My truck starts fine when cold. After running for 15 minutes or so it dies and will not start back up. Engines spins, have gas, but no spark. It will fire once in a while when you release the key from the start position to run position. It also fires sometimes when you turn the key from run to off after trying to start it. After it cools back down it will start again.

Have replaced the control module, coil, and pick-up coil to no avail. Here is the thing that is stumping me. When engine is cold and the key is in the on position I have voltage to both the positive and negative sides of the ignition coil. After the engine warms up and dies I have no voltage to the negative coil terminal and the coil gets hot. I'm guessing that some component is failing due to heat and shorting out. Could the ignition resistor wire do this? Or is there a temperature switch in the system causing the problem?

I guess I should also add that this is a 351M. I took off the original 2bbl intake and installed an eldebrock performer intake and 4bbl carb several years ago. No more erg valve either. Gave me a lot more power, but this ignition thing is driving me nuts.

Please help, I have been dreaming of reading the same wiring schematics over and over for the last 3 nights!!!

 

have you tried an new ignition. try hot wiring it by pass the key switch.

 

Thanks for the reply fordfury469. I put a new ignition switch in about 3 years ago. I haven't tried to bypass the ign. switch yet because I can't see how that would fail only after the engine has warmed up.

 

Coil's bad. Bypassing the ignition isnt going to fix the problem. The longer your truck runs, the electical components get warm, has nothing to do with the temp of the motor. Seeing as how when its cold you hve juice on the negative post, and after its hot, you dont..there is your problem.

Okay The coil is nothing more than a transformer, that takes in 12v, amps it up, and when the distributor, tells the coil to (by shorting the ground side) it releases its stored up voltage thru the plug wire. this is the best explination I can give, some will probably use different termanology, but tis my way.

Good Luck!

 

Thanks for the reply Scott.

I just put a new coil on it this morning... same problem. Then I changed the pick-up coil... same problem. I understand how this stuff is supposed to work. I was an NIASE certified mechanic in the early '80's and was an aircraft mechanic in the Air Force for over 20 years.

Something in the primary ignition circuit is failing due to heat is my guess. But I can't figure it out. What do you think about the resistor wire? And do you know where it is?

Brad (AKA browntruck)

 

No its Wrapped somewhere under the dash, I think. My 73 has been totally rewired and no longer has a resistor. and my 75 hasnt given me any need to go lookin for it. YET.

Well Logically approaching this..Measure the voltage to the coil cold, then hot. Its possible, thou unlikley, you got a bad ignition box. There isnt much left.

Good Luck!

 

could be a wire to, more heat you get the more resistance there is. does all your electrical work after it does lights etc.

 

Thanks again Scott, I already changed out the ignition box also. I know that there is not much left. That is why I am questioning the resistor wire. Guess I could get an external ballast resistor or a coil with the resistor built in, and run a new solid wire from the ignition switch and then see what happens. What do you think?

 

Yes, everything works except the ignition circuit. That is why my brain keeps telling me to check out the resistor wire.

Thanks fordfury

 

just a thought, have you gotten a good look at the area surrounding the exhaust on both sides? could be a pesky wire has gotten itself into a place where as the exhaust heats up (and expands) it is contacting the wire and grounding it out. I saw something like that on my truck with a speaker wire that was used as the tach signal lead. when it grounded out the motor just died...

 

anything goes now.

 

Good thought Mr. Wilson. I don't think so, but I will be sure and double check in the morning.

Thanks

 

Well im not sure how youd bypass the stock electronic ignition stuff. You can try the wire from pos. side of batt to pos side of coil but im not 100% sure that'll start it. I know on my 75 it will, but its a stand alone pertronix setup.

LIke I said before check the voltage at the coil cold, and after it shuts off on you. If there is a difference, + or - .1 or .2v then i think you'll be hunting down the ballast resistor. I do know that ford uses a Wire, not a block like chebbie.

 

you cant by pass the stoc electronic ignition just the key switch.

 

the clip-on thing that attaches to the coil itself. the big plastic C-clip thing.that was the problem on my F-600, stumped me and two shops.