Brake system fault

[Captain Cyrus]

Hey guys, I’ve got a little mystery I could use help with.

The bus is a 2000 Thomasbuilt FS65 Freightliner with CAT… (and if anyone has resources to find a repair/service manual for this model, none of the Thomasbuilt dealers are able to find one for me so far)

The problem: Engaging the key I get a dash audio and visual alarm for the brake system (!). Physically the brakes are working fine. When pressing on the brake pedal continuously the brake lights turn on for 5-6 seconds then turn of, and repeat. The brake light relay on the under dash relay board is ticking over in concert with the lights going on and off. Measuring its output it is alternating discharge between 12v and 6v.

What I have done so far: Replaced the relay switch. Checked for continuity on all the fuse and fuse breakers on the fuse/relay board. Re-ran a new wire from relay to rear lights and tested on a separate 12v circuit to eliminate any potential ground faults on that side of the system. Wrote Thomasbuilt central for help finding electrical schematics, who has told me to contact my local dealer (their default response). Contacted several dealers, local and not, to try to find electrical schematics…. to no avail. Scoured Skoolie forums for answers.

If anyone can help me figure this out I would be super grateful. It’s a complete show stopper. I’m stranded until I can fix it. Not having schematics is the weak link in my chain and the electrical for the brakes on these machines is not, shall we say, the simplest of designs.

[BamaBus]

Was the melted ABS sensor issue ever resolved?

[ewo1]

Originally Posted by BamaBus

Was the melted ABS sensor issue ever resolved?

Nice catch!
Good memory!

Previous thread -> Here

I didn't realize there was another thread already that quite obviously could be tied together with this thread...

Melted wires and all!
A short like Cadilackid suggested in the other thread???

Hey captain, please keep this all down to one thread, you got two already on the "Brake system-Fault & Braking Electrical Fault" question.

As you can see, many are here to help but it's easier to help if you consolidate this to just one thread!

Melted wires could possibly be why the relay is clicking???

[Captain Cyrus]

Roger. Still figuring out the system for threads here.

The ABS sensor was replaced quite easily and the melting was due to the brakes being partially stuck closed after sitting for a long period with the parking brake on. The first couple miles of driving after that period caused the melting but the new sensor is working fine.

I may finally have some drawings coming to me for this model bus, that will hopefully shed more light on the electrical complications of the braking system.

In this way I can potentially isolate different sections of the system to see where the fault may lay.

[Captain Cyrus]

Ok. Managed to get an electrical schematic for this bus from a dealer in Massachusetts. Does anyone here have experience reading these schematics and can tell me what would be making the braking system alarm and the brake light relay to be picking back and forth while the brakes are engaged?

[BamaBus]

I don't see a reason for this issue but I do see that wires 379B and 380 are connected to the DRL (daytime running lights) module and the park brake relay. I had an issue with a bad DRL module in my 99 FS65 causing issues. I removed it and my problems went away.

[Captain Cyrus]

Thanks BramaBus,

Did you remove the module from the system and just terminate the wires?

[BamaBus]

Once you get to it you could just unplug it to see if it helped. It was in the center of the dash, had to remove some plastic panels for access. I removed mine just in case I needed the part number to replace it.

[ewo1]

Originally Posted by BamaBus

I don't see a reason for this issue but I do see that wires 379B and 380 are connected to the DRL (daytime running lights) module and the park brake relay. I had an issue with a bad DRL module in my 99 FS65 causing issues. I removed it and my problems went away.

Going on what Bamabus pointed out, I see a diode on the coil side of the park brake relay, which has the 380 wire (relay coil) going to the DRL. (wire 309, relay coil) that goes to J8 of the brake module. I would go ahead and try to locate that diode and test it. If it were shorted or leaking I could see where you might get the relays to bounce.

That diode would most likely be at a connector and not "in-line".

From a troubleshooting point a view, this to me would be a "Must verify".

The stop lamp relay is fed from the 12a fuse on wire #36, to the stop lamp switch, (brake pedal switch), relay activates and turns on the stop lamps.

Wire 500 sends sends 12v to "A" of the brake module.


There appears to be some sort of "Ground" switch there and it appears to be controlled by the diode?
that ground switch goes to the brake module, J8

I would take a guess to say that if the diode at 309 were bad, loose or internally leaking then it might make the brake module or the ground switch "trip" and maybe that's why you getting your issues?

The other theory might be the brake module itself is going bad?
I like the diode theory!

It would be real helpful if you had a part number for that brake module!

[ewo1]

It also looks like that brake module might control a brake motor, thru contact position L and M on the module. It your brake pedal pulsating when you apply the brakes?

From your post on 09/16...
I have read on another forum that there is a brake system module under the dash somewhere that needs to see voltage from both brake lights and brake booster to activate.

Wire 500 feeds the 12volts to the brake module from the brake lights!