Fuel Sending Unit Problem? - Diesel Forum - TheDieselStop.com
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99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the 99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

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Old 10-26-2009, 12:14 PM   #1 (permalink)
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Fuel Sending Unit Problem?

In my 99 Ford 7.3 the fuel gauge on my dash is stuck on Full. When I turn the key off, it goes to empty, but during driving and when the key is on it is all the way to full. Is this my sending unit or in the panel? Ford quoted me around $500 for a new sending unit.
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Old 10-26-2009, 05:26 PM   #2 (permalink)
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Check the wiring back to the tank. If it's broken, the fuel gauge will read full. The sender is a resistance deal, with 160 ohms being full and 15 ohms empty, so a disconnected sender reads full when you turn the key on.

If you can get at the sending uint plug, measuere the resistance across it and see what you come up with.

It's also possible that the float got stuck up against the top of the tank. If it happened when you filled up, and have run some of the fuel out, crawl underneath with a rubber mallet and whack the dickens out of the tank. That may dislodge a stuck float. Worked for me the first two times I had a problem (4x4 launches on the drag strip without a completely full tank).
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Old 10-26-2009, 05:47 PM   #3 (permalink)
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I am going to try beating on it and see if that does anything. I think I would have to take the tank out to actually get to the sending unit to test it.

It was actually on empty, I left town for 2 weeks and came home and it was on full.
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Old 10-26-2009, 06:07 PM   #4 (permalink)
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You shouldn't have to buy a whole sending unit to fix your problem. I think you can take out sending unit and replace the the resistor unit. I know on the older trucks, it was held on to the sending unit with a couple of screws. I did this on my brother-in-law's truck and got the piece from Ford.
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Old 10-27-2009, 12:06 AM   #5 (permalink)
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I am going to try beating on it and see if that does anything. I think I would have to take the tank out to actually get to the sending unit to test it.

It was actually on empty, I left town for 2 weeks and came home and it was on full.

You don't think someone was kind enough to have filled er up while you were out of town?

I dunno if there is a connector somewhere between the tank and the dash, but if there is, that is another place you can ohm out the resistance of the sending unit. I "think" if the wire is gounded anywhere it will show full. Also, does the guage show "super full" - like its pegged above full, or something else?
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Old 10-27-2009, 10:23 AM   #6 (permalink)
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My 2002 F350 did that once after I fueled up, but my gauge only showed half a tank. I disconnected both sets of battery cables for 15 minutes and the gauge reset it self. Good luck.

Last edited by blue1ton; 10-27-2009 at 10:26 AM.
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Old 10-27-2009, 04:19 PM   #7 (permalink)
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I "think" if the wire is gounded anywhere it will show full.
Actually an open circuit anywhere between the sending unit and the instrument panel will cause a full reading.

As I recall, the connector on the fuel tank can be disconnected with the tank in place and pulled over far enough to probe with a multimeter. The only trouble is you need 2 elbows in each arm to reach up there, and you have to use the braille method to see what you're doing.

I've never actually tried it, but have had the tank down a number of times and there's a fair amount of slack in the wiring.
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Old 10-27-2009, 10:54 PM   #8 (permalink)
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"Open Circuit"? Does that mean a break in the wire, or a wire that is touching ground? Or simply that the circuit is not complete, "through" the sending unit?
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Old 10-27-2009, 11:01 PM   #9 (permalink)
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any neighborhood kids and a water hose around while you were gone. A friend of mine kids played "gas station" one day using water to pump "gas"
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Old 10-28-2009, 09:52 AM   #10 (permalink)
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Same problem with my sending unit - been ignoring it for a long time (use the trip odometer for miles driven)....but I need my 'quick reference level' back for heavy towing.

I'll check the wiring, but I'll run the tank low and drop it that way I can check the float as well.
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Old 10-28-2009, 10:44 AM   #11 (permalink)
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On a lot of trucks it is easier to unbolt the bed and raise it than remove the tank.
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Old 10-28-2009, 07:12 PM   #12 (permalink)
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"Open Circuit"? Does that mean a break in the wire, or a wire that is touching ground? Or simply that the circuit is not complete, "through" the sending unit?
A short is a wire touching something it shouldn't. An open is a break in the wiring, or an incomplete circuit.

On the sending unit low resistance is empty tank and high reistance is full tank, so an open circuit (infinite resistance) is "really full".
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