This has me completely stumped. My 94 XLT, 2.3 engine, manual transmission was running fine, then suddenly wouldn't start (at 1:30AM in an airport parking lot). It has a new battery and both coils are new, cranked fine but wouldn't hit at all. When I went back the next day to troubleshoot, it started and ran fine. I made it about halfway home, it quit suddenly and wouldn't restart. I had it towed to the shop I usually use, they diagnosed a failed fuel pump, and replaced it. It ran fine for about 8 miles, quit, restarted, then quit again after one block, fortunately in a parking lot.

I dragged out my tools and found neither the EEC relay nor the fuel pump relay were activated when the ignition switch was on. Fuel rail pressure was zero. I replaced the fuel pump relay with a jumper wire, the rail pressure came up to 40psi, but it still wouldn't start, even when I replaced the EEC relay with a jumper. Cranking with the fuel pump relay removed did not bleed off the fuel rail pressure, so I concluded that the injectors weren't firing. According to my wiring diagram, the injectors are powered directly off the EEC relay output (jumpered to 12V, at this point), and grounded through the PCM. I figured it must be the PCM, and replaced it. The truck started and ran fine, so I went to get a ride back to retrieve it.

When I returned, it wouldn't start. Fuel rail pressure was zero, even when I jumpered 12V directly to the EEC relay output, the fuel pump relay output, or both. I wasted a couple of hours checking voltages and looking for bad connections, then the truck started and ran fine, for no apparent reason. I drove it home (4 miles). The next morning it started, ran for about 1 minute, then quit and wouldn't restart for the next four days. During this time I determined that the fuel pump would not run, even when I applied 12V directly to the output side of the inertia switch connector. I checked continuity through the fuel pump to battery ground, and got about 1 ohm. I verified that the fuel injectors have 12V when the EEC relay output has 12V. I found that the EEC relay coil base has 12V at both input and output sockets, which explains why the coil isn't being energized - the output socket should be connected directly to ground, according to my wiring diagram. Even though the EEC relay isn't being energized when the ignition is "on", it sometimes buzzes in "Start".

After four days of frustration, the truck again started and ran, for no apparent reason, and is still doing so, though I haven't driven far. When it's working properly, with ignition "on", I measure 0.75 - 0.95V at the coil output (ground) socket of the EEC relay base; with ignition "off" I got 3 - 4 ohms between the socket and battery ground. The fuel pump relay coil output (ground) socket has 1.3 - 1.4V when the ignition is "on", but it grounds through the PCM, so I don't know what the correct value should be.

If my wiring diagram (Haynes) is correct, the EEC relay coil ground should be at 0V and close to 0 ohms to battery ground. However, the diagram also shows the fuel gauge powered off fuse 16 - there is no fuse in slot 16, but the gauge works, so I have doubts about the wiring diagram. I'm fairly sure that something is feeding 12V into places it shouldn't be, off and on, but I'm out of ideas where to look for the cause.

Anybody have any suggestions?

 

It sounds like a possible ground circuit problem. From the schematics I have it shows the PCM case and fuel pump share a common ground point, i.e., G104. However, I’m not sure of its location on a 94. Check in the engine compartment on the inner fender by the PDB or radiator support on the drivers side.

 

Thanks. Back when I did this kind of thing regularly, partial voltage most anywhere was a dead giveaway for a ground problem. Today, with computers and cross-connections, who knows? At any rate, I removed, cleaned and checked all the grounds I can find, and it didn't solve the problem.

Between 94 and 01, Ford must have moved the grounds and Power Distribution Box from the left to right side of the engine bay, otherwise your directions were spot on. There are three grounds: one on the radiator support crossmember and two on the inner fender. The wires to the fender ground closest to the PDB, and to the one at the radiator are apparently common. With all the ground wires disconnected, the black & black/green wires to the second fender ground are separated by 1.5 ohms from the other two. The orange wire to the second fender ground has no connection to any of the other wires. None of the ground wires are common with the low side of the EEC Relay coil, being separated by 8 ohms (radiator and first fender ground wires) and 5 ohms (black/black/green wires to second fender ground).

I have a couple of pages of notes on voltage and resistance readings, but the bottom line is that, when the truck runs normally and the relays are removed, there is low voltage (0.5 - 2 V) on the "ground" sides (both coil and power circuits) of the sockets that the EEC and Fuel Pump Relays plug into. When it doesn't run, there is about 10 V at the "ground" sides with ignition "on", dropping to <5 V at the EEC Relay coil "ground" in "start" (clutch pedal up, starter not running). My wiring diagram shows the EEC Relay coil "ground" (black/white wire) going directly to vehicle ground, but I haven't found any point that is common with that wire, once it dives into the wiring harness. I have been know to cut into the harness to trace a wire, but I'm not quite that desperate yet.

In the course of all this, I opened the bottom of the PDB and found that someone had cut the "ground" wire from the Fuel Pump Relay coil (blue/orange, connects to PCM terminal 22) and spliced it into the "ground" wire from the EEC Relay coil. I reconnected it properly, found the truck would not start, and thought I had another clue. Later, the truck started and ran normally.

I will be grateful for any suggestions. Even if you're as confused as I am, does anyone know where I can get a COMPLETE and ACCURATE wiring diagram?

 

rpsptd,

If you want to PM me your Email addy I'll send you what I have on the Ground Distribution, Engine Control, and Power Distribution diagrams.

 

I haven't put enough miles on to be sure, but I think it's fixed. The schematics made it possible for me to trace the mystery low voltages to a bad connection at an in-line connector under the PDB. Grounds all read zero volts and have zero ohms separation from the battery ground. The truck runs fine. Thanks.