I have a 94 4.0 2wd 5-speed that will not start. I drove it to work one day and it would not start that evening. I've since trailered the truck home to work on it and I suspect I have a bad PCM, but am looking for help to make sure I've ruled everything else out first.



The truck will crank over just fine but makes no attempt to start. The fuel pump does not prime when the key is cycled. The CEL does not light when the key is on, but to be honest I can't be 100% sure the bulb isn’t burnt out. The battery light does comes on when the key is on, though. The inertia switch is not tripped. The fuel pump relay and PCM relay have been bench tested and they are good. Both relays audibly click if removed and plugged back in with the ignition on. The PCM fuse is good (in fact, all fuses in the PDB and under the dash are good). The fuel pump will run when connected to an alternate 12v source. The truck will stumble for a few seconds if starting fluid is sprayed into the throttle body but even with the fuel pump constantly running off an alternate power source the truck won't run. (I suspect the PCM is not firing the injectors). Is there any other diagnostic tests I can run before buying a new PCM?



Thanks

 

You have tested everything pretty well

1994 4.0l uses a separate ICM(ignition control module) called EDIS-6 for spark, it doesn't need PCM for start up so engine should start if fuel is added manually, like you have found
On my 1994 the EDIS-6 is on the FRONT side of rad support, opposite battery, you can see it looking up from under bumper
Not your problem just mentioning location

EEC-IV seen here: Ford EEC-IV

The Blue Capacitors often leak after 20 years or so, that can corrode the circuit board causing problems
Pop the top on the PCM(computer) and have a look inside, it is out of warranty, lol

MIL(CEL) gets power(12v) with Key on and the PCM pin 17 Grounds it to light it up, like it grounds fuel pump relay to power the fuel pump

Pin 1 on PCM should have 12volts all the time, thats from the PCM(EEC) fuse, it's for KAM(keep alive memory), like radios have for pre-sets and clock

Pins 37 and 57 are the Power up 12volts from the PCM(EEC) relay, both pin 1 and these pins share the same fuse, but pin1 is on the power side of relay

You can test this wiring after pulling out PCM just to be sure, but yes, it does read like PCM has packed it in

Good testing

By the engine fuse box you will see the VIP(OBD) test port
Drawing here: http://www.therangerstation.com/tech...ry/OBD_I.shtml

The slot for fuel pump is labelled in drawing
If you GROUND this slot, it will cause fuel pump relay to close when key is on, it is connected to same wire(pin 22) that PCM uses to ground relay to turn fuel pump on
PCM does not, should not, ground fuel pump relay for more than 2 seconds when engine is not running, it is a safety feature, each time PCM boots up it will ground pin22 but only for 2 seconds.
You can also use this test port to see if you can get PCM into test mode so it flashes CEL

 

I pulled the pcm and opened up the case hoping to see or smell something out of spec but it looks just fine. I'm not fantastic with electrical circuitry but would like to keep testing before I by a new pcm. I need to go back and read what you posted a few more times before I understand but I think you've given me some good info. I was able to get the fuel pump to run by grounding the slot in the test connector. Any suggestions on what I should check next? Is there a way to bench test the pcm somehow?

 

Check all the Ground wires for the PCM, they are not shared internally, so one bad ground wire could disable fuel pump and injectors, since they both use ground to operate

So with PCM connector off test each wire in that connector for 12v(key on) wires 37 and 57 and Connection to Ground, 16, 40, 60

wire 22 should show 12v with key on, thats the 12volt that comes from EEC relay and travels thru fuel pump relay to PCM, when PCM grounds that wire fuel pump relay closes

Also test 17 with connector hooked up to PCM, it should be a Ground if PCM is booting up
So connect meter to 12v and pin 17, turn on key and you should see 12v on meter, or use a test light, it should light up like CEL bulb when PCM starts to boot up

 

Thanks for the guidance. Is there a pinout available somewhere? I searched here and TRS forums and couldn't find one.

are you saying pins 37,57 are hot and 16,40 and 60 should all be ground?

 

I posted it earlier post: Ford EEC-IV

 

That is immensely helpful, thanks. Not sure how I missed that previously.

 

i started poking around and noticed I was not getting any ground at the pins I should be, nor am I getting any power at the pins I should be. Then I started studying this pinout and realized it must be wrong. Some of the "not used" slots in the diagram are clearly being used in my truck. For example, I have a ground at a pin that is not even used according to this diagram. I need to keep looking and find another pinout.

 

Ok, I found what appears to be the correct pinout here:

Part 1 -1994 Ford 4.0L Explorer And Ranger PCM Pin Out Chart

im getting ground at pins 2, 6, 8, 10, 15, 16, 20, 40, and 60. I find it odd that pin 10 (ac compressor) is grounded, but maybe that's because the compressor locked up last month and I haven't fixed that yet, lol.

My my concern is that I'm *not* getting ground at pin 49. This chart labels this simply as "ground" without any other description. I'm also missing ground at pins 9 and 46, but I am not as concerned with these as I suspect they only get grounds under certain circumstances.

Is it a problem that I do not have ground at pin 49?

thanks for your insight!

 

You won't have a ground at pins 9, 46 and 49, they are the sensors Reference Ground not a vehicle ground

Pin 10 is for the WOT(wide open throttle) Relay, it turns off AC Compressor when vehicle needs full power for passing or ??, i.e. driver presses gas pedal to the floor(WOT) and PCM cuts power, via WOT Relay, to AC Compressor
With key on you may see 12v at 10, but they changed WOT wiring over the years

Yes the diagram I posted is the basic pin out, it has the grounds and power pins noted, nothing extra.
While it is good to have all the pins listed when troubleshooting a specific issue, for your needs you just want to see if computer is booting up, if not then wasted effort on other things, although always good to learn new stuff

If you get power with key on(pins 37 and 57), and grounds are good, then CEL(pin 17) should go to Ground if computer is booting up, check for that
In previous post you said CEL wasn't coming on, but were not sure if it was bulb issue, so test for boot up

If you want to test the CEL Bulb, turn key on, bulb gets 12v via dash wiring, and then Ground pin 17 wire, thats what computer does on boot up to turn on CEL
Or test 17 for 12v with key on, if filament in bulb is good you will get 12v(a little less), if burned out then no volts

 

Perfect, thanks for the detail. It looks like I am most definitely getting power and ground to the pcm. Just to me sure I understand correctly, it is safe to back probe pin 17 using a test light with the pcm plugged in and key on? I just want to make certain I don't fry the pcm by applying power through the test light somehow.

Based on on my findings I would venture to guess that I will not get ground there as the pcm is not booting up. Which should be a pretty definitive sign that the pcm is bad.

 

Yes.
You can test pin 17 for 12v with key on, also Ground pin 17 and CEL should come on, if bulb is good.

With 60 wire connector in place and key on pin 17 should light up test light if test light has 12v on other end, or show 12v on meter in same set up
So safe to test that way

Here is the 1994 instrument cluster wiring
Malfunction light indicator is the CEL
It gets 12v from B12
Then B8 runs to pin 17, so if grounded CEL comes on
This wire also runs to Data Link connector, thats the same OBD fitting you jumped to ground fuel pump

 

i can ground pin 17 (well, the green/pink wire that runs to the obd connector) and get the CEL to come on. I think that plus knowing I have good power and ground to the pcm at the harness is pretty definitive that the computer is toast. Thsnks so much for the diagnostic help. I'll post an update after I get the new pcm in.

 

Good work on the troubleshooting

 

I bought a used pcm from eBay for $89. Plugged it in and the truck started right up. Huge thanks to RonD for all the guidance.

 

You did all the work

Good job and thanks for the update and kudos

 

While the truck runs great, my turn signals and tail lights don't work. Does the pcm have anything to do with those circuits? I can't imagine it does, I'll have to start checking fuses and such I suppose.

 

You are correct, PCM doesn't have any connection to exterior lights

1994 light Wiring diagram below

 

D'oh. Found an unplugged wiring harness under the bed. Must've pulled loose when I moved the bed to replace the fuel pump (which is what I originally thought was wrong). All good now.

 

You da man

 

Update part 2. I put about 30 miles on the truck this weekend and all was well. Drove it to work this morning no problem. I'm sitting in traffic about a block away from work on my way home and it dies, and won't restart. Fuel pump does not come on when I cycle the key. Check engine light does come on though. It will also sputter for a second like it wants to start if I let it sit 5-10 minutes. I'm leaving the truck here tonight and will mess with it tomorrow. In a weird coincidence I suspect the fuel pump actually did go out this time. When I ground the fuel pump test port of the diagnostic connector the fuel pump relay clicks but the pump does not run. Maybe I was too rough with it when I took it out to bench test it a few weeks ago?

 

What a bummer

You could try inertia switch reset but yes, does read like fuel pump is bad or it's wire got disconnected

 

I had the same issue with my 94 2.3 Ranger and was in the same spot. Your PCM diagnostics were invaluable,thank you.