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Fuel Pump Power


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25 replies to this topic

#1 CornerHard

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Posted 24 February 2008 - 03:15 AM

A bit of a dumb question, but I haven't found the answer yet by scouring the FSM or searching around here: Where does the power wire to the fuel pump come from, and how is it wired? Today my fuel pump stopped working, and testing voltage at the fuel pump showed that it would get just a tiny blip when starting to crank the engine, then go back to 0V. Does the power come directly out of the Fuel Pump Control Unit?

#2 MilesFox

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Posted 24 February 2008 - 07:29 AM

there is a relay above the steering column directly above the ecu. the BROWN one

it will trip with a signal from the ecu caused by a pulse from the distributor.

if you plug in the green test (static timing) clips the pump/relay will cycle onand off every 2 seconds.

if the test clips prove to work, then you need to look at the distributor, mainly to see if it is turning or not(broken timing belt)

turnong the key to the on position will kick on the pump for a second until it sees a signal from the disty to keep going

it is possible the fuel pump is getting voltage but is not grounding. the ground side ties in withthe rest of the body ground. you can jump a new wire directly to the body if it is necessary

#3 Skip

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Posted 24 February 2008 - 08:33 AM

It is a total guess not knowing what car/year/fuel system

could be a rev sensor (control unit)
could be a relay
could be??

The system most prevelent is a relay
controled by the ignition switch.

The ground being provided by the ECU

You say "control unit", I don't think you have an ECU.

?????


Why do so many people these days
think we know what car their working on??


#4 brokebill

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Posted 24 February 2008 - 09:53 AM

[quote name='Skip']Why do so many people these days
think we know what car their working on??
______________________________

you mean youre not omnipotent?!

#5 Skip

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Posted 24 February 2008 - 10:00 AM

No Bill it's not that.

At my school of higher learning,
Mind Reading 101
will not be offered until next semister.

;^)

#6 brokebill

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Posted 24 February 2008 - 05:21 PM

just dont let our incompetence sour you!

Be a lot of us walking within a month!

#7 Cougar

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Posted 24 February 2008 - 05:46 PM

I assume the OP is working on his '83 Soob which is stated in his profile area. It is easier when they state what it is in the first part of the post though.

I made a reply to this in post #25 of another thread.

http://www.ultimates...?t=82917&page=3

#8 CornerHard

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Posted 24 February 2008 - 08:01 PM

Sorry, it is indeed the 83 GL wagon listed in my profile, and no, I don't have a timing belt :) I thought the FPCU was a black box above and behind the hood release?

#9 CornerHard

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Posted 24 February 2008 - 09:42 PM

Ok, I traced the fuel pump wiring back to the FPCU, and it was having the same issue at the source - the voltage blips to about 3v when you start cranking the engine, then it goes back to 0v. I tried wiring the black power wire from that connector directly to the fuel pump wire, and with 12v, the fuel pump started working normally again. I assume this means my FPCU is bad?

#10 Cougar

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Posted 24 February 2008 - 09:59 PM

It does sound like it is bad.

#11 GeneralDisorder

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Posted 24 February 2008 - 10:28 PM

Could still be the ignition switch as well. Sometimes they have a "sweet spot" that if you go past voltage will stop flowing. Also could be ignition related - if you aren't getting spark, then there will be no tach signal, and the FPCU will not power the pump.

GD

#12 Cougar

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Posted 24 February 2008 - 11:20 PM

Checking for voltage at the coil will tell the story there.

#13 newsoobdude

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Posted 25 February 2008 - 12:07 AM

At my school of higher learning,
Mind Reading 101
will not be offered until next semister.

;^)

Yeah, I've failed that class more times than not yet everyone at work thinks I gradiated with honors in it :lol: Good info to know everybody, unfortunately, I have no input though on the situation, just knowledge gained.

#14 CornerHard

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Posted 25 February 2008 - 02:47 AM

Thanks for all the tips guys (and GeneralDisorder - I'm amazed that you always have something useful to say in every one of these troubleshooting threads I've posted on :))

I have 12v at the coil, but I'm not getting any sort of tach signal or spark. I just got my distributor back from having it rebuilt at Philbin, and now it doesn't seem to be working :confused: Before it would work, but the car just wouldn't run. Now I have these fuel pump problems as well :(

I was under the impression the fuel pump should get full voltage if I'm cranking, regardless of the tach signal, though?

#15 Cougar

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Posted 25 February 2008 - 04:22 AM

The REV sensor will shut down the pump if there isn't any tach pulses. You now need to see if the pickup in the disty is working. Also make sure then is voltage on the minus side of the coil.

#16 CornerHard

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Posted 27 February 2008 - 12:37 PM

It looks like that's what it is - if I turn the ignition to "on", then take the black and yellow harness wires from the ignition coil and tap them together to create a fake RPM signal, my fuel pump and tach start working again.

#17 Cougar

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Posted 27 February 2008 - 12:54 PM

Good sluthing CornerHard. I wonder what happened to the disty while it was in the shop? Are you sure you got the same unit back?

#18 CornerHard

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Posted 28 February 2008 - 01:21 AM

Thanks for the help, Cougar. I suppose I don't really know if it's the same one or not, since they rebuilt just about everything in it.

I'm trying to understand what's going on here, though:

The wires coming from the car harness:

Black wire (power supply): +12v and almost perfect continuity with ground
Yellow wire (tach signal): No voltage or ground

The wires coming from the distributor:

Black wire (+ terminal): some continuity with ground
Yellow wire (- terminal): slightly less continuity with ground


The ignition coil fires when there's a change in the current flowing through the two terminals. The distributor either causes flow through the coil periodically or interrupts flow through the coil periodically - I've heard conflicting reports as to which way it works. What I'm wondering is, why is the power supply wire also a ground? It seems like almost all the current is just going to flow through that circuit and not go through the coil at all. Is the power supply wire supposed to be a ground? What changes about the distributor wires as the rotor goes around? Could I wire a straight 12v to the positive terminal or is that where the external resistor is needed?

#19 Cougar

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Posted 28 February 2008 - 10:54 PM

The coil generates a high voltage when the current running through it is interrupted. The sudden stop of current flow makes the coil's electric field collapse and the high voltage is produced.

I think you have a reluctor type circuit in your disty. If it was working before had it worked on then the shop should fix this for you. The replacement parts are expensive.

#20 CornerHard

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Posted 29 February 2008 - 01:06 AM

Yep, the shop warranties their work, so I may end up sending it back. So the yellow wire on the distributor is the one that has its ground interrupted by the reluctor, correct? That would cause the current to stop flowing through the ignition coil.

#21 Cougar

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Posted 29 February 2008 - 10:33 AM

Yes, I think that is correct. One other thing, in my previous post it may have been more correct to say the magnetic field of the coil collapses instead of electric field. Though the two are related to each other. (Where's OB99W when you need him?)

#22 WoodsWagon

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Posted 01 March 2008 - 05:15 PM

DId the engine sputter and die, and choke alonge a bit before it stopped, or did it just die? If the timing belt snaps, the disty won't get turned, the rpm signal won't get sent, and the pul pump won't get turned on. My dad was chasing a fuel pump problem , had all the diagrams printied out and laminated, and spent lots of time on it before he gave up on the car. I fixed the broken T-belt, and the fuel pump issue remedied its self.

#23 GeneralDisorder

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Posted 01 March 2008 - 06:49 PM

I fixed the broken T-belt, and the fuel pump issue remedied its self.


That's nice and all, but EA81 land doesn't have timing belts.

GD

#24 CornerHard

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Posted 17 March 2008 - 02:15 PM

In my case the fuel pump wasn't working because the distributor wasn't sending an RPM signal. Apparently, the car won't even prime the fuel filter unless there's at least some signal. Once I got the distributor working, everything was good again.

#25 GeneralDisorder

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Posted 17 March 2008 - 07:35 PM

In my case the fuel pump wasn't working because the distributor wasn't sending an RPM signal. Apparently, the car won't even prime the fuel filter unless there's at least some signal. Once I got the distributor working, everything was good again.


It should prime for about 1.5 seconds after key-on regardless of tach signal. Of course that's not long enough to get much fuel flowing to the filter - but the pump should still run.

incidentally - what was wrong with the distributor? Was it something the shop did?

GD




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