Hello,

I have what is at this point a Frankenstein 1985 F250 with an EFI 5.0. The original harness was a rotted-out mess, so I basically scratch built a new one, incorporating the remotely mounted TFI module that Ford finally went to just a few years before discontinuing the design altogether. The truck ran/drove great, but for some odd reason an unusual problem has come up whereby the ignition cuts out completely, as if a switch is being thrown, but comes back to life once you lift your foot off the gas. I did have a stored code indicating a SPOUT failure, so thinking about it had me suspecting a wire harness issue, though pulling here and there pretty good on it while the engine was running had zero effect. I have a permanently mounted fuel pressure gauge on the dash and it always reads good, even when the ignition cuts out (though pressure climbs for those several seconds when it does). What in the EEC IV system would cause this engine speed/load kind of failure? An ECU issue?

The truck also has the very peculiar issue of dying when you pull the SPOUT to time it...unless you keep the engine speed at 1500 rpm or greater...or happen to have an LED test probe fitted between the SPOUT line and battery 12+. I did try an A1 Cardone ECU to try and solve that issue, but it did the same thing. I loop checked all the wires, followed Ford's troubleshooting procedure for closed-bowl distributors, different ignition modules...No change.

As a side note, one of my jobs is at an auto parts store. At my store we have a pretty cool Borg Warner ignition module tester, and with it I found every Ford TFI ignition module we had in stock, both the Borg Warner and also a house-brand generic, tests BAD. The only ones that don't are Motorcraft. At least at first. One I bought and had hardly put a few thousand miles on did test bad while diagnosing my dies-when-pulling-SPOUT-issue.

Any suggestions, advice, or criticism is much appreciated.

Rick

 

Sounds like a bad ECU computer.

 

There are 2 different types of TFI modules for these trucks with different signals used and two different formats for each.. a remote mount and distributor mount version. So my question is what vintage harness did you design/use for this and what TFI module is being used? Also note that the PCM and TFI module much be correct for one another so you could have a couple different mismatches going on. These differences may also explain why all those modules at the parts store fail the bench test.

 

I went from a grey push-start distributor mounted ICM to a grey push-start remote mounted. For the wire harness, I started with a naked 60 pin connector, a bunch of pins and wire and went from there. The ignition side of it is per '94-95' factory Ford closed bowl schematic, with the exception of a 22k Ohm resistor added to the IDM circuit, which my original '85 vintage ECU requires. I used shielded wire for the PIP, SPOUT, and IGN GND wires, with the drains grounded in the cab at the firewall. My original harness also required diodes on the IAC and NDS circuits, which I have also retained.

One other thought I've had, which perhaps I've not tested properly, is that if there were a ground issue either at the distributor or ICM, the circuit could look for a ground through the SPOUT, and when removed, so is the circuit ground. I did note that the eight pin distributor connector has multiple ground pins, whic I find odd (as with the ECU as well). To test this possibilty thst something wasn't right I clipped on some leads from the distributor housing and the ICM heat sink and ran them straight to the negative post of the battery. It made no difference.

I have been wondering...what does the IGN GND circuit do that wouldn't be served by the 'regular' ground of the components, anyway?

 

Note that the shielded wire for the PIP, SPOUT, and IGN GND wires shield can only be grounded at one end or you will have problems.
The ground point for it is inside the distributor.

 

Yes. I read up about shielding and the big take away I got was that if you have a wire carrying a small signal and want to protect it from ambient EMF interference you run it in a shielded wire grounded at one end. If however you have a wire carrying a strong signal which may be a generator of noise, such as an electric motor lead, or a spark plug wire, and you don't want that EMF getting out and effecting other wires or components, you run it in a shielded wire grounded at both ends.

I did my drain ground at the firewall, not the distributor. I never came across anything that said which end you used mattered, because the sheet metal of the truck and the distributor housing are at the same potential. I did check that with an Ohm meter.