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1987 - 1996 F150 & Larger F-Series Trucks 1987 - 1996 Ford F-150, F-250, F-350 and larger pickups - including the 1997 heavy-duty F250/F350+ trucks

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  #1  
Old 03-05-2012, 01:28 AM
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Rear fuel pump dead on 91 F250, cant figure out why

so i'm working on my 1991 Ford F250, 460,5 speed 4x4, the rear fuel tank need to be replaced due to leaking. I will admit, the pump did get run dry a few times, also front pump needed replaced in front tank. I got the new tank up and into the truck (haven't gotten to the front tank yet) but now i have no power at the rear fuel pump. I have power at the front fuel pump, i have taken a known good fuel pump and hooked it up to both the front pump plug in (it hums, as it should) and to the rear pump plug in (no hum, dead to the world) i noticed a decent amount of corrosion in the rear fuel tank plug, so i am currently replacing the plug. I have swapped the rocker switch in the dash for a known good switch, I'm running out of options. Wiring has never been my strong point, so help me out.

Thanks,
Lee
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Old 03-05-2012, 01:14 PM
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Time to break out the meter.

Fuel System diagram:
Click the image to open in full size.

See if you have voltage on the Brown/White wire that feeds the rear fuel pump at the switch, then again at the connector to the FDM.

You could have lost the return (ground) for the rear tank. It's the Orange wire that runs to ground G100, located in the engine compartment on driver side radiator support.
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Old 03-05-2012, 07:49 PM
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Egads, i hate wiring bugs like this ... can i get a step by step?

So, im going to put on the new connector ( cuz the old one is trashed) then i'll get out my test light (cuz it's what i have, but it has a 20 ft cord on it) and i'll check for power at the brown/white stripe wire (i've done this once, but i'll do it again)

after that, where do i need to check for power at??
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Old 03-05-2012, 08:02 PM
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Personally I do not think a test light is going to get you very far with this issue. I highly suggest a cheap digital volt meter (DVM). It will pay for itself in a VERY short time.

If you insist on using that *^&*&*#$%^ test light make sure you it lights when you have one end on chassis ground and brown/white wire at the rear fuel pump (FDM). Your tests so far indicate there is no voltage there. No way to use it to verify the integrity of ground G100.
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Old 03-05-2012, 08:11 PM
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ok, fair enough, im off to Sears to buy a digital multimeter tomorrow. ...

i'm a brake, suspension, exhaust guy at work, so bear with me... I can rebuild engines, manual transmissions, and differentials, but i have VERY LITTLE knowledge on electrical systems...

so, i'll drop a line with new multimeter in hand ...
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Old 03-05-2012, 08:14 PM
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Harbor Freight my friend..............Digital Multimeter - Save on this 7 Function Digital Multimeter

I am an electronics and network engineer by trade. I can do engines and differentials. Auto transmissions are a mystery to me
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Old 03-05-2012, 08:23 PM
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i got about $40 in sears gift cards ... im an hour away from any sort of civilization in any direction, and tomorrow im heading to a place with Sears but no Harbor Freight ...

I see you're from IL, what part are you from? i'm an hour straight east of East Dubuque ...
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Old 03-05-2012, 08:36 PM
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I hate to disagree with rla2005 but never use a digital volt meter (DVM) in the fuel circuit.
You will just chase your tail for hours on end. Always use a test light in the fuel circuits.
The digital volt meter (DVM) is needed for a lot of testing with your truck but not the fuel wiring. You need to load the circuit with an incandescent bulb type test light and not an LED type.

A digital volt meter (DVM) will show about 8-10 volts on the power wire when it is a dead circuit. This power comes from pin #8 of the computer and this voltage makes a lot of posters chase their tails for hours on end. A test light will not light on this voltage.
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Old 03-05-2012, 09:59 PM
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Bill, I was hoping you would chime in. I am not a fan of the test light. Yes I know an open circuit or open ground will usually trigger a phantom voltage reading on a DVM, but I can walk someone through that.

I see no way without a DVM to check the continuity of the ground return for the fuel pump without using a DVM. That is another reason why I profess using one versus a test light. I suppose if you jumpered the return at the FMD to ground and the pump turned on that would work. I am just more familiar with using a DVM or an ancient iron vane VOM for troubleshooting this kind of thing.

If you have a better procedure then please share it with us.
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Old 03-05-2012, 10:02 PM
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hooray! i have two gurus to help me make this old truck run!!!
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Old 03-05-2012, 10:11 PM
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Quote:
Originally Posted by Bootlegger's Deluxe View Post
hooray! i have two gurus to help me make this old truck run!!!
Do you have a mouse in your pocket? Otherwise there is only one real guru here and that is Bill. I'm just a guy who reads a lot
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Old 03-05-2012, 10:29 PM
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Well if he does not have power at the plug at the tank testing for a ground is not going to help.
We need to get power at the plug first. So ground pin #6 of the self-test plug and turn on the key with the rear tank selected. Take the test light with the long wires on it and put the ground on the battery NEG (-) Post. Check for power on the pump wire (Brown wire with a white stripe).
Then my way is divide and conquer, so if no power at the tank check at the selector switch for power on the rear tank pump wire.
If you have power there check at the safety wall plug, C202 and inline plug C133.
Let us know what you find.

Click the image to open in full size.

Click the image to open in full size.

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Old 03-06-2012, 07:55 AM
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Is the self test plug you are referring to the OBD 1 scan tool hook up? Is grounding it to a nearby fender sufficient?
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Old 03-06-2012, 08:23 AM
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Quote:
Originally Posted by Bootlegger's Deluxe View Post
Is the self test plug you are referring to the OBD 1 scan tool hook up? Is grounding it to a nearby fender sufficient?
Yes it is the same plug that you hook the OBD 1 scan tool into. You need to ground it to a good ground. You may have lost your body ground to the that fender so I would use the block or the Battery NEG post. If the pumps run OK that way with the outside ground then jump pin #2 to pin #6 of the self-test plug with the key on to test the ground through the computer to the battery.
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Old 03-06-2012, 01:34 PM
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See, Bill has all the photos that is why he is the guru
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