1977 F-150 - 351M - Automatic tranny

I need some help before I blow it up!

I've had this truck for about 2 years now and it has been running fine. I decided to sell it since I'm moving soon. Guy comes to test drive it and it wouldn't start! It would crank but not fire. Checked for spark...no sparky sparky. I got a sweet package deal on a starter solenoid, voltage regulator, coil, and ignition module. Started changing them one at a time and trying to start in between. When I changed the ignition module, which was the last one I did, the truck fired right up! Ran great for 2 weeks then the same potential buyer dude came back and guess what?!?! Fired right up! He drove it 5 miles away and it died while he was turning around in a parking lot! I towed it home and figured maybe there was a bad wire frying the ignition module. Nope...checked every darn wire and they are all good. Tested the module and it is good. After hours of troubleshooting I said what the heck lets see if it starts. It did...but it wasn't firing all cylinders. Then a couple days later I tried to start it again and it wouldn't fire at all. I had a new distributor lying around so I threw it in just for s***s and giggles. Still won't start. I know this has been extremely long but I wanted to put as much info as I could. I'm at my wits end! Anyone have any ideas

 

Check for power at the ignition module. The RED wire coming from the truck (typically goes to WHITE on the module) should be hot in START. The WHITE wire coming from the truck (typically goes to RED on the module) should be hot in RUN.

 

Both are good

 

What do you mean both are good? What voltage did you measure, and under what conditions?

 

8VDC on red in start position and 12VDC on white in run

 

sounds like the truck is fighting back the power that wants to sell it. overall im not quite sure but i do know that for a while my truck would often just die. rebuilt the carb and now no more cutting off

 

8 volts on the RED wire coming from the truck, or the RED wire of the ignition module (assuming you have a later-model module with the swapped colors I described above)? If it was the RED wire coming from the truck, was the starter spinning, which would explain the sub-12 volt reading?

If every update from you requires more questions from me to clarify what you're actually working with, we're never going to get anywhere.

 

RED from truck, yes starter is spinning, yes there are 12VDC at the center post of the coil sending power to the distributor, yes there is fuel getting to the carb, yes the carb opens when depressing the gas pedal. can't think of anything else right now.

 

Connect a test light from the TACH TEST (negative terminal) of the coil to clean, unpainted metal on the engine (ground). Crank the engine over with the key, and the light should BLINK. Report your results.