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1987 - 1996 F150 & Larger F-Series Trucks 1987 - 1996 Ford F-150, F-250, F-350 and larger pickups - including the 1997 heavy-duty F250/F350+ trucks

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Old 03-30-2013, 02:35 PM
joel_93f250 joel_93f250 is offline
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1993 F250 bad PCM? Symptoms

I am new to these forums and have a question regarding my 1993 F250. My truck has always ran like a top. My problems started about a week ago. I was driving and suddenly lost power (as though my truck was in limp mode). The check engine light turned on, engine sputtered (some exhaust smoke which smelled heavily of gas), transmission would not shift out of whatever gear it was in. I limped the truck back home and checked for codes. I tried jumping the connectors and hooking up my obd1 Ford code reader. nada.. No codes. The reader just showed zeros and would not perform any engine tests etc. (even with a check engine light) The next morning I start the truck and it runs, drives, and shifts perfect. About 10 min into my drive same sequence of events happens.

Also, either of my fuel pumps will run continuously when the key is in the on position. The transmission also shifts hard into park/drive (when it would start). I purchased a used PCM on ebay and replaced and truck now just cranks and will only fire for a second of two before stalling. It seems as though this problem gradually got worse. The fuel pump will not run with the PCM disconnected. I am beginning to wonder if the second PCM was good.
Any thoughts or input would be greatly appreciated.
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Old 03-30-2013, 03:54 PM
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Open up the first PCM and see if the Caps are leaking. It sounds like bad Caps.
You might be able to solder in some new ones and it should be good to go.

The second might be bad or not for your truck.
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Old 03-31-2013, 10:04 PM
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Thanks for the advice. I have opened and inspected both PCMs and they both appear to be fine inside. They also both have matching codes T2X1. Would there be anything else that could cause these symptoms or should I try for a third PCM?

Could a bad PCM / EEC relay cause these problems?
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Old 03-31-2013, 10:52 PM
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Yes it indeed could (and has been known) to cause issues before. It sounds like classic symptoms of a failing computer though. Possibly a failing TFI and/or PIP module too, though.
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Old 03-31-2013, 11:42 PM
joel_93f250 joel_93f250 is offline
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Thanks for the reply.
Interesting- I had not thought of TFI and PIP module.

I am curious which of the following could cause my fuel pump to run continuously with the key in the on position..
bad TFI/PIP module, EEC relay, or bad PCM.

Not being able to check or receive any codes while my check engine light is on also baffles me.
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Old 04-01-2013, 12:47 AM
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Bad PCM really. It's possible a bad ECU relay could make the computer do something weird and enable the fuel pump output.
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Old 04-01-2013, 09:30 AM
DPDISXR4Ti DPDISXR4Ti is offline
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Quote:
Originally Posted by joel_93f250 View Post
I am curious which of the following could cause my fuel pump to run continuously with the key in the on position..
bad TFI/PIP module, EEC relay, or bad PCM.
EEC relay or PCM. I'd try the relay first since it's easiest to swap - I always have a spare in the glovebox.
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Old 04-01-2013, 09:43 AM
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I would change the PCM Computer first as you know that it is bad and the relay could not do that.
Do not wast money on changing the wrong parts.

With check engine light on (MIL) you know the PCM Power relay is OK and sending power to the Computer and the coil of the fuel pump relay.
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Old 04-01-2013, 10:09 AM
DPDISXR4Ti DPDISXR4Ti is offline
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I would change the PCM Computer first as you know that it is bad and the relay could not do that.
I've had a bad relay cause exactly the symptom he is experiencing (fuel pump runs continuously) so I wouldn't eliminate it entirely as a possible cause.

That said, I agree that the additional symptoms point to the PCM as the culprit. My suggestion was based on the premise that he should have a spare PCM relay in the glovebox regardless.
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Old 04-01-2013, 10:17 AM
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The PCM relay only supplies power to the coil of the fuel pump relay. It can not turn the fuel pump off and on. It can keep the fuel pump from running if it does not supply power to the coil of the fuel pump relay with the key on. But then nothing else would run either.

A stuck closed Fuel pump relay can keep the fuel pump running with the key off.
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Old 04-01-2013, 11:11 AM
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Quote:
Originally Posted by subford View Post
The PCM relay only supplies power to the coil of the fuel pump relay. It can not turn the fuel pump off and on. It can keep the fuel pump from running if it does not supply power to the coil of the fuel pump relay with the key on. But then nothing else would run either.

A stuck closed Fuel pump relay can keep the fuel pump running with the key off.
A chattering PCM relay, or one that does not make a complete connection can prevent the processor in the computer from starting up. If the processor doesn't start, the fuel pump keeps running.

The relays cost next to nothing, and it's a good place to start rather than dropping $150 on a new computer.
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Old 04-01-2013, 11:24 AM
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Being it is a 1993 all he has to do is swap the horn relay for the PCM power relay and the marker relay for the fuel pump relay.

Click the image to open in full size.

/
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Old 04-03-2013, 02:05 AM
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Thanks guys for all your input.

I did try swapping the regulators out (pcm and horn) and the fuel pump continues to run non-stop with the key in the on position. Both computers I have appear completely fine inside and they both have the same exact tuning code.

I feel like the fuel pump running non-stop, check engine light (with no readable codes), transmission stuck in gear, and now truck won't start should all lead to one problem. Anymore advice or anything else that could cause these problems?
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Old 04-03-2013, 07:27 AM
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Everything you post points to a bad computer.

You say the fuel pump does not run with the computer unplugged. So you know it is the computer turning on the fuel pump. You also know from this that the computer has a good ground. The only way the computer would be not at fault is if the computer was being reset over and over with the key in the run position. I have seen this with a 5.0L engine with bad wiring to the O2 sensor but I do not know what engine you have.

Even if the fuel pump relay was stuck closed making the fuel pump run all the time this would not stop it from starting and running.

Beside you know the fuel pump relay is working and the fuel pump relay coil wire is not shorted to ground or the fuel pump would not quit when you turn off the key.
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83 F150 Flare Side 351W AOD
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Old 04-03-2013, 11:59 AM
DPDISXR4Ti DPDISXR4Ti is offline
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Both computers I have appear completely fine inside
But have you tried running both of them?
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Old 04-03-2013, 11:59 AM
 
 
 
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