1995 Ford Explorer Fuel Pump Control

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Funny you should refer to him.
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Okay... Contrary to your statements, neither the CPS or Hall pickup in the distributor are 'dumb or passive circuits'
They are solid state, multi-element, as is CERTAINLY the TFI. And they ALL likely have signal shaping circuitry within.
Which work by means of positive feedback, IOW 'hysteresis'. The effect is that the output of the shaping circuit remains at a state until the input signal reaches a level high enough to overcome the feedback of the output, at which time the output switches and reinforces the input, then it remains at THAT state until the input drops low enough to overcome the feedback, again.
Now, the same circuit as takes in a ramped or sine signal and outputs a square wave can with little modification be made into an oscillator.
The modification is usually done by adding capacitance or a separate negative feedback element. and they obviously require no external signal to oscillate
Now, suppose that the housing/coating for one of those MODULES had cracked and moisture might have invaded... if it happened to contaminate at just the right area of the circuit, that might have altered the characteristics just enought that until, the substrate warmed, negative feedback -say from an adjacent circuit trace- would occur and the circuit would oscillate. Of course that is only hypothetical, but you can take this to any circuit designer, describe the scenario and i believe he'll vouch for the theory.
AGAIN: a very rare problem, right?
And i want to point out to you AGAIN that the OP implied that the FP Relay turned OFF when the key was turned off... else there would have been mention of the battery running down and difficulty in actually getting the FP to shut off.
So AT THE LEAST, the PCM Power relay controls the state of the FP relay.
Which means that, given the engine ALSO wont start at the coincident point, that something earlier in the chain must be holding the driver transistor on and at the same time, altering the known events that drive the logic so that the engine wont start.
Whether that is happening inside the PCM, itself, is unclear.. it could be... but it sure aint happening at the FP relay. And BTW.. when a driver transistor shorts, it's DONE FOR! and most circuits have no fail-safe that prevent the short from looking like a short when the main supply is off, remove power, it's still shorted. That's how you can find it with an ohmmeter.
Now.. let's look at the other assumptions. No Codes. What does that tell you? It tells me that the PCM doesnt know there's anything wrong. Which means the fault COULD still be in the PCM but narrows it down quite a bit.
Now... I'm done. if you dont understand my thinking, then go back to your Fords Tech Manuals and branching troubleshooting charts.
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wrote:

Yea they are. The hall device is nothing more than the typical hall device material, it can be a thin sheet of metal or a simiconductive material with three or four wires attached ( depending on application ) to it no resisters, no capacitors nothing. Some hall devices do have there own amplifers, this one does not. The TFI module does that. With out a magnetic field it does nothing. It is no more complicated than a resistor, your would not call a resistor complicated would you?

The TFI is a solid state circuit, but is as dumb as rocks. In it is an amplifier and pass through circuit for PIP ( the signal the hall makes which is very weak ) and a transistor and it's supporting circuits to ground the coil. It has no logic chip, and can not make decisions it is dumb.

The hall effect device does not work by means of positive feed back period.

On a different type of device, lets say a transistor? yea, but not a hall device. It does not work that way.

Read his second post.......again I'll cut and paste it again.
"Thanks Jim, I suspected that was the case since I didn't see a pressure input in the Chilton wiring diagrams and one of the three dealers also said it was a timed even as well. Now to the problem, on ocassion the engine won't start and I've noticed during those times the fuel pump runs continuous in key on position. After cycling the switch several times and I hear the pump stop then I can start the engine. Otherwise I can let the pump run 2-4 minutes and try repeated cranking without the engine starting. It appears the PCM is not completing its start sequence. Is there other test I can perform or should I consider the PCM history. I will add during the on/off switching the voltage is stable to the PCM and other indicator lights on the dash appear normal while in this continuous pump on mode. thanks again"
"Now to the problem, on ocassion the engine won't start and I've noticed during those times the fuel pump runs continuous in key on position."
The mention of running down the battery was as in your case an assumption.

No it does not. The driver may just may have failed or is failing. He may have other problems. You are assuming on false reasoning.

If that's how you check a transistor, so be it.

I didnt say the PCM IS not at fault, I even made several post saying it could be, I just wanted him to know for sure which is it the relay or PCM. I also stated it could be other problems.

I dont work for Ford or at a Ford dealereship.

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And there ya go... Havent there been one or two cases where the CPS was flaky?
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And as soon as no crank signal, PCM turns the pump off. Your point? The TFI will not cause the pump to continue to run period. A stuck relay will though...........
You have a problem with no fuel pump run when engine crank, I might go along with your little theory. But I'll bet you get some kind of ignition related code. Like 211 PIP circuit failer maybe a 212 or a 214. My statement stands. PCM sees PIP, PCM turns on relay. PIP goes open relay turned off.

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WHAT-EVER!
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Exactly. Your a little over your head, go back to the shallow end.

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..
Okay...I'll be looking forward to your ideas on how to help him.
BTW, how long have you been on here?
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When you turn the key to run the PCM readies it self to run the engine. It runs the fuel pump to make sure there is enough fuel pressure does a static check on the out puts and inputs and then waits for a crank signal. It is the PCM that turns on and off the relay.

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Ok, its a timed event controlled by the PCM. But the PCM, on ocassion, does not turn the pump off even before cranking. I know ahead time if the engine will start by listening to the pump. If the pump goes off as normal the engine will start when cranked. The question still is, what would keep the pump on beyond the 2 second period? It appears either the PCM is bad or some input is keeping the PCM from completing the timed sequence. It is a 4.0 with EI however before cranking, before I have a RPM or PIP signal the pump should turn off. Since it doesn't have a pressure input its not fuel related, the computer can't make it to the next step in its logic start sequence. What static input test are performed, would a high or low input on the O2, MAF, TP, etc. cause the logic lockup. I assumed a lot of folks had seen the problem and the answer would be apparent.
If we solve why the pump continues to recieve an on signal before cranking then we solve the problem.
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You are only assuming that. The only way you will know this is by using a volt meter. Put a voltmeter on the negative side of the primary circuit of the relay. If the PCM is keeping the relay grounded you will see a voltage drop ( a voltage reading of .05v or less ) if you see 12v the ground is open meaning the PCM is NOT keeping the relay grounded. Don't believe me? Do a google search on DC series circuits. You will find that it is true in all DC circuits that when the circuit is open source voltage will be present on all parts of the circuit up to the open. That will be before and after the load ( the negative wire or ground is after the load ). The PCM is the only device that turns on and off the relay, and it is unlikely the ground has got a short. The PCM only tests the continituity of the circuits it can see, the tests can not prevent the engine from running. Dont take running steps before you walk. You may have more than one problem.
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pedantic and F343532g irrelevant!
It is not the relay or the relay driver.
The coincidence of the engine not starting while the FP remains on... and the engine running fine in other cases call for the examination of INPUT circuits OR the possibility of internal logic failure in the PCM.
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Backyard Mechanic wrote:

not that at all. The PCM is toast. I would first check the grounds and powers to the PCM since you have a wiring diagram, but I would bet its the PCM. Very strange problem for sure.
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Ford PCM's rarely fail but I think you're probably right about this one being toast. Bob
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