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Fuel Gauge Problem

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Dave Richards

Dave Richards

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Had a very pleasant 80 mile drive yesterday.

Drove to a family members home for Mother's Day, as I drove the fuel gauge acted normally, that is it bounced around as I went up and down came to stops, etc.

In the evening, as I started the car, I noticed the fuel gauge right to Full, in fact, it went past full to the pin!

Turn ignition off, fuel gauge goes down to other pin, turn ignition on, gauge is pinned past full.

I did a search of the forum and came up with something about having a good ground, ok I'll check that.

The gauge does light up with the rest of the panel.

I suspect someone here probably knows exactly what the problem is, and I'm sure curious to know as well, any ideas?

Dave
 
Hi Dar100, Try connecting an aligator lead from the tank sender's body to the ground in the boot for the lights. If the guage now acts ok, then add a wire permanently in place of the jumper. Additionally, make sure the guage grounds behind the dash are tight--Fwiw--Keoke
 
Hi Dave,
As has been mentioned, a good ground on the tank is important. Also, a good ground on the dash gage itself is important. Emphasis on both grounds.

Basically, with the wire to the tank unit disconnected, an ohmmeter connected from the tank unit to ground should show near zero ohms with the tank empty & around 70 ohms with the tank full.

With the disconnected end of the wire from the gage to tank grounded the gage should show empty. With this wire left open circuited, not connected, the dash gage should show full plus.

Here is a very comprehensive article on the MGA gauge which is the same as the AH gage:
https://mgaguru.com/mgtech/electric/circ_f1b.htm

If you really want to get into gage calibration & repair start here:
https://mgaguru.com/mgtech/electric/fg_01.htm

Good luck,
D
Ps - did you ever get my private message that I sent a couple of weeks ago?
 
Hi Dave,

My fuel gauge does exactly the same thing. While this is not likely the answer that a "real mechanic" would favor, I have found that when the needle is pegged on full while driving, when I know that this is not the true measure of fuel in the tank, if I tap on the glass of the gauge the needle will free itself and either go to the true level (if sitting still) or resume its dance up and down (if under way). So far I am satisfied with this "repair". As a side note, I have learned (the hard way) that if my fuel gauge shows just under 1/4 full I am out of gas.

Wana
 
Thanks Dave Russell, I didn't get a chance to get to the car today, but will try tomorrow. I didn't get your PM, but did send you one earlier today...
 
Dave -

Sounds to me your guage isn't grounded properly. Just fiddle with the thumbnuts and that should fix it.

Frankly speaking I always look forward to fiddling with my thumbnuts.

Failing this particular joy of life, also check the connection to the guage.

If no joy there, take the sender out and make sure it's not stuck in the up position (my BJ8 just did this to me). Make sure to have a new gasket before yanking the sender.
 
Obviously check the ground as mentioned, but don't overlook Dave Russell's other point. If the green/black wire between the sender and gauge is off or broken, the gauge will read past full.
 
The green/black wire is tight to the sending unit, the ground at the gauge is secure.

I'm going to try the jumper now. Also the green/black wire shows continuity to ground. I suspect the green/black wire has touched ground somewhere between the gauge and the tank.
 
About 30 ohms from sending unit to ground, (with the green/blak wire disconnected) the tanks about 1/2 full.

Tank has continuity to ground at chassis, same with sending unit.

A jumper from the sending unit to ground: no diffrence.

A jumper to replace the green/black wire: no difference!?

I'm going to follow Dave Russell's link to the MG fuel gauge, unless anyone else has any ideas...

Well, that didn't pan out much, I have no desire to take the gauge apart, does it seem likely the gauge is the problem?
 
Dave -

If all else fails, take the sender out and check to make sure it's not stuck in the up position. Of course make sure you have a spare cork gasket on hand before you start pulling the sender out of the car.

Just had this same problem with my BJ8 and chased it finally to the stuck sender. A little WD-40 and some wiggling back and forth got it loose and working again.

I bought a spare sender while I was at it, but that ended up being a waste of time, now I have a spare sender in the closet!
 
Thanks Alan for the reply, sorry to see you're sick!

Is it likely the sender unit would send the gauge needle past full to the pin? Intuitively, it seems like an electrical issue.
 
Hmmm, maybe you are right.

Another thing to check is to see if the little tank sender lead is grounded to the sender... if so there will be some galvanic corrosion around the lead...
 
Dave , what happens to the meter when the Grn/Blk wire is disconnected from the sender and then grounded?. What does the meter show with the Grn/Blk wire just disconnected from the sensor? --Keoke
 
Keoke said:
Dave , what happens to the meter when the Grn/Blk wire is disconnected from the sender and then grounded?. What does the meter show with the Grn/Blk wire just disconnected from the sensor? --Keoke
Click to expand...

About 10 ohms. BUT, not when the grn/blk wire is disconnected from the gauge, then it doesn't register...no connection.

What do you think?
 
healeynut said:
Hmmm, maybe you are right.

Another thing to check is to see if the little tank sender lead is grounded to the sender... if so there will be some galvanic corrosion around the lead...
Click to expand...

I checked this. The post has 37 ohms resistance with ground, which seems consistent with a 1/2 tank of fuel.

With the grn/blk wire connected to the post, the tester is pegged to continuity.

It's a riddle to those without the knowledge!
 
I do not suspect the sender because you've measured its resistance to ground at 30 ohms with half a tank. You can expand this test by connecting your meter across the sender terminals and lifting/lowering the float with a coat hanger (or similar) to verify the sending unit's operating range.

I put together a document a couple of years ago for someone else trying to troubleshoot a similar gauge. Please take a look at:
https://home.mindspring.com/~purlawson/files/OldFuelGauge.pdf

Move your tests to the gauge itself and focus again on the green/black wire. You're not looking for a short of the green/black wire... that would produce a reading of Empty. You're looking for an 'open'. Look for a break in the green/black wire, run a new test wire between the tank and gauge if necessary.

Return to the gauge case ground itself also. I know you said it's secure, but make sure that both ends of the gauge's ground wire (gauge and chassis ends) are clean and making good contact. Add another ground just as a check.
 
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