Gas gauge woes

[raymakr]

Hi Everybody,

Does anyone have a miracle cure to get a GPW Jeep gas gauge to read correctly? I installed a USA made gauge, after sanding the back edge and both sides of the dash and the area where the support bracket attaches. To make sure it was grounded I added a second ground strap attached to the sending unit base and Jeep body. The bond strap for the gas gauge is attached to bare metal. I swept the sending unit arm
outside of the tank to see if the two were compatible and the needle moved just fine. With the key on, the gauge needle pegs to full, a sign of poor ground. That or possible short in the wiring. On a short drive, the gauge fogged up. Maybe my fault for sanding the rear edge.
On gauge number two, non USA made, again swept the gauge outside the tank and needle moved just fine. With sending unit installed gauge 2 needle went to half full but the tank is completely full. The fact that the gauge needle moved over makes me think there is no short in the wiring.
I bent the JMP sending unit arm down upon advice and it made no difference. I have no idea who made the tank. It came from Mikes Motors in Santa Rosa. It came with a shorter sending unit that looked like a CJ unit. It only ever registered three quarters full before dying.
Not really comfortable doing a blind sweep inside the tank for obstacles.
Would I be better off buying the new JMP gas tank, or try a different sending unit? Sorry for the long posting. Thanks in advance.

[Bill H.]

Yes, get a new sending unit from JMP. Ron has them. There were a lot of junk sending units from years past.

['42 Peep's Motorpool]

Hi Ray,

I bought a gauge that would peg out to read 'FULL' - a sure sign of a poor ground in a gauge like this (assuming it has the original type internals). In my case after extensive testing I concluded the problem was inside the gauge. Knowing that I'd be voiding any return policy, I chose to open it and fix it myself. The problem was as I suspected, the internal ground from gauge insert to the outer case was interrupted (in my case, there was no connection). I installed a 'jumper' to solve the problem, but opening a case doesn't mean it can be fixed, and it will certainly cause at least some damage to the bezel ring.

No idea what gauge you have, but it may be a similar fix. Many gauges (original and new) seem to have grounding issues, regardless of source.

But first test the sending unit, they are easy to test!

You can see a little about my gauge halfway down on my restoration thread. viewtopic.php?f=144&t=322802&start=180

Good luck!

['42 Peep's Motorpool]

Yes, get a new sending unit from JMP. Ron has them. There were a lot of junk sending units from years past.

Yes, the JMP sending units are great!

[YLG80]

+1 on Peep’s advice. When I restore a fuel gauge, I tin solder a ground wire inside the housing.
Yves

[raymakr]

The sending unit I am using is a new Joe's Motor Pool unit. I think my issue is with the tank. As both gauges worked
outside the tank. the needle moved from empty to full with full swing of sending unit. Any one have any advice on who sell's the best gas tank? I am thinking about the JMP tank. Doe anyone know of a shorter sending unit that would fit the WW2 Jeep gas tank. Thanks for the responses.

[bryanbdp]

You might have to bend the sender to fit tank, it's near the center baffle. Mine seemed to stick on empty, but as I drive it loosens up and reads properly.

I soldered a wire to the outside of my gage and ran it to ground.

[JAB]

The sending unit I am using is a new Joe's Motor Pool unit. I think my issue is with the tank. As both gauges worked
outside the tank. the needle moved from empty to full with full swing of sending unit. Any one have any advice on who sell's the best gas tank? I am thinking about the JMP tank. Doe anyone know of a shorter sending unit that would fit the WW2 Jeep gas tank. Thanks for the responses.

Is the tank grounded? In the correct factory configuration the metal fuel line from the tank to the fuel filter is the ground. If you cut the line to add a filter with rubber hoses then you cut the ground path. The tank straps might make an occasional ground, but not reliable.

And yeah...I ordered a new JMP gas sending unit from Ron yesterday because my brand new 25 year old still in the box unit from Tiawan works perfectly.... until it's installed. I think the leverage on the float (it's gripped toward the end) twists the arm and lifts it off the resistor wires when it's installed. Like yours, it works perfectly on the bench. I drilled out the rivets on another old one I had and bent the arm to put more pressure on the resistor wires but it gave very jerky readings on the gauge as I think it caught on the wires. Pop rivets were all it needed to go back together. I finally gave up and ordered the JMP.

[raymakr]

Thanks Jeff,
Yes the metal line from the tank to the fuel filter is solid. The bond strap to the firewall is present and though its not tinned the paint is removed so the strap goes to bare metal. I have installed a second ground strap from the base of the sender unit to a footman loop screw. I do have the felt strips on the bottom of the tank. I do know the second ground strap works as the second gas gauge needle did not move without this second ground strap. I think I will be bending the sending arm to the left hopefully away from the center baffle, If that does not work then I think I will replace this older gas tank. I know Ron sells the JMP tanks and I hear good things about them. Maybe this will be the solution. Thanks for your input.

[JAB]

Why would a new tank change things? Unless maybe the float is hanging up on the baffle? You could try bending the arm just enough to clear the baffle. Another thing; Mine was fluctuating from full to empty so I thought maybe I was pinching and shorting the tank wire to ground under the seat or that the circuit breaker was bad and (or being over taxed) and breaking. I jumpered out the circuit breaker with no effect (didn't blow up the tank either!) and the wire is routed OK, but you might want to check that before your spend so much on a tank and be right back where you started.

[raymakr]

I will take a guess and estimate this baffle is to the right of the opening for the sending unit? But to the left of the tower. So I will bend the arm to the left away from this baffle? Tomorrow I will make a bend and see what happens. I don't want to have to buy a new tank, though I have a feeling brand new might be a huge improvement over what I now own. Thanks.

[YLG80]

Hi Ray,
Do you measure the grounding by means a multimeter switched in ohmmeter from the gas tank sender housing to the gauge housing?
Yves

[raymakr]

No, what would that reading be? For that matter, what should a 12 volt gas gauge read in ohms if it is good?

[Wolfman]

The reading Yves is referring to would be zero or close to it. Like when you touch the two gauge probes together. The gauge and sending unit housings should be connected together through a good ground.
As for the sending unit arm. Take the sending unit out. Hold it over the tank hole with the mounting holes in the tank and unit aligned, with the arm laying on the top of the tank. The arm should be parallel with the tank sides and the float not close to the tank side or baffle. If the arm angles toward the front or back side, it needs to be bent so the arm is close to parallel. With the sending unit out, you can see the baffle through the sending unit hole with the help of a flashlight. Don't use a match !! Just joking.

[raymakr]

So today, I removed the sending unit and bent it at the upper end carefully as it appeared to angle slightly towards
the baffle. I then installed it and checked and no change.
So I remembered another G503 post when that person attached the second ground strap to the actual power post of the sending unit and it worked. Well I did the same thing and now the gas gauge needle goes to full as is the tank currently.
So I am amazed that the ground strap attached to the power lug for the gas sending unit makes the gauge work and I hope it causes no problems sitting on the gas tank, but I will take it for the moment.
Interesting and frustrating journey. My first MB Jeep was twelve volt. I remember the gas gauge would not work, and I remember it took two sending units before it worked accurately.
I did not sand anything on the gauge or the dash and I did not have the bond strap on the gas line or a second ground strap going to the sending unit power lug. Maybe I should have kept that MB.
Many thanks to everyone for their advice. Have a great weekend.