Any thoughts?!

1994 f-150 5.0

First we had starting system issues (no crank) intermittently which was cured with a new starter. Immediately afterward it would no longer stay running:


Issues:
Engine stalls constantly
Coil spark is not consistent, but does spark intermittently for minute then nothing.
When the engine does run, it stalls when the headlights/parking lights/rt turn signal are turned on.
The dashboard shows the rt signal dimly lit, but not blinking.
With ignition in the on position, but not in start position, the headlights (while turned off) have slight trace voltage going to them (they are dimly lit) which is coming through the 20 Amp Ignition Circuit @ Main Fuse Panel even if the main headlight fuse is removed.

My theory is a tiny short between light circuit and ignition circuit that is interrupting the voltage to the ignition coil or ignition system causing inconsistent spark. When the engine does run turning on the lights will cause a worse short circuit stalling engine due to the added voltage running across that short.

Any ideas on "how" to test my theory or other ideas at all?

 

It is not a short circuit. Most likely, you have a bad ground and its the one used by the ignition system. With the normal ground path open, the current is finding is way home via the lighting circuit.

Since this all started when the starter was replaced, start there and check for any ground straps that weren't reconnected or that have corroded or broken.

After that, you'll need a digital voltmeter to take some readings.

 

What can happen; When replacing the starter, for whatever reason you may have lost the main ground wire to the engine block. So when you tried the new starter it used one of the little grounds instead. When the starter uses one of the little grounds it burns it up. Now you have the big ground hooked up, but your little grounds to the body are burnt up and not working.

 

Thanks for your experienced thoughts and I will go back to checking grounds because I was also suspicious of a ground issue.

Another concern:
This is my sons truck and some other guys were trying to jump start with a high amp booster box and tinkered around under the hood before I got to it. So its very possible the problem lies outside the starter swap.

I will update as I get time to look at it and much Thanks!!!!!

 

Well, so far the ground(s) cleanings took care of the trace voltge through the headlights from the ignition circuit. However, no spark or start from ignition coil which I replaced just to be sure. Ignition control module tested good on test machine. I have voltage to the ign coil.
Now im reading and considering about the PIP sensor in the distributor.

 

The ignition system of your 94 5L is virtually identical to the one in my 95 Mustang 5L. I just went through a PITA ignition system problem that appears (crossed fingers) to have been fixed.

I had an initial "Crank no spark" symptom (as you now do). Several observations and tests for you to make:

1. Observe CEL when cranking. If the PCM is getting and processing a PIP signal, the CEL should go off during cranking. If it stays on, the PIP signal isn't present or isn't getting processed. Mine was staying on.

2. Check for the +5Vref power at the TPS. If the +5Vref is missing due to either a faulty regulator in the PCM or the power line is shorted to ground, the system won't work. Mine measured okay.

3. Test for a PIP signal at the output of the distributor. Gray/orange wire. The PIP signal should alternate between ground and battery voltage as the distributor rotates during cranking. Mine wasn't, it was staying at ground.

4. The PIP signal goes to both the ICM and the PCM. Disconnect the ICM and retest. If PIP is back, the ICM is bad (shorted input for PIP signal, probably not your issue). Repeat with the PCM to see if it's shorting the signal to ground. Mine was, a replacement PCM fixed the "no spark" issue on my 'Stang.

5. If there was no PIP with both the ICM and PCM disconnected, then the problem is focused around the Hall effect sensor in the distributor. Verify power supply. Verify IGN ground. Verify that the signal line for the PIP signal from the distributor connector (the vehicle's harness) is not shorted to either power or ground.

6. If those checks show no wiring harness fault, replace the distributor (or the sensor, if you're up to that operation).

In my case, I was still getting an intermittent no spark after PCM was replaced. A new distributor seems (so far) to have fixed it.

Good luck!