Fuel gauge troubleshooting

Fuel gauge read fine for a couple months.

One week ago, ran over big pothole, fuel gauge read a 1/4 less than it should (still read accurately despite being in the wrong place)

Today. Low on fuel, around a 1/4 (real reading), go to refuel. Check gauge after filling, shows 1/2 tank.

Same situation, gauge reads accurately but the needle is just at 1/2.

My question:
Wiring issue?
Gauge issue?
Sender issue?

I hope it's wiring... don't want to replace that sender...

The original style of fuel sender units fail at the retention bolt which holds the inner bracketry to the round mounting disk. It is just the weight of the fuel sloshing against the float and working on the mounting area. Once that gets loose, readings are erratic. If you change to the later style fuel senders that are a aluminum tube with a float that slides within it, you will not be sorry. You problem might be elsewhere, but I don't think so and this is a problem area for these vehicles.

dilvoy said:

The original style of fuel sender units fail at the retention bolt which holds the inner bracketry to the round mounting disk. It is just the weight of the fuel sloshing against the float and working on the mounting area. Once that gets loose, readings are erratic. If you change to the later style fuel senders that are a aluminum tube with a float that slides within it, you will not be sorry. You problem might be elsewhere, but I don't think so and this is a problem area for these vehicles.

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As far as I can tell the gauge still reads the consumption of fuel correctly. It's just where the needle starts.

Like if I could move the needle back up to the full position with just the internals of the gauge, it would be fixed.
Kinda like a recalibration of a speedometer.

I had mine in question......I crawled under my machine late at night with a real bright flashlight and shined it at the tank, I could see the level of the fuel move in the tank as I shook the truck......my gauge read full and sure enough so was the physical level in the tank......just a thought on how to roughly check your gauge's reading......

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erasedhammer said:

As far as I can tell the gauge still reads the consumption of fuel correctly. It's just where the needle starts.

Like if I could move the needle back up to the full position with just the internals of the gauge, it would be fixed.
Kinda like a recalibration of a speedometer.

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OK, Look through your TM's and find the trouble shooting section. It gives easy to follow step by step instructions for finding the common problems on the Humvee.

If you unplug the wire 28A from the gauge it should read full scale to the Full side. If you connect a ground to the 28A port on the gauge it should read below empty.

You need continuity on the cables and good clean grounds. Other than grounding issues, the old float arm style senders are usually the problem.

dilvoy said:

The original style of fuel sender units fail at the retention bolt which holds the inner bracketry to the round mounting disk. It is just the weight of the fuel sloshing against the float and working on the mounting area. Once that gets loose, readings are erratic. If you change to the later style fuel senders that are a aluminum tube with a float that slides within it, you will not be sorry. You problem might be elsewhere, but I don't think so and this is a problem area for these vehicles.

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Interesting... I didn't know this was a thing and just looked it up (part 5716625 for anyone curious).

Is this style sender less susceptible to fuel tank sloshing than the float style? The bouncing fuel gauge when driving has been one of my pet peeves on my truck. Stupid, I know, but spending the $90 for this style sender to get it to stop would be worth it to me.

(Thankfully I've got the access door so it's no big deal to swap.)

Bump.

Anyone know about the performance of the new style sender?

mine has never bounced with the tube style sender.

Action said:

mine has never bounced with the tube style sender.

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Thanks Action! That's just what I was hoping to hear.

Adding one to the to-do list so I can finally have that stop bugging me.

Hey guys I just changed my sender ( the floaty white one that looks cheap fro. ebay) and right after I packed it back Iits barely coming above E after puttong a good 12 gals in it.
Unfortunately I foundout my generator betz are snapped so cant fill it up until I replace that but is that right? Did I flip the wires at the sender side?
It did read full when it was disconnected.
PS i dont have access hatch so its a 2 hours job

Mine started bouncing around on its own (truck not bouncing) and yesterday just went to reading full all the time.
I assume poor connection or float failure.

Do not know if I have the newer or older style unit. 1988 model truck.

New style sending unit and access cover.

I have the hole in the floor. Popped it open, wiggled the wires, all is good now.

For anyone looking for an alternative, theses magnetic reed switch senders from the marine/industrial world are perfect. https://rochestergauges.com/product/9800-marine/

I use one in my Deuce and one for a remote gauge on my diesel tank.

papakb said:

Ah, the good old American smack it with a hammer fix! Second only to the All American coat hangar antenna!! LOL

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No the American way is to find a way to use flex tape or flex seal...I fell a few steps short of that! ��

pjwest03 said:

For anyone looking for an alternative, theses magnetic reed switch senders from the marine/industrial world are perfect. https://rochestergauges.com/product/9800-marine/

I use one in my Deuce and one for a remote gauge on my diesel tank.

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This is the only type fuel level sender used in the power generation world now. Its very good. In the beginning, we had many problems with it, as the military was using JP4 fuel. The fuel ate up the floats. After that got fixed, the switch started to have problems with the contacts. The opening and closing of the contacts caused a tiny spark at the contacts. The contacts at some point in time welded itself together. After that was fixed, life was good. About the only problem I see with them now, is that if there is any way metal filings, or chips can get into the tank, (and yes I have seen this!) the floats tend to pick it up. The float rod is normally adjustable. Loosen the nut on top, and shove the tube down to the bottom of the tank. Pull it up a tad, and lock it down. That's where it can pick up metal, as its close to the tank bottom. If the meter fluctuates, of moves strangely, its probably the tube too far down in the tank. The vibrations of driving on the road will make it shake. I highly recommend this type of float switch.

pjwest03 said:

For anyone looking for an alternative, theses magnetic reed switch senders from the marine/industrial world are perfect. https://rochestergauges.com/product/9800-marine/

I use one in my Deuce and one for a remote gauge on my diesel tank.

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I really like this option. I recently purchased a new fuel sender since my gauge doesn't work. At the same time I planned on replacing the fuel tank. The noticeable difference is the new style has a baffle around the sender. I think this one would be more likely to vary the gauge reading with the fuel sloshing around. It would be pretty easy to build a baffle that would prevent this. I'm pretty sure this will be an upcoming project on my HMMWV thread. Thanks

UPDATE. More information about the newer style HMMWV sender, it is also a magnetic sender. I posted more about it here https://www.steelsoldiers.com/showt...M998-Avenger&p=2209303&viewfull=1#post2209303