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MG Car Repair/1980 MGB gas gauge


The gas gauge on my 1980 MGB stops at half full when I fill the tank with petrol (gas). I have installed a new(replacement) tank and have changed the floater et al twice but to no avail. The needle on the gauge stops at half.  I am not a professional mechanic.  Please help.

Hi John,

In the MG dealership we use to do a preliminary test when we had anything wrong with the gauge reading. The first thing we did was turn the ignition key to "ON" not "Start" and read the gauge. (yours says 1/2 full) then we remove the power wire off of the tank sending unit and the gauge should go full deflection in either empty or full then ground the wire on the tank sendig unit case and the gauge should go all the way in the opposit direction. This it just a preliminary test of the wiring from the gauge to the tank unit.

Next jack the right side of the car up high and remove the sending unit from the tank. Put a shop rag in the hole to keep gas fumes from where you are working.

Now connect the two wires back on to the sending unit. You need both wires on the sending unit for it to operate. With both wires on the sending unit have someone turn the key to "ON" not Start. Now manually operate the arm of the sending unit and watch the gauge movement. You should be able to move the gauge needle from enpty to full and back to empty. The gauge needle may move slowly but that is normal.

If you do not get full deflection by manually moving the float arm, then you must remove the fuel gauge and test the voltage applied to the light green w/green tracer wire on the gauge with the key "ON". It should be about 10 volts. If it is not, then either the wiring has been changed or the Voltage stabilizer is bad and must be replaced. If the stabilizer has failed then the temperature gauge must be reading wrong also because the stabilizer supplies 10 volts to both the fuel gauge and that temp gauge. The Stabilizer is either mounted on the fire wall behind the instruments or on the back of the Speedometer or tach. (small two connector metal box)

Let me know,


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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