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

[1088logtruck]

Having to find a short to ground on a ground switch circuit is something I'd never had to deal with till this definitely makes it difficult

 

[Ronmar]

Well if you remove K6, and disconnect the brake switch terminals, you should measure 12V on pin 86 of the relay socket and you should see no path to ground on pin 85 Of the K6 relay socket. If you do it is shorted… I think that wire follows the bundle along behind the heater…

 

[Ronmar]

I have put together test lights and used meters that have the continuity buzzer that I hook up thru the shorted circuit. Then I start moving wiring, separating, pushing pulling tapping while looking/listening for any indication of a change in the circuit from the tester to get me closer to the fault…

 

[1088logtruck]

So I did get it fixed well took the heater out and didn't find any damaged wiring at alll....but then after putting it back together I had 12v at the brake switches and the brake lights worked.


Now I am putting a red arc brake controller and I was pulling brake light signal for it from the output of relay k6. And when I tried to test it the truck just died as soon as I hit the brakes. And I'm basically back where I started. No power to brake switches and now every time you have lights switch on and main pwr on it will blow cb70 almost right away. ....

Anyone have any ideas on what happened?? I feel like I probably pulled brake light signal from the wrong source and fucked something up.....but now if I do get this fixed where do you get signal for the brake controller from.?

 

[1088logtruck]

Basically now when the trucks master switch is on or it's running it'll stay running till you turn the lights on then it'll blow cb70 and shut off.

 

[RRaulston]

Basically now when the trucks master switch is on or it's running it'll stay running till you turn the lights on then it'll blow cb70 and shut off.

CB70 terminal 4 feeds two 1586 wires, one to px15, (main light switch) and px17 (ignition switch). Could the main light switch be bad?

 

[1088logtruck]

CB70 terminal 4 feeds two 1586 wires, one to px15, (main light switch) and px17 (ignition switch). Could the main light switch be bad?

Ya main light switch was bad you were right but I bypassed it to a regular toggle....now I have power at 2 of the brake switch terminals again but as soon as I hit the brakes it blows cb 70 and cb76 and shuts off....also noticed the lowest of the three 4 pole diodes on the panel melted a little bit.
Really insane building out a whole knuckleboom log truck and the brake lights are what has the whole thing stalled. About ready to burn this thing to the ground and say **** it.

 

[1088logtruck]

Also of the 2 output wire from relay k6 one has short to ground the other is open. If I remove the grounded wire it won't blow a fuse but still no brake lights. The px15 k wire is short to ground.

 

[RRaulston]

Also of the 2 output wire from relay k6 one has short to ground the other is open. If I remove the grounded wire it won't blow a fuse but still no brake lights. The px15 k wire is short to ground.

px15 is a connector (Main Light Switch), not a wire. (TM 9-2320-366-34-1 (3 of 40)) K is wire 1404. I think 1 k6 wire goes to the light switch and one goes to a large connector on the bumper... Which is grounded? The light switch wire or bumper (j51) wire? Damn, i'm getting confused...

 

[RRaulston]

px15 is a connector (Main Light Switch), not a wire. (TM 9-2320-366-34-1 (3 of 40)) K is wire 1404. I think 1 k6 wire goes to the light switch and one goes to a large connector on the bumper... Which is grounded? The light switch wire or bumper (j51) wire? Damn, i'm getting confused...

If you unplug J51, does the fuse blow?

 

[1088logtruck]

I got it....but ya ik px 15 is the connector. I meant the k wire on it was shorted. But it ended up being that I had the trailer plug wired wrong and the brake wire was grounded in the trailer plug. It's all fixed again and I pulled 12v signal for the brake controller off of the output on k10 instead of k6 like i had it. Now it's all working again . Thankyou for the help.

The rest of the wiring I'm doing (strobes, work lights, pto switch, radio, egt boost and trans gauges) won't be tied into the trucks system except to pull power and ground so I shouldn't have any more problems with this stupid brake circuit haha.

 

[coachgeo]

.... ended up being that I had the trailer plug wired wrong and the brake wire was grounded in the trailer plug....

It is always something STUPID right.. Welcome to my world. If my brain made no mistakes... my brain would not be working at all.

Glad you found it. Sounds like you didn't fry anything if your back to functioning.

 

[1088logtruck]

Well I did fry the light switch but I wanted to rewire that to a regular rocker switch so I did that and then found my mistake and rewired the brake controller signal from k6 to k10 and now it's all good.

 

[j_boucher]

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.

So Im having a similar issue as of yesterday ... Previously they were working great But I noticed the other evening the brake lights stopped working,

Current items work fine

All Parking lights work
All Blinker lights blink when left or right signal is turned on
All Hazard lights blink when turning on hazard lights
All Bulbs are fine
Headlights work fine

I have power to both brake switches
When brake pedal is depressed I can hear and even felt the relay working (K6)



Is power suppose to run to the other terminal once the brake pedal is depressed or does it just activate the relay to send power to the lights ? I have no power to other terminal when the pedal is depressed but as stated before I do hear the relay activate.

Thanks in advance

 

[Ronmar]

What truck do you have? The brake circuit is different between the A0 and A1…

 

[j_boucher]

I have a A0

 

[Ronmar]

K6 is a good sign, means the switches are OK. from K6 it passes thru the light switch and activates K10. K10 switches the light output from flasher to brakes. Flashers working means K10’s de-energized contacts(30-87A) and all the rest of the light circuit is also OK. So I suspect you have someth9ng a iss with the circuit thru the main light switch. That one is easy enough to bypass, and turn it into a brake light system more like a normal vehicle(brake lights work at any time)…

here is the A0 light circuit I drew showing it all on one page

 

[j_boucher]

K6 is a good sign, means the switches are OK. from K6 it passes thru the light switch and activates K10. K10 switches the light output from flasher to brakes. Flashers working means K10’s de-energized contacts(30-87A) and all the rest of the light circuit is also OK. So I suspect you have someth9ng a iss with the circuit thru the main light switch. That one is easy enough to bypass, and turn it into a brake light system more like a normal vehicle(brake lights work at any time)…

here is the A0 light circuit I drew showing it all on one page

View attachment 931224

So how would I bypass it if needed ? I did a test on DA1 and it also had power . Maybe I should check the K10 relay again

 

[j_boucher]

Should I also check the K30 and K31 relays And btw thanks for the diagram

 

[Ronmar]

K-30 and 31 are for feeding 24v commercial trailer lights via the 10 pin rear connector. You can remove them if you wish…

pull K10 and connect a meter from pin 86 to 85(ground). You should see 12v there when you step on the pedal…

You can bypass the light switch by jumping pin K to C which makes the brakes live whenever you press the pedal…