Fuel gauge PEGGED at or above max - Wiring or sending unit in tank?

I am trying to get all loose ends tied up on my older trucks with the state of the world. Parts seem increasingly hard to find and and I want everything fully functional. I plan to take it to the shop for some other things and might let them tackle this but was wondering if this issue was something easy or not. I figure it might be a wiring issue.

Anyway, the truck is a 1999 F250 Super Duty 5.4L. I have posted about other issues before and this one was more of a nuisance issue so wasn’t at the top of the list. The fuel gauge showed pegged at max or above max like the voltage is at the full end of its range. I am guessing it is either a ground somewhere or broken/loose connection. What are some easy things to check or would the best guess be the sending unit in the tank?

you might have a sending unit that is bad.

From what I found the sending unit has low resistance when full, high when empty, so being out of range on the full side might be caused by a short to ground in the wire to the sensor.

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So the answer is yes it could be either.

To get at the sending unit, you will need to get a new gasket, plus there is the hassle of removing the bed or dropping the tank. I’d start checking it externally first.

With power off, i.e. key out of the ignition, if you can reach the connector on top of the fuel tank, then disconnect it. Check the hot lead on the connector wire to ground. If low resistance, then the problem is external.

Turn the ignition to run, but do not start. You should get 12v on the hot wire and the fuel gauge should read empty. If so, go ahead and get the gasket cause you’re going inside.

The float arm may be stuck not allowing it to drop like it should. You may only have to bend it a little.

Take your dashboard off? I did, for a different problem, could have tested the gas gauge and its signal.

Gas gauges generally work on the same principle as oil pressure and temperature gauges.
The needle pegged out means low or no resistance in the gauge circuit as mentioned by texases.

It’s best if possible to access the pump connector or possibly a harness disconnect if fitted with that. If the needle drops to E with the pump disconnected the problem is likely the sending unit.

I had no desire to pull the bed when the pump failed in my Sonoma a few years ago. I printed off a pic of a Sonoma gas tank on eBay and did some measuring of the tank and the truck using common reference points.
Used a Sharpie to mark the spot and cut an 8 x 10 inch hole in the bed. Right on the money; what are the odds. Replaced the pump and used a heavy piano hinge on the bed rectangle/lid. A bit of RTV and with the bed liner no one knows. Any potential future pump problem is a fast, easy fix. Remove the bed liner, flip the lid up, and done in minutes.

Did the same thing with my Lincolns. Changing the pump is a 10 minute job after cutting an access hole under the rear seat.

If you decide to replace the sending unit and have to drop the tank, I might go ahead and replace the fuel pump too, assuming it’s never been replaced. Sending unit and pump may be sold together as an assembly anyway. I’ve dropped tanks on a couple of different vehicles to replace the pump. Not a real big deal, in my opinion, but I’m not in a part of the country where they salt the roads.

I’d start by disconnecting the external connector to the sender and see if the gauge stays low.

I got along just fine without a gas gauge. In my Riviera when I had a new tank put in they screwed up my sender. A new one was north of $300 so I said forget it. It triggered the low fuel light which was on continually. I just reset the computer when I filled up so I knew how many miles I went and remaining miles to empty. Never ran out.

Not sure if newer cars use a 7 volt regulator on the fuel guage but it was relatively common for those to fail on older cars.

Did that with my Focus. 100% worth the effort to cut the hatch, granted I only needed to change the pump once (at somewhere between 185-205k miles)

Thanks. I plan to disconnect the harness at the pump and see if this changes anything first. If not I am going to just take it into the shop and have them change the pump and sending unit. If all that labor has to be done, I am not going to NOT replace a pump that likely has 200,000 miles on it. Everything will be replaced including the gasket, sender, and pump. I think the sender is part of the pump assembly on these as well.

I know it’s frowned upon to take your own parts to a shop for them to install them…but if I didn’t furnish the parts myself, I’d at least ask them what brand pump they’ll be putting in there. This is not in an effort to save money on parts, though. Just the opposite. I wouldn’t want them putting a parts store brand pump in there in an effort to lower the cost of the job. I’d want a pump from an oem supplier.

Isn’t that backwards? If nothing changes that could mean the problem is a short to ground in the wiring. I’d then check for that.

I was thinking the same thing. I trust the shop and will ask them about this before the work starts, if I have them do it. I have always heard you want to use OEM pumps unless it is an easy job and you don’t mind getting stranded.