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MG MGA - Fuel Gauge Question

Fuel Gauge problem

I start out assuming it’s a fuel gauge problem but it’s probably not. 2 years after having the gauge rebuilt by Nissenger I finally got my fuel gauge to “work”… it’s always been on “E” … now it pegs on “F” with a tap on the face of the gauge. It did not do this spontaneously… I looked for the 1000th time at the wiring diagram and finally decided I had not connected the gauge’s green wire correctly so I re-connected it directly to the green wire in the harness, ie, to the fuse. Well now my gauge pegs on full… even after a 100 mile drive last Saturday. I know the car’s gas mileage is not that good! I don’t know which is worse… driving around on empty or full? Today I ran a special ground wire to the pole of the gauge holding it to the dash panel but the gauge still pegs full. I don’t have any special equipment like Barney Gaylord uses… so is there anything I’ve missed or should try from here on out? There is only a “B” and a “T” on the gauge so there can only be so many alternatives. Should there be current on the “T” side when the ignition is off, for example… The gauge remains on “E” until the ignition is turned on and thumped?
thank you
danny jacob

Since thumping the gauge makes it change to full, and you have checked all the wiring, it sounds like a problem inside the gauge. Do check that the gas tank is grounded, but that would not cause the gauge to need to be thumped.
As far as gauge accuracy, our A stays on full until at least a third of the tank is used.
John H

You may have a bad ground on your gage or tank. thiumping the gauge might indicate the gauge's ground may not be good. Check out the info on Barney Gaylord's site.
R. L Carleen

The green wire from the fuse should be connected to the b terminal, and the power on it should be switched off by the ignition key. If the green wire to the fuse is not connected, the gauge will read empty. If the case on the gauge is not grounded properly, the gauge would read full all the time. If the fuel tank or the sender were not grounded, the gauge would read empty.

Another possibility is that the wire to the sender is shorted to the chassis somewhere. Disconnect the wire at the sender, and the gauge should read empty. If it does not, then disconnect the wire at the t terminal on the gauge. If it then goes to empty, the wire is shorted to the frame somewhere.
Jeff Schultz

If the gauge has been damaged by being incorrectly connected, then it may well read full, even though it's not "working" correctly.

I experienced this after a cable fire (the body shop re-installed the loom incorrectly after repairing a rear end shunt). I got the car back, and was on my 900km way to my sister's wedding using wedding cars tunbridge wells, when the fuel pump started to make a lot of noise, even though the gauge was reading full. I coasted off the Autobahn into a gas station, and the engine died as I crossed the threshold of the gas station, allowing me to coast to the pump with NOTHING in the tank!

The gauges can be fried very easily if connected inncorrectly in this way. I fear you may be looking at another rebuild.

dominic clancy

you all have given me a clue about my previous handy-work. Should the sending unit be completely insolated from the tank by the cork washer? One of my 1st problems with the car was a serious leak around the sending unit. One of the screws was stripped so I’ve jury-rigged larger ones into the holes. The leak is cured but I’m wondering if there is somehow direct contact between the sending unit and the fuel tank now that prevents the gauge from working properly. This leaking sending unit is like re-opening a fresh wound… I hate to pull it apart if a potential contact problem is unrelated. So, will wait for advice!
danny jacob

Just realized that my last post was wrong. On an MGA, a sender wire shorted to ground will cause an empty reading, and an open sender wire will read full. The metal body of the sender should make connection with the ground. This normally happens tru the fuel tank, but corrosion can make a bad electrical connection from the tank to the frame. Try running a wire from the body of the sender to a good frame connection.
Jeff Schultz

Danny

The sender should be grounded to the tank. It helps to have an earthing wire running to one of the flange fixing screws to make sure. Ensure the tank is earthed too.

Now are you sure the sender is the right way up? My car's was upside down, I noticed from the two previous owners bills, neither got that one sorted!

The sender's exterior terminal must be at the bottom (might save you removing it to check)

When the sender was upside down, my gauge showed full all the time the ignition was on.

Apologies if you know all this - but I've been here too!

Pete
Pete Tipping

For the gauge to work properly the sending unit has to be grounded on the tank, and tank has to be grounded to the chassis. The tank is mounted with rubber packing bands in the mounting straps and the filler pipe is connected with a rubber hose. Original ground connection was through the steel fuel pipe from the tank to the fuel pump, which was in turn grounded to the frame with a black ground wire.

If the steel fuel line was cut and connected with a rubber hose you can lose the ground connection for the sender unit. A good fix is to install a grounding wire from one of the mounting screws on the sender unit to the ground lug on the frame nearby where the wiring harness is (supposed to be) held with a P-clip at the top of the frame arch.

You don't need any special equipment to calibrate the fuel gauge, just a very cheap pair of 68 ohm resistors from Radio Shack. See here: http://mgaguru.com/mgtech/electric/fg_10.htm
Barney Gaylord

Hmmmm. Gremlins, eh? If you have the time, here is what I would do. Run the tank as empty as possible. Remove the Sending unit. Remove the gauge from the dash board. Using wire with alligator clips (purchased from radio shack), and a 12 v battery charger ( the 1 amp model works well for this), connect the sending unit, guage and charger from your wiring diagram. move the sending unit float up and down and see if the guage moves. If nothing happens, you either have a bad guage or sending unit. I think you can test the sending unit with a OHM meter. If that checks out, the guage is the fault.
On the other hand, if everything works perfectly on the bench, it is your wiring in the car. Either way, you will know.

This is how I tested mine during restoration. I discovered a broken wire in the sending unit and had to replace the whole thing. It has been working fine every since.

My 2 cents.

Dennis, Az
Dennis

At full my gauge reads full. At empty it reads 1/2 full. Is there an adjustment that can be made? Thanx.
Steve
Steve Meline

Barney has excellent web pages on calibrating the gauge. The links to the fuel gauge pages are at the bottom of this page. http://www.mgaguru.com/mgtech/electric/electr_1.htm
Jeff Schultz

This thread was discussed between 08/05/2006 and 10/05/2006

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