77 kw w900a

Should be on the internet somewhere
But it also should only be about five wires so you won’t really need a diagram
It should be very easy to follow the wires and use a test light

start by checking all the connections
Especially where they pass through the cab firewall

 

I don't even know if I'd trust a schematic anyways. If its anything like my 1980 was, its had over 40 years of modifications, repairs and hacks done to it. The one schematic I did find online didn't really match what I was looking at anyways.

Honestly, wiring wasn't overly complex back then. Shouldn't be too hard to open up the dash a trace the circuit.