Hello All -
I got a question the local Ford dealers can't answer, three different
dealers = three different answers.
1995 Explorer Question. If you don't crank the engine the but leave the
switch on, the fuel pump will stop after 2-3 seconds (normal
condition). What terminates pump? Is it a timed event controlled by the
PCM or is the PCM responding to a pressure transducer or some other
event? This is not a relay question, I realize the relay starts and
stops the pump, but under what condition does the PCM send the off
command to the relay?
This is a timed event on your car.... the time is closer to 1.5 to 2
seconds. At key on, the PCM is a busy little boy.... he peaks at the BARO,
the ECT, the IAT and a few other little things as well as turning the fuel
pump on briefly to charge the fuel rail (the spray pattern of the injectors
relies, to a great degree, on the pressure drop the fuel experiences across
the injector orofice). I firmly believe, but haven't proved it beyond a
doubt, that some strategies will also pulse ALL the injectors briefly to
purge any air or vapours. This strategy may also be temperature dependent.
Once the PCM "sees" rpm signal (PCM strategy *may* dictate a minimum value)
it will once again turn on the fuel pump relay.
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.
Jim Warman wrote:
I'll have to defer to BYMs prognosis... To my detriment, I have seen this
concern discussed before but, since I so very rarely get to work on
something this "old", I don't ever pay that much attention to the
That the fuel pump stays running yet the car refuses to start, this thought
has a great deal of merit. Look at it this way... if we remove the relay and
short the load terminals so that the pump runs continuously, this should
have no affect on starting or running. If, however, the PCM is seeing a
spurious rpm signal, it will command the fuel pump "on" and, at the same
time, be incapable of supplying spark at the correct time (if at all)..
The TFI will not cause the problem. But it can cause the PCM to not see a
crank signal wich would then cause the PCM to not turn on the pump. YOu are
not verifing the fuel system by reling on your ears use a guage to check if
the fuel pump is running or not.
Ok, I'll bite. How does a TFI module that has only a connection to the PCM
for ignition via a square wave voltage and one return square wave for timing
purposes keep a relay that is turned on and off by a PCM keep said relay on?
That square wave is called the PIP signal. No pip, no crank signal to relay
ground command from the PCM. It's that simple. Do not make it more than it
Not speaking for BM but I have seen that before. The problem wasn't the TFI
module but the ignition PU. This was back when the SBDS first came out and I
was looking at the inputs at the 60 pin connector. The fuel pump was running
and the injectors and coil were firing which flooded the engine. IIRC the
PIP signal was indicating several thousand RPMs even though it wasn't
turning over at all. Anyway a new pickup solved the problem.
Of coarse a 95 Explorer doesn't have a TFI and if it was getting an errant
signal from the CPS I would thing injectors should be clicking also.
NOTE: This does not obviously apply to the problem at hand...
You're getting on my nerves. The pickup is a hall-effect device, the
thing is electronic. People who DEAL with causal failure would not find
the premise impossible... happens all the time, though rarely; And that
is NOT a contradiction. Think 'millions'
You dont know WHO 'Bob' is, You dont know that he isnt a pro in the
shop...yet you call him out with a statement that any experienced
electronic tech would find amusing.
You dont know who I am. You dont know my background, yet you dismiss me
out of hand.
That is risky territory
Yet your diagnostic step -measuring voltage at FPR- ignores the event
chain you yourself described. Not to mention suggesting the OP check his
This is a problem solving group, not a teaching group. It's all good
that you are able to point out mis-perceptions... fine!
But we dont dont need lectures on how it works.. or step by step analysis
By the way, the last time we had someone on here who dealt with the group
the way you do, it took years but he was finally harassed off the group
by constant complaints to his various ISP's
in fact this is getting to smell a lot like...
Yeh, I'm a Krusty old Geezer, putting up with my 'smartass' is the price
you pay..DEAL with it!
I don't care if I am getting on your nerves. That hall device will not do
what he described period. The permanent magnet does not alternate if
polarity and can not shunt it self, why do you think there is a vane wheel
between the semiconducting material and magnet? So no it does not happen
all the time. Hall devices do not fail in that manner.
No I don't know who Bob is, and yes I called him out. I believe his
statement is a fabrication. Any experienced tech would.
I don't care your background. I've given you every opportunity to clarify,
explain your statement. Why should I take your word as gospel. I don't know
you from jack shit. My training and experiance tells me what I called you
out on is crap. If it's not, again, explaine your self.
The information I gave the OP tells him how he will find out for sure with
out any doubts as to what is the reason the pump stays running. PCM or
relay. A simple check that will take less than 5 minuets. Far better that
disconnecting connectors on parts you know nothing about. Do you check air
pressure in tires by letting it out or do you use a gauge? The answer and
reason should be obvious.
Go right ahead, complain all you want, I've done nothing wrong. I will come
as go as I want. Now with out your patented self important crap like "you
don't know who I am" Explain how that hall device will keep the relay on.
You feel it will, why, I want to know.
Well you're the one who called ME out and said it's not possible. You prove
it... Keep in mind probable and possible are not the same. I never said it
was probable, only that I had seen it once in the last 20 years. I've
repaired a LOT of cars in that time. If it was a common failure I'd have
seen it a lot more than once.
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