I have a 93 F150 with an E4OD Transmission, mass air conversion, MSD Ignition, 5.0 HO out of a 94 Mustang and an ECM out of a 96 f250 5.8 to power it. I did the conversion about a year ago. Runs awesome!

Then about 2 months ago the truck acted like it had a dead short in it. While driving all of the gauges quit, transmission shifted out of over drive, the radio quit, the engine died. Total power failure. It only happened for a split second but it did it several times. It does it so fast that you can just keep driving (although it's a little weird feeling). It feels like your turning the truck on an off with the key. It never completely died and stayed off, just off for a split second, on for a few seconds, then off for a split second again. Then all the problems went away and it drove fine.

Drove for another month or so with no problems.

Then about a week ago it did it again on the highway for about 10 miles. EVERYTHING OFF, EVERYTHING ON in a split second. It did it about 20 or 30 times in a row. I got to an exit ramp stopped at the light (it stayed running) and then pulled in to a parking lot. As soon as I got in the parking lot it quit completely. No power at all. When I turned the key on, nothing, no radio, dash lights, gauges, nothing. It was like there was no battery in the truck Opened the hood to see if it was about to burn to the ground, didn't see anything wrong, (no burnt electrical smell, no smoke). Got back in, it started right up and I drove it for 3 weeks with no problems.

Today I started it up, let it run for a few minutes and it quit again. Total electrical failure.

Does this sound like it might be a malfunctioning ECM?

The problem is so random that I'm having a real problem diagnosing it.

Any Ideas?

 

There are only a couple sources of power into the truck from the positive battery post, and just a few grounds. I am not familiar with your specific year, but I bet there is a single connector that is causing all your grief. When you pull it apart it will be full of corrosion. Here's a link to a the EVTM on CD:

http://www.google.com/url?sa=t&rct=j...80642063,d.cGE

These are invaluable in working on these trucks!

 

A bad PCM will not cause the entire electrical system to shutdown.

I suspect bad battery cables or connection to the battery. Start by cleaning the battery terminals and checking/cleaning the block ground and small grounds bolted to the inner fender.

 

If I were guessing I would say you ran the battery NEG post ground to the frame and nowhere else. The frame is not a good ground point and only used for the trailer if you pull one. Then you would have to ground the frame.
Why I say this is if you lost the battery for a few seconds the alternator would keep power up for at least 30 seconds with no battery if you have the battery wired right. So you lost the ground to the alternator or as Ford calls it an AC generator in the Ford diagrams and everything else.

 

Good Thoughts / Leads.

It has a pretty new battery and cables in it. I'll work my way back from the Battery and check connections. I have a complete wiring diagram from when I did the mass air and ECM conversion so I should be able to locate the critical connections. I rebuilt the entire wiring harness from the ECM forward, maybe I didn't get something plugged back together as well as I should have and now it's worked its way loose. Seems like no matter how hard you try some of those wonderful little tabs will break when you take the connection apart.

I think you're correct, something has to be disconnecting versus shorting out as there is never any heat smell, burnt fuses, melted wires, or draw on the battery.

Thanks

 

I don't think the PCM will cause a complete electrical system shutdown, engine & trans yes, but not everything. The fact that things like the radio quit says that it is more global. One thing that would narrow it down further would be if the dome light quits at the same time, but given the brief nature of the problem that might be hard to check.

I'm with tempest. Thoroughly check the main positive and negative cables as well as some key grounds. Check the following to make sure the are clean and tight:
- battery terminals - If you have the temporary (not the use of the word temporary) battery terminals that clamp on to the end of the wire check those extra carefully.
- negative cable connection at the frame down below the battery
- positive connections at the starter solenoid
- ground strap between the engine block on the passenger side and the firewall
- negative connection from the battery terminal to the core support

If all that is good, check your fusible links. The problem won't be a blown one, but you might have a damaged one that is intermittent. The reason I mentioned the dome light above is that if the dome light stays on when you loose power it would point to the fusible link that provides power to the dash, key switch, and other circuits. You'll need to flex and wiggle each fusible link while the engine is running to see if it is intermittent.

edit to add: Got interrupted while posting. Subford's point about the grounds is what I was talking about in this post.

 

I wasn't able to work on the truck yesterday, but based on your thoughts I had my daughter check the dome light.

She went out to the truck and opened the door. The dome light came on. When she put the key in the key chime came on (since she had the door open).
She turned the key on to see if it would start and everything died, including the dome light.

I checked it when I got home after work and everything was dead. The truck is outside and it was dark so I really couldn't check much.

I'll see what I can find when I get home from work today.

Any thoughts based on the dome light going off?

 

Most likely you have either a bad battery or battery post connections.
But could be bad battery cables also.

 

You can do a check and dirty check of the battery. Hold the terminals of a volt meter on the battery posts... the posts, not the cable ends or the clamps... and have somebody turn the key. If the voltage drops noticeably then the battery is bad. Otherwise, your up against cables and/or connections. Work your way through the list I made post #6 above. Be methodical and don't jump around.

 

X2

If you don't have a meter, a test light will also work. In fact, I prefer a light for tests like this.

 

I've always used meters and have no experience with test lights. Will the bulb dim noticeably if the voltage drops by several volts?

 

Absolutely.

Test lights are old school filament bulbs and varying the voltage will make a substantial difference in brightness. Tried it with an LED bulb and it does not work as well. Another advantage to the filament bulb is that it is not polarity sensitive.

Meters work just fine, but the digital readout of modern meters takes a little more effort to interpret. An analogue meter or a light is much more responsive to quick changes.

 

Thanks. I'll have to play around with one.

I'm a marine electrician and am frequently looking for things in the fraction of a volt to trace down wiring problems so I've always used good digital meters. One nice feature of nicer meters is the high/low capture which makes catching transients very easy.

 

No substitute for a good DVM when you are looking for fractions of a volt. I was repairing circuit boards component level when that was still a practical occupation. Made the transition from analogue to digital meters sometime in the 80's.

One trick I'll use is to turn on the headlights, then crank the engine. This often shows the problem to be poor battery connections, but if the lights stay bright, you know you need to troubleshoot farther down the cranking circuit.

 

Or a bad battery which was why I wanted him to put the probes directly on the battery posts themselves. That eliminates connections and focus on the battery. Tests good? Then move onto connections, etc.