the brake pedal and the PX15 plug loose from the switch The wire from Pin K is still showing ground , Its completely loose and moves freely withing the harness , I cant spot a short anywhere ??

Whats the chance the wire is grounded inside the connector itself ? It looks like it could not happen due to the straight through pin orientation but could this be possible ?

And what is taking this plug apart like , I have nightmares about these things lol

Either unplug the lights, or remove the bulbs or you will see the wire grounded through the element.

Ok So Forget everything I had just posted this morning lol .

I started second guessing my findings and on a hunch I went back out to look at the harnesses once more , I was wrong on the output wires from the brake switches , The output with the 2 wires going into the ring loop I followed up and it splits 2 directions one wire does go up and supply Pin K with the brake signal , the other wire runs across the dash to the Circuit board .

The second wire output from the brake switch is a single wire and it goes up the harness to the circuit board , The plug that has the input wire for relay K10 has 2 wires going to the terminal 86 on the relay , Now I dont know if these 2 wires represent the 2 the come from the brake switches and they both connect together at this plug as it is still resting ground , I will look farther into it !

Doghead, the circuit he is having the short circuit on is the one that drives the brake light relay(K10). The relay drives the actual lights, which work fine as they also act as turn signals and hazard flashers...

I have found where 2 wires go into the one connector that triggers terminal 86 on the K10 relay , I am not sure if this is where the 2 wires from the brake switch outputs come together or not I am assuming at least one of these wires is comming from Pin C in the PX15 plug but Im not sure where the 2nd wire would come from ?

The brake switch wiring comes together on the other side of the main light switch from relay K10. It has to, as the main light switch then routes that switch command to either the normal brake lights(activates K10) or the blackout lights.

The drawing shows 2 wires coming off of pin C(one to K10, one to feed the trailer connectors which I marked in green). If you only have one wire coming off of pin C, what they may have done is sent it to feed K10 and made that wiring split where the wire connects to the PDP Feeding K10.

Ok Thats starting to make more sense , Yes one wire from my brake switch runs up to Pin K on the PX15 plug , For some reason the wire lining up with Pin C On the plug is going to the dimmer rheostat for the dash lights ??

I can jump Pin C in the Cannon plug to Pin F and get both brake light to illuminate , Jumpin Pin H gets all the running lights and tail lights to illuminate , Pin K still represents a shorted condition

I think you are misreading the plug, B goes to the rheostat.

9071083A1 said:

I can jump Pin C in the Cannon plug to Pin F and get both brake light to illuminate , Jumpin Pin H gets all the running lights and tail lights to illuminate , Pin K still represents a shorted condition

Click to expand...

Well with the Px15 plug disconnected, if pin K measures a short circuit, then your problem is along that wire connected to pin k, back toward the brake switches...