Thread: 78 302 starts right up then shuts right off. Help please!

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Here is the deal. Your ignition switch supplies power to the coil in two positions.

* When you turn the key to "on" or "run" (first positon) it is supplying less than 12 volts to the coil, usually something like 4.5 to 7 volts. This reduced voltage is used keep the motor running but not so high as to burn out the coil or module.

* When you turn and hold the key in the "start" positon, a full 12 volts is being supplied to the coil via a wire that is connected to one of the little posts on the fenderwell mounted starter solenoid. That 12 volts is supplied to the coil as long as you hold the key in the "start" position, but quits once you release the key into the "on/run" position. (The Ford solenoid has 4 posts on it.....1 fat cable, power in..... 1 fat cable, power out to the starter.........1 skinny wire, power in from the ignition switch when turned to the "start" position.........and 1 skinny wire, 12 volts out to the coil when cranking.)

What is happening, probably, is that your coil is getting that 12 volt shot of juice, enough to start it, but as soon as you release the key it is NOT getting the lower voltage to keep the motor running.

Easy to track down. Just turn on the key to the "on/run" position and put a test light or volt/ohm meter on the + post on the coil and the other lead to ground. I bet you see no voltage there, but you should be reading that 4.5 to 7 volts. Ford uses a special wire that reduces the voltage from 12 at the ignition switch down to that lower voltage..............it is called a resistance wire. You either have a bad wire or bad ignition switch. If you have no voltage at the coil when you do this test, then go to the ignition switch and see if you have 12 volts coming out of it when you do the same test with your tester. If you get 12 volts coming out of the switch but none at the coil, you are losing it somewhere in the wire, possibly a break or it is burned out.

Another test you can do is run a piece of electrical wire from the positive battery post temporarily to the + side of the coil and start the car again. It will probably keep running as long as you have that wire in place, but don't do it too long as 12 volts is too high for it to get for a long period and it might damage the module or coil. What you have done by running this temporary wire is you have bypassed the entire ignition switch and resistance wire.

Don