I have a 390 FE V8 swapped into my 1978 F-150 with a c6 transmission 2wd, and right now im having some problems with it. To give some background ill start off by saying what it started doing and everything ive done to it. I've replaced multiple parts, but this is now an electrical problem as far as I believe. Ive replaced the starter sylenoid twice, re-done the some wiring, replaced battery cables, replaced voltage regulator, new cap and rotor, re-done the timing, new ignition switch, and new alternator. When I try to start it, it will keep cranking, even after I turn the key off and take the key out it keeps trying to run. I'd love to have it running soon, but I'm pleading to anybody who could possibly know what could be wrong please let me know. Thank you

 

Cheap Chinese made solenoid ? I had one go bad in 1 day from Autozone

 

OK, this makes me wonder:

"When I try to start it, it will keep cranking, even after I turn the key off and take the key out it keeps trying to run."

So the engine cranks with the starter. But with the key off, it continues to:

A) keep cranking WITH the starter?

or

B) keep turning on its own WITHOUT the starter?



WITH starter, you have an electrical & solenoid problem. (plus something else, as it isn't firing)

WITHOUT the starter, you have a timing or valve closing issue. You may have an electrical issue as well, as with the key off you should have no spark. It is possible for an engine to "diesel" with the key off, but that is usually a hot engine.

 

I agree it's probably the solenoid. I had one do that on my Mustang ( Cheap Chinese replacement ) I had to jump out a few times and beat the thing with a screw diver handle to get it stopped..

 

Remember there's no real reason to purchase modern, crapola-quality solenoids for our trucks. They've been manufactured for at least 50 years and they made a metric crapton of them. And the new ones are often junk. Even the "jobber" quality aftermarket replacements made in the 60s have a nice heft to them and use copper where it's needed. NOS, boxed, is for sale by the usual suspects for not too much money. Maybe $20 or less. Cheaper than two or three el-cheapos and a burned up starter!

 

I've had this happen, and it turned out to be a bad battery and/or battery cables, or can even be a bad starter.

Reason being, if the voltage drop is so much that it causes the load to go through the roof, the solenoid contacts will actually weld themselves together. Whack the solenoid with something hard and it stops.

I too, thought it was the solenoid, and replaced it, brand new Standard brand, 25 years ago, did the exact same thing.

My battery was marginal, and the cables were not much better. Again, too much voltage drop, and the solenoid arcs so much that it welds the contacts together.

Check the voltage of the battery with everything off. A 100% charged battery should sit right at or above 12.6 volts. Then, connect the volt meter between the + side of the battery, and the starter cable right at the starter. Crank and read the voltage. If the voltage drop is more than say a few volts, you have a bad connection, or even a bad solenoid at this point.

Then, check from the (-) side of the battery to the engine block. Again, more than a volt or two of drop, you have a bad ground.

 

I was leaning towards ground also.
I wonder if the truck will start if you hot wire it and jump the solenoid...

 

I forgot to ask before I fart around with a lot of wiring, could it also be a compression or fuel pressure problem? Also realized that the hose on the vacuum advance was not hooked to the carburetor...oops

 

I had the same crank on problem when I reinstalled the 428 in my mustang. I had the ignition wires on the solenoid swapped backwards.