Eknoz,

Old truck, schematic maybe hard to find. I would check all your cable connections at the battery, engine and starter. Take a voltage reading at the negative of the battery to the cable connection at the starter. Then switch from the negative at the engine to the battery positive. See if you have a voltage drop. Also, there is an anti restart controller in the system. If I can find a schematic that can be read will send. Check all connections at the starter also. Good Luck.