1975 F250. One day truck would not start - no spark. Replaced electronic ignition module and we were good to go.

Drove about 10 miles and plowed snow for a couple hours, on the way home the truck died. Fusible link from starter solenoid was melted. Also found all right turn bulbs were blown out(I hit the right turn when the truck died).

Replaced link but still no start, replaced ignition module again and it ran. Amp meter looked like it was really running high so I shut it off and disconnected the main lead from the alternator so I could test the output.

I started the truck up with the main alternator lead unhooked and a few seconds later all the dome light bulbs went up in smoke due to voltage spike, by smoke I am talking blown glass and sparks. Back of instrument cluster also melted where it feeds the amp meter. I now have no power to anything so something in the harness also melted as the fusible link is good. I need to find what melted but I also need to find where this electronic frying voltage is coming from. I am hoping I shut it down quick enough to save the new module but we will see.

Can a coil feed back through the power wire? Can the alternator feed high voltage through the F or R terminals as I only unhooked the big batt connection? The truck has a external regulator.

Any ideas?

 

There are only two devices on the truck capable of generating voltage: the alternator, and the secondary winding of the coil. Since the secondary winding of the coil only goes to the spark plugs and not the remainder of the electrical system, the alternator is producing the excess current. The only explanation for the alternator producing excess current is because the regulator thinks the system's voltage is less than 14.4 volts, so it's cycling the field current in the alternator for too long.

The most likely cause of this problem is a bad voltage regulator, or bad wiring in between the regulator and the alternator (including the ground at the regulator). If the truck would run, I'd tell you to use a multimeter to measure the voltage across the battery terminals while the truck is running, and see how high it really is. In your case, though, you're going to have to track through the wiring harness and see where you've lost continuity. Either way, a multimeter is a must-have for electrical troubleshooting.

Before you try getting the truck started again, however, I'd go through your fuse box and the wiring at the solenoid and make sure that all the fuses are the proper rating, and that the fusible links are present. If the voltage across the bulbs was high enough to make the glass shatter, I would have guessed the current draw would have been enough to blow a fuse. This catastrophic of a failure should not have happened. You need to go through the fuses and links as a safety measure; consider yourself lucky that the truck didn't burn to the ground. When I bought my '79, several of the original fuses were wrapped in tin foil. Stuff like that causes problems like this.