I have a 1991 F150 302 4x4 manual. It has the following mods done:
MAF conversion
BBK throttle body
Edelbrock plenum
x code cam
superchips custom tune, no mods done after most current dyno tune
shorty headers
MSD distributor and coil
underdrive pulleys

Im having a problem where the truck shuts off like you turned the key off while driving. It will also randomly do this while idling, but really does not happen on acceleration. It seems like an ignition issue, because it happens on open or closed loop. If I unplug the TPS to force a check engine light, the light goes out when it cuts out and comes back on after a few seconds (just like you killed power to the PCM, or turned the key off). I took the column apart to check for shorts and found nothing. I replaced the cap (it was cracked for some reason) rotor plugs wires and coil. I also replaced the gray module on the distributor. When it dies, there is NO loss of fuel pressure (always stays between 32-38 psi). Can anyone please point me in the right direction? I really appreciate the help!!

 

Also, when I hook a voltmeter to the two wires that plug in to the coil, I have 12v with KOEO, but running the voltage jumps all over the place from 0 to 13v. Why does the coil have two power wires and what voltage should they be getting?

 

any ideas?

 

I can answer part of that one. Them are not both power wires(the small ones that hook onto the coil). One comes from the ignition switch and that is the power wire. The other small wire goes out to the distributor, it will have power through it when hooked to the coil with the ignition switch just turned on, or with the engine running.

When the engine dies, will it restart right away.

 

ok got it now, so will the ignition power wire always be energized even when the engine is running and the distributor is supplying voltage to the other side?

 

The ignition power wire, the one that comes from the ignition switch, will always be energized anytime the ignition switch is turned on and/or the engine is running.

You may misunderstand, the distributor does not supply the power to the other side(the other small wire that hooks onto the coil). That small wire still gets it's power from the ignition switch and will have power to it THROUGH the coil anytime the ignition switch is turned on AND the two wires are connected to the coil. If you unhook the two wires from the coil, of course only the small wire that comes from the ignition switch will have power to it.

This is kind of hard to explain, I tried to make it clear, any more questions just ask.

 

Interesting.

The PCM gets its power through a relay controlled by the ignition switch. If that relay is flaking out or the socket is dirty or not making good contact it could cause that problem. So could the fuse that feeds the relay its power, the power diode, or the wires that connect the battery to the fuse, the fuse to the power circuit on the relay, or the power circuit on the relay to the PCM.

Another possibility would be the control side of the relay. It looks like you thought of that and started checking the wiring in the dash, but did you check the connections at the PCM relay too?

 

Just to follow up with you guys the cause for this issue ended up being a faulty ignition switch on the column (the electronic part). The 2 halves of the switch actually started coming apart and caused intermittent loss of connection. Hope this helps others.

 

My truck is shutting off as well. If I hold the ignition switch forward (which I think, prevents it from losing connection), it will run. But when I take my hand off of the key, the engine will turn off after a while. Do you think replacing the ignition switch will solve the problem? It sounds like I'm experiencing a similar issue to you. Thank you.

 

it's quite possible you have the same problem, and it would be well worth your while to visually inspect the ignition switch to see whether the two halves are separating (fairly common). if it looks ok, try loosening the mounting bolts, slide the switch up the column a millimetre or two and re-tighten.