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!