Fuel Pump Resistor

[FuturaGuy]

Went on a scrounging mission to the local junkyard (correction: "automotive recycler") the other day to get the mating half of the oval shaped 4-pin connector used on the fuel pump/sender on '84-'86 LTD's. It's also used on other Fords including Tauruses from '86 thru '95 and some Tempos. I got one off an unidentified Ford tank in the yard's storage shed. When I got the connector home and cleaned it up a bit, I found there was electrical continuity, with 1800 ohms resistance, between the POWER and GROUND sockets. None of my Ford wiring diagrams show a resistor in parallel with the pump motor. I think it might be there to prevent the motor from over-revving at no load condition but I'm guessing. Does anyone know why that resistor is there and whether or not I'll need it since I replaced the low-pressure Ford pump with a high-pressure Camaro pump?

[Chuck W]

Replace that ground wire(it's a resistor wire) with a standard length of wire to a good chassis ground. the resistor wire is not needed (or desired) once you switch to the single in-tank pump.

[LTDScott]

Hmm I didn't replace any wire on mine when I swapped in the Camaro pump, but my approach was not as scientific as yours.

[Chuck W]

On my 83 Tbird (originally dual pump set-up) did have that resistor wire. When I installed the single 255 pump in the tank I did away with that wire and just grounded it to the chassis with a "clean" ground wire.

[FuturaGuy]

I got some more info on the resistor in the fuel pump circuit and replacing the low pressure pump with a pump from a Camaro. The service department at the local Chevy dealer printed out the portion of the wiring diagram showing the fuel pump circuit and Chevy doesn't use a resistor in parallel with the pump. I also called Airtex, who makes the pump, and talked to one of the engineers there. He told me only Ford uses a resistor in parallel in the pump circuit and only on certain models. Airtex tests their pump motors at no load and they don't over-rev. Bottom line is if you're using the Chevy pump in your installation, you can cut off the resistor and wire the pump without it.

I wouldn't call this being "scientific", but just good engineering. I only want to do this job twice, once on paper and once for real.

[LTDScott]

Well now you've got me wondering if my car does have that resistor or not.

[Chuck W]

Well, it's not a "resistor" but a 1 Ohm resistor wire...
If the car has the dual pump set-up, it will have this resistor wire in the ground wire for the in-tank pump.

[LTDScott]

I'll just c/p the response I made to Futuraguy here....

I had read about that resistor on turboford.org when people were talking about converting old SVOs and GTs to a single pump. But I never gave it anymore thought. My car seems to run just fine, and I think the Camaro pump is a bit overkill for my car, so maybe the resistor may work in my favor.

Although when I dynoed the car, it was a little lean at WOT...

I'm assuming the only way I can check for the presence of the resistor is to drop the tank and measure the resistance between the negative terminal of the pump connector and ground?

As if I don't already have enough things to worry about with that car.

[Chuck W]

So..are you saying this Camaro pump has a resistor built into the pump itself? :?
Was your car a dual pump car to begin with?

[FoxChassis]

Yeah, the 5.0L CFI LTD and Mustang had/have a pump in the tank and a pump on the frame rail. Others probably did too.

[LTDScott]

So..are you saying this Camaro pump has a resistor built into the pump itself? :?
Was your car a dual pump car to begin with?

No, I'm not saying that at all. I'm saying that if my car has the resistor wire (yes, it was originally a 2 pump car), then it still seems to be working well enough with the Camaro pump.

I was sort of thinking out loud that since the Camaro pump is 255lph, it's overkill for my engine, so maybe the resistor wire might work in my favor by slightly lowering the pump output. But that's purely speculation.

[Chuck W]

OK, that's what I thought. Then yes, you are probably running that pump at less than full voltage if you did not bypass the resistor wire.

[FuturaGuy]

I hate to disagree with you, guys, but the resistor in parallel will not drop the voltage to the pump or cause it to pump less gas. It would if it were in series with the pump but that's not the case.

The only way I know to tell whether you have the resistor wire, other than looking at it, is to drop the tank, pull the conector off the pump and measure the resistance between the power and ground sockets. At the same time you could turn on the power and measure the voltage across those same terminals. This is starting to resemble hard work.

We still don't have an answer to my question at the beginning of this thread. What was the resistor there for? Once it's there, it doesn't affect the Camaro pump. Like Chuck, I just clipped the resistor off.

[Chuck W]

The resistor wire is in series with the pump.....it's the ground wire.

Power comes into the pump and exits to ground through this resistor wire...the front high pressure pump does not have the resistor wire for a ground like the lp in-tank pump does....

The only thing I can think of is we are talking about two separate things...

[LTDScott]

The resistor wire is in series with the pump.....it's the ground wire.

Power comes into the pump and exits to ground through this resistor wire...the front high pressure pump does not have the resistor wire for a ground like the lp in-tank pump does....

The only thing I can think of is we are talking about two separate things...

I'm glad you mentioned that. Futuraguy has been PMing me about this, and I wasn't quite getting exactly what he was talking about because it was my understanding that the the resistor is in series because it IS the ground wire.

I'm no electrical whiz, but if that's the case, you'd want to measure the resistance between the negative terminal on the connector and the ground itself. When Futuraguy mentioned having to check the resistance between power and ground terminals on the connector, I was lost. But it seems like we're not on the same page here.

And if the resistor IS the ground wire for the pump, what is the net result of having the resistor there? I would think it'd be reduced pump output, but again I'm no electrical whiz ;)

[Chuck W]

The net effect is reduced voltage to the intank pump.

If you look at the wiring diagrams, the front pump has a clean ground and the in-tank pump has the resistor wire ground. Both pumps are fed from the same 12V source and voltage is going to take the path of least resistance to ground. The pumps are actually wired in parallel. 12V in-splits to each pump-back together to ground. One branch of the circuit has a resistor in it, the other does not.

Anyway, if you are leaning out I bet you are not getting full volatge to the pump. I bet you can remedy it w/o dropping the tank, but I'm not 100% familiar with the LTDs. I know on my 83 Tbird, the wiring actually goes to the tank from the trunk and I was able to deal with the wiring from there (my intercept of the ground wire was after the grommet inside the trunk)

[LTDScott]

Well I'm not really leaning out. A/F ratio is 13.7, but I'd prefer a little richer.

http://www.dynoperformance.com/jpgraph/graph_af.php?dynorun_id=939&width=680&height=450

Where exactly is the resistor part of the wire? On my LTD, the wiring does indeed come in the trunk floor, but if the resistor portion is between the tank connector and the trunk floor, I would have to drop the tank.

[LTDScott]

Well I'm not really leaning out. A/F ratio is 13.7, but I'd prefer a little richer.

http://www.dynoperformance.com/jpgraph/graph_af.php?dynorun_id=939&width=680&height=450

Where exactly is the resistor part of the wire? On my LTD, the wiring does indeed come in the trunk floor, but if the resistor portion is between the tank connector and the trunk floor, I would have to drop the tank.

[Chuck W]

Not 100% certain.

Strip off some of the tape around it. It will say "Resistor wire" or something of the like on it and the insulation will feel a little different.
I'd have to look closer when I get home. I think it's back closer to where the ground wire attaches to the chassis, so I just cut mine just after the grommet and spliced in a new wire.
I just pulled all the excess wiring out of my car a couple weeks ago, including finally pulling the old wiring for the front pump, so most of it's not there any more on my car.

[Chuck W]

Not 100% certain.

Strip off some of the tape around it. It will say "Resistor wire" or something of the like on it and the insulation will feel a little different.
I'd have to look closer when I get home. I think it's back closer to where the ground wire attaches to the chassis, so I just cut mine just after the grommet and spliced in a new wire.
I just pulled all the excess wiring out of my car a couple weeks ago, including finally pulling the old wiring for the front pump, so most of it's not there any more on my car.

[ponydude]

Will it hurt to leave the frame rail pump when using a 255 in the tank?

[LTDScott]

Will it hurt to leave the frame rail pump when using a 255 in the tank?

I'm not sure why you'd do that. Most people convert to the single pump setup to gain extra room for a second muffler for dual exhaust. Leaving the frame pump would defeat the purpose.

If you're looking to put a higher flowing pump on the car and don't mind leaving the frame mounted pump there, you can get a high pressure version of the frame pump from Walbro.