M35A3 Starting Issue

Last year, back in October, My M35A3 refused to start when I tried to depart from the Southeastern Military Vehicle Rally. With the truck in neutral and the accessory switch on, engaging the start switch did nothing. The instruments came on, but the low air buzzer did not, even though the pressure was very low. I tried at least a dozen times and nothing. Some friends were about to help me when all of a sudden, the problem disappeared and the engine started right up. No problems since, until yesterday. Many attempts to start and nothing. The starter relay did not engage, but all of the instruments came to life, but again, the low air buzzer didn't sound. I was in the process of troubleshooting, when all of a sudden, the problem disappeared. I am all but convinced that the problem lies with the accessory switch partially failing due to the fact that the low air buzzer is also non-functional when the issue occurs. I am obviously getting 24VDC on pins A and C of the accessory switch, or the instruments wouldn't come to life, although I haven't measured it. However, there is no 24VDC at the starter relay when the start switch is engaged. To me, this indicates that the accessory switch is failing since the low air buzzer also fails to energize with the accessory switch on. The only thing the low air pressure buzzer and the engine start function have in common is the accessory switch.

May not be connected to the buzzer but do you know if you're getting 24v past the solenoid mounted on the engine block? They tend to go bad and keep power from getting to the starter. I'd also check that all the wires on the starter were tight.
I start at that end and keep backing up one connection at a time.

LowTech said:

May not be connected to the buzzer but do you know if you're getting 24v past the solenoid mounted on the engine block? They tend to go bad and keep power from getting to the starter. I'd also check that all the wires on the starter were tight.
I start at that end and keep backing up one connection at a time.

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I've disconnected everything and cleaned the connections. I have 24VDC at both the starter relay and solenoid. There are two separate wires labeled 11 feeding the battery/accessory switch. The switch sends power to the windshield wipers and heater blower on one leg and the other goes to various places including the start switch and the low air buzzer. The problem is either with the battery/accessory switch or with the wiring in my opinion. Unfortunately, everything is now working so the only thing I can do is clean connections, look for broken or mouse-chewed wires or replace the battery/accessory switch. I've ordered a new switch. Hopefully, the problem will permanently disappear.

I've had this issue a few times and normally if you vigorously flip the switch on and off a few times while holding the start button or switch it'll start working. Yes, it most likely is the contacts in the master switch failing.

I've had this issue a few times and normally if you vigorously flip the switch on and off a few times while holding the start button or switch it'll start working. Yes, it most likely is the contacts in the master switch failing.

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That was the first thing I tried, but no joy. My NOS switch just came in today. Not looking forward to replacing it as there is virtually no room. I'm also replacing the starter relay. I know it doesn't have anything to do with my current problem, but it is extremely corroded. Part of the mounting bracket is completely corroded away. The only thing not corroded are the wiring contacts that I keep clean and cleaned again when this problem surfaced. I found an NOS Prestolite starter relay. I actually have an NOS complete starter that looks brand new in my spares that I found years ago for $50.00. I didn't need it, but at that price who could pass it up?

I installed the new battery / accessory switch. Changing the switch was a royal pain, laying on my back with my head under the dash. Everything works and the engine starts fine now. The problem is that right before I installed the new switch, everything started working. I have no idea if changing the switch cured the problem or not, but I intend to drive the truck a lot on local runs and not to shut down the engine until I return home to gain some confidence that the problem is cured.