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Old 08-08-05, 02:37 PM   #1
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Other than old/faulty wiring, what causes low voltage to fuel pump?

So I THINK ::knocks on wood:: that I may have found my start-rev-die problem.

Today I went out and tested the voltage at the fuel pump and was horrified at how awful it was! With the test connector jumpered and the key to on, the pump was barely seeing a constant 5 volts! When cranking, voltage was anywhere from 3.8-5volts (a time when there should be the full 12v to the pump, if I understand correctly).

So, looks like I need to do the fuel-pump rewire. My question is this, is there anything else besides old/bad wiring that would cause such poor voltage? I don't want to run down the wrong alley yet again.
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Old 08-08-05, 04:45 PM   #2
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Assuming you're getting full voltage at the battery, it's just a matter of something in between the battery and the fuel pump not passing the current without a lot of resistance.

Off the top of my head, some places to check: The fuel pump relay (under the dash), the voltage dropping resistor pack (not sure where it is, I'm a NA guy), and other places you can come up with. Also check the ground - I'm not sure where the fuel pump is grounded, but a bad ground will cause trouble as well. Did you measure the voltage across the terminals, or from hot to (a different) ground? This will give you an idea as to if there's a ground problem.

Really, as long as there's enough voltage at the battery, the only thing that can cause bad voltage at a device is a bad connection somewhere.

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Old 08-08-05, 05:43 PM   #3
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Syonyk is exactly right. I have found the voltage at cranking at the fuel pump to be around 9.5 to 10.5 volts while cranking and more like 12v when running. I don't think you will get twelve while cranking with the starting pulling so many amps.....how many volts does she show while running? The fuel pump relay is under the dash on the right side of the steering column...you could try unplugging and replugging the connector there...sometimes connectors can build resistance especially if they have gotten wet with something.
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Old 08-08-05, 11:57 PM   #4
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Thanks guys.

I checked the resistor pack and it checks out as per the FSM.

Good call on checking to ground as well as across the terminals, I'll have to try that.

Harley, I don't know what the running voltage is since the car won't stay running, it will just start, rev up, and die :P
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Old 08-09-05, 02:12 AM   #5
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if im not mistaken dont the turbo cars have a relay that switched the fuel pump from high to low voltage?

or is that the resistor pack you guys are talking about?
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Old 08-09-05, 04:28 AM   #6
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capn - Yes TII have that it is the resistor pack.

MountainTurbo - I highly doubt the issue of reving up and dieing is you fuel pump. Maybe a factor but I dont feel it is the pump. Look at AFM.
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Old 08-09-05, 06:05 AM   #7
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The TII resistor pack drops the voltage, but it should only drop it to 9v or so at idle, not 3.5v.

An extremely low fuel pump voltage can certainly cause a "Start/rev/quit" condition. It gets enough fuel to fire, but not enough to keep running (the start procedure is kind of a bunch of good guesses on fuel, and it's pretty tolerant - rotaries flood easily because it uses so much damn fuel).

You can't check the voltage with the engine running, but you can certainly check the steady state voltage of the fuel pump. Jumper the fuel pump test connector, turn the key on, and check the voltage at the pump while it's running (the test connector, similar to it's name, lets you run the fuel pump with the engine not actually turning, for testing things like this, priming the fuel system after it's been opened to air for work, and other assorted things).

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Old 08-09-05, 09:48 AM   #8
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Did you checl the fuel pressure first? I would check that before I do the rewire.

Also, if you really think this is your problem and you do not want to check the fuel pressure...it is not that hard to run a thick *** wire from the battery to the fuel pump positive.

However, you can make a fuel pressure tester for 10 bucks at home depot.
Fuel Pressure tester...cheap!
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Old 08-09-05, 10:22 AM   #9
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The TII switching resistor drops the pump voltage at CRUISE also...
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Old 08-09-05, 10:50 AM   #10
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Quote:
Originally Posted by Wankel7
Did you checl the fuel pressure first? I would check that before I do the rewire.

Also, if you really think this is your problem and you do not want to check the fuel pressure...it is not that hard to run a thick *** wire from the battery to the fuel pump positive.

However, you can make a fuel pressure tester for 10 bucks at home depot.
Fuel Pressure tester...cheap!
Yah, I was going to do that, but I didn't want to spend 40 bucks on a checker I was going to use once. So your idea for a 10 buck checker is perfect!

The only thing I think is going to be an issue is if the pump is only getting 5 volts, it's probably not supplying full pressure.

Quote:
You can't check the voltage with the engine running, but you can certainly check the steady state voltage of the fuel pump. Jumper the fuel pump test connector, turn the key on, and check the voltage at the pump while it's running (the test connector, similar to it's name, lets you run the fuel pump with the engine not actually turning, for testing things like this, priming the fuel system after it's been opened to air for work, and other assorted things).
Yup, that's how I tested it, as well as testing while cranking. With the test connector jumpered, the voltage to the pump was only a constant 5v. The battery is fully charged, showing ~12.5volts.
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Old 08-09-05, 11:11 AM   #11
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So, since you have spare time, buy a spool of 18-22 gauge wire and run it from the battery to the Blue wire on the fuel pump connector and see what happens when you try to start the engine.

I'd make the ignition key or the Circuit Opening Relay suspects since you say the voltage is five or so volts with the fuel pump check connector jumpered.

You MIGHT pull the ENGINE fuse out and turn the key to ON. Then put your meter lead on the side of the fuse that feeds the ENGINE fuse and see if the voltage is five or so volts there. IF it is, then I say ignition switch problem OR black wire to the ignintion switch problem.

Personally, with what you now know about the low voltage at the pump, I'd buy that roll of wire and hard wire it and start the engine. I'm talking about just running the wire outside of the car if you have to. Not a permanent install. Just to get this mystery over with.
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Old 08-10-05, 01:42 PM   #12
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Ok, so I might be back to the drawing board.

I bought everything to do a fuel pump rewire, but before installing I decided to check voltages one more time. I disconnected the fuel pump, jumpered the check connection, and tested voltage at the fuel pump connector. 11.96volts. D'oh.

So, I reconnected the fuel pump, re-jumpered the connection, tested again (with fuel pump now connected and so obviously running). Voltage was 6v....

So, am I just checking it wrong? Is it normal for the voltage to drop to 6v when checking with the pump running? From what I recall, it should still show 12v if that's what the pump is getting, right?

So is it possible my fuel pump is fragged?
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Old 08-10-05, 01:45 PM   #13
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You've got a bad connection somewhere. Voltage will only drop as a result of resistance if there's current flow.

I won't go into a full basic electric circuits primer here, but it shouldn't be dropping below 10-11v (at worst) while running. Rewire it.

-=Russ=-
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Old 08-10-05, 01:49 PM   #14
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So even though the fuel pump connector is getting 12v, and only when the pump is connected it drops down to 6, I should still re-wire it?
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Old 08-10-05, 02:27 PM   #15
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If you are going to make big power.....you rewire without asking

It is a realibility thing. And constant voltage is nice for tuning since it could affect fuel pressure if it gets uber low in voltage.

James
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Old 08-10-05, 02:51 PM   #16
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There's a bad joint somewhere producing too much resistance. It only drops voltage when there's current flowing (that's how resistors work, they turn energy into heat). If it's dropping to 6v when running, there's a problem.

You could troubleshoot it with a multimeter at various points when the fuel pump is turning, or you could just rewire the damn thing.

Just rewire it already!

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Old 08-10-05, 03:24 PM   #17
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Quote:
Originally Posted by MountainTurbo
So even though the fuel pump connector is getting 12v, and only when the pump is connected it drops down to 6, I should still re-wire it?
When the plug is connected to the pump, and the pump is dragging the voltage down to 6vdc, I'd take the meter at THAT time, and go to the battery and make sure the voltage at the battery is still around 12v and not down to the same 6v your seeing at the pump plug. I'm saying check the batt voltage at the same time the pump is running.

Personally I'd just run a direct wire to the pump from the battery and have at it. If the voltage still drops to the 6 and the battery voltage is about 12v at the same time, then suspect the pump or a bad ground.

Just in case: at the fuel pump plug, its the LARGE BLUE wire that is the input on the harness side of the connector and it, the LARGE BLUE, mates with a Black/White on the pump side of the connector. Just in case you were measuring the fuel gauge wire. I doubt you were. Just in case though.

Yeah, just rewire it, either permanently or for trial purposes. End the mystery.
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Old 08-11-05, 01:14 PM   #18
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Quote:
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When the plug is connected to the pump, and the pump is dragging the voltage down to 6vdc, I'd take the meter at THAT time, and go to the battery and make sure the voltage at the battery is still around 12v and not down to the same 6v your seeing at the pump plug. I'm saying check the batt voltage at the same time the pump is running.

Personally I'd just run a direct wire to the pump from the battery and have at it. If the voltage still drops to the 6 and the battery voltage is about 12v at the same time, then suspect the pump or a bad ground.

Just in case: at the fuel pump plug, its the LARGE BLUE wire that is the input on the harness side of the connector and it, the LARGE BLUE, mates with a Black/White on the pump side of the connector. Just in case you were measuring the fuel gauge wire. I doubt you were. Just in case though.

Yeah, just rewire it, either permanently or for trial purposes. End the mystery.
K, with the pump running, the battery is still at 12v.

Sooo.....

Ran a wire outside the car from the battery and spliced it in. Re-connected the negative battery cable, instantly I could hear a big difference in the fuel pump sound. Turn the key, car fires.... revs... dies. Dang.

Notice a giant puddle under the car, smells like gas... D'oh. So, fixed that leak, tried it again....


The car runs! It has to be babysat a lot, I have to keep light pressure on the gas pedal to keep the revs up. It doesn't sound quite right though, lots of breakup, pretty lumpy. When I can get a buddy over here I'll have him check timing and for vacuum leaks, but I'm getting closer!!!

It's really weird though, if I press the pedal all the way down, the car sounds like it just shuts off completely, and it dies. If I release the pedal before the revs drop completely, the car will catch and run some more. Any ideas on what would cause that?
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Old 08-11-05, 01:19 PM   #19
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Timing, TPS, Water thermo sensor, adjust idle.
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Old 08-11-05, 03:28 PM   #20
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That must be some really honked up original wiring for the pump.

I think I'd go to the Fuel PUmp Relay and Resistor in the right front of the car and pull the plug off it. Turn the key to ON and check the voltage at the Blue/Red wire. It comes from the Circuit Opening RElay. There are two Blue/Red. They are really one and the same. Should be 12v or batt voltage. There's no Load with the plug off so this check isn't bullet proof.

There's a better place to check. Well better from one way of viewing and worse from another. There is a connector b/t the Front and the Rear harness. It's called X-13 in the 87 manual and is located SOMEWHERE b/t the interior fuse box and the front left door jam. I've never seen it....never looked for it myself. Pull that one apart and measure the voltage on the two BLUE wires on the Front harness side of the connector, with the key to ON.

You need that jumper in the fuel pump check connector to do both of the above.

Just a note: I don't think that I could fiddle around looking for the wiring problem if I'd been working on trying to get my engine to run for the last month or so. I'd probably half *** things with the wiring so I could get the engine up and running right. I'm NOT saying that is the right way to conduct thiings, it's just the realization that I wouldn't be able to constrain myself. (sorta humor)
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Old 08-11-05, 03:48 PM   #21
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I may be wrong here HAILERS but he has NA right? There is no fuel pump resistor pack on NAs. Right? The TII's have one behinde passenger head light.
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Old 08-12-05, 01:57 PM   #22
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Actually ice, it is a TII, sorry if I didn't specify before.

Hailers, I do have 12V at the fuel pump connector with the fuel pump disconnected, so wouldn't it be the same 12v at all those other places if they were disconnected? It's only with the load of the pump that the voltage drops.

Question: If my buddy installed the CAS drive gear backwards, would my timing be off 180degrees? Could that cause the car to die fairly instantly when you floor it?
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Old 08-12-05, 09:16 PM   #23
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Well then I was *** backwords this whol thread :-D
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Old 08-12-05, 09:16 PM
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