shorted PCM? Fuel pump continues to run

Poppy · 12-04-2011, 03:34 PM

My 1990 bronco with a 302 EFI isn't running correctly.

Today I drove it just 1/2 mile, and she started bogging, so I brought it back home. The CEL was flashing... staying on solidly, went off, and the truck ran fine, then it started flashing again, and then stayed on.

I pulled codes and got a 56 and 69 they are transmission codes.

I figured I would start her up and let her warm up so that I can properly pull codes, and noticed that the fuel pump did not shut off after a couple of seconds.

I checked the data link self test connector and the Tan/Green wire that when grounded triggers the fuel pump relay. That wire was shorted to ground. I pulled the relay, and still shorted to ground. I pulled the connector to the PCM, and the short cleared.

I'm thinking bad PCM. Could it be anything else?

Poppy · 12-05-2011, 05:41 AM

Here is the wiring diagram for the 1990 EEC and Fuel pump relays.

1990 Ford Bronco 1990 Bronco EVTM picture | SuperMotors.net

Pecos Slick · 12-05-2011, 06:15 AM

Check the EEC power relay, the contacts inside may be stuck together.

Poppy · 12-05-2011, 07:39 AM

OK.. I pulled the EEC relay, and checked the trigger side of the fuel pump relay, and the short to ground is still there.

This is probably coincidental, but I noticed that my alarm is not working. I can't set it with the remote, nor is the remote start, or remote door locks, (the door locks do work with the door switches). I never carry the remote, so it is possible that this has been this way for a year.

Dominick · 12-05-2011, 04:43 PM

Did you check your ignition switch and or wiring gor a short?

Poppy · 12-06-2011, 07:43 AM

Unless someone has an easier test, I plan to pull the EEC and see if pin 22 of the EEC (the grounding pin for the fuel pump relay) has continuity with any of the other grounding pins. If it does, I'll assume that the EEC is shorted out, and that the problem is inherent to the EEC.

There is a problem with that logic however, it is based on the assumption that the default is for the grounding circuit to that pin is OPEN, and that the PCM closes that circuit to send a grounding signal when appropriate.

819fva · 12-06-2011, 09:11 AM

Check the wiring diagram of your alarm system. Is it an after market system? The alarm system may have relays also. Did this alarm stop working the same time as the fuel pump issue? Very rare the ecm shorts out.

Poppy · 12-06-2011, 01:46 PM

Sorry... I don't know anything about the alarm system.
Also, since I never really use the alarm, I have no idea when it stopped working.

I believe that it is more of a starter interrupt system than anything else, but I understand that it is hard to ruel that out without additional information.

Dominick · 12-06-2011, 02:56 PM

Poppy · 12-07-2011, 05:09 AM

Thanks Dom,
You come up with great pictures!

Poppy · 12-11-2011, 12:46 PM

I finally got around to pulling the EEC in my Bronco. It came out through the cab. I had to pull the driver's side kick panel out of the way, and then only one screw held it in place. 10mm socket at the connector to disconnect it in the engine bay.

I took it apart, and don't see anything obvious, I might be able to take it to an electronics hobby shop for the guy there to take a look-see at it on Monday.

I checked pin 22 Fuel Pump Enable output to grounds 20 40 and 60. To pins 40 and 60 it had continuity at 101 K ohms. Then when I checked pin 22 to pin 20 it acted like I was charging a capacitor or something. When I put the pos and neg contacts to the pins it would read -3.00 k ohms, and then it would gradually increase to about 30K ohms, and then become an open circuit. If I re-tested it would read open circuit. If I reversed the polarity it would follow the same pattern... -3.0 ohms and increase to +30K ohms. then open circuit.

My guess is that is how the EEC charges the fuel pump relay for a couple of seconds and then kills the ground unless it recieves a PIP signal.

OK.. so I went one step further and found that pins 40 and 60 act the same way as pin 22 when tested against pin 20

I'm guessing that this is the way that the EEC is supposed to act.

I didn't check the case ground in the truck, I'll really kick my self in the ass if that is all that it was.

Poppy · 12-11-2011, 03:41 PM

I vaguely recall that to test an electric motor start capacitor that you check it with a volt meter, and watch the numbers go up, and then reverse polarity and watch it happen again., OR maybe the numbers start high and go to zero. At any rate I figured that is what is happening when I tested pinn 22 to ground 20 (case ground). That is probably the timer/delay that allows it to be grounded for the first second or two. Until the EEC/PCM senses a PIP signal and continues to ground it. Also it seems that when pin 22 is grounded by the PCM the other grounds 40 and 60 get grounded. so the other sensors that rely on that get their grounds.

OK... I took another look at the EEC today, and may have found the problem.
The entire EEC circuit board is covered in a rubbery feeling, maybe greese. There is a little access panel in the back of the EEC module case that has a multi circuit port where a tech can access the EEC with some kind of scan tool. It seems that some of that greasy rubberoid material may have rubbed off, and MAYBE it was shorting at the case at that point. I wrapped it in electrical tape, put it back together and installed it.

I loosened the EEC ground to the truck and retightened it, it looked ok.

So I started it her up and she idled nicely for 20 minutes, no CEL.
I just took her for a short 2 mile ride to get some snacks.
She drove beautifully!

Yes... the fuel pump only primes for a second or two and shuts off like it is suppossed to. Yahoo!!!

Here are pictures of the EEC and my tape job.
http://www.fordforumsonline.com/foru...o-eec-pcm.html

Without an EVTM, troubleshooting something like this would have been impossible for me. One of the best $20 investments I ever made for that truck.