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My 78 is having starting problems. It cranks and has gas but wont fire sometimes. 3 times now the engine fired up AFTER I let go of the key.
I suspect the ignition switch so I tested it. As near as I can tell, the power terminals sometimes go dead when the key is in the start position. (Can't be positive of this due to crude testing procedure).
Is this a common failure in an ignition switch? Could it be something else (like the starter relay)?
When the starter relay closes there is 12 volts direct to the coil for starting, from the "I" terminal of the relay.
Since your truck wants to start when you release the key to run the problem is in that circuit.
It could be a bad connection to the "I" terminal inside the relay or it could be a bad connection at the "I" terminal, the coil + or it could be the wire itself that runs from the relay to the coil.
Ignition switch has nothing to do with it as it only closes the starter relay, in this circuit anyway it does a whole lot more for other circuits.
So if the starter is turning and the truck only fires when you release the key to run, I would bet on the starter relay but check the wire first.
The relay I have was out of a box of spare parts from another engine. Guess it's time to replace it with a good one. I'll check the wires first though!
And I thought I was gettin' smarter. Dang! Wrong again.
Another possibility, keep forgeting about that "new fangled" electronic igniton. I always thought that the module would go completely dead not just on one circuit. Neat.
I guess the best way would be to measure for voltage at the coil + when you are cranking over the engine. No voltage starter relay or wiring, 12 volts no start could be ignition module or wiring.