Fuel Gauge Repair and Near Death Experiences

The offending fuel gauge unit partially removed for the repair.

I decided there is no way I could commute in this without a working gas gauge.  Mine was either pegged beyond full no matter how much gas I had, or bouncing wildly between empty and full as I drove around - neither was helpful.  The way the gauge works is that inside the gas tank is a floating aluminum cylinder attached to an arm, which as it moves up and down with varying levels of gas the acts as a variable resistor by moving some internal parts.  This assembly is called the Sending Unit, and is connected through a long wire to the back of the fuel gauge where the varying resistance is supposed to position the needle between E and F relative to the height of the float, and therefore the level of fuel.



I knew that the gas tank in my car is a replacement, along with the sending unit so I assumed the new sending unit was working.  However, the MGA Guru says that replacement tanks often have the original metal gas line replaced with a section of rubber tube on the way to the fuel pump presumably because that's easier than refitting a metal pipe.  This then causes the fuel gauge to malfunction since the electrical connection is broken because it is no longer grounded.  That could explain the odd behavior of my gauge sometimes waving wildly and sometimes pegged.  I took a quick peek under my car and sure enough there was a short section of rubber tube.  So, I took a wire and touched a bit of bare metal attached to the tank to a grounding screw and the needle moved off the peg to near the "F".  When I took the ground wire off, it jumped back to the peg.  So, I needed to make a permanent ground connection.

I tried loosening one of the screws attaching the sending unit to the tank but gas instantly started to ooze out after a quarter turn of the screw.  In the end I bought a small hose clamp for $0.50 and connected it to the bit of metal pipe coming out of the tank before the rubber hose.  I took a spool of wire, stripped one end and clamped it in the hose clamp, snaked it along as best I could with other wires running through clips attached to the underside of the car, up to a grounding point on the frame.  I first tried one near the battery, but when the socket slipped, I hit the positive post of the battery with the wrench and the sudden grounding to the frame and caused a big spark, a minor weld point on the handle, and a pretty good jump out of me.  Oops.  I decided to use another ground point a little further away from the battery.  Good thing there weren't any gas fumes around - that could have been bad...  There was my near death experience.

When done, despite my quick trial looking like it'd worked, the needle was still pegged beyond F.  An hour or so of head scratching, tracing wires, etc. ensued.  I decided to measure the resistance from the sending unit wire.  It appeared to be above 100 ohms, and a stock original is supposed to be 70.  I have the original sending unit and it measures 70.  So I pulled the gauge out of the dashboard and put 70 ohms across, and the gauge read full; 35 ohms, it reads half; 0 ohms, it reads E.  Perfectly calibrated, but for the original normal resistance, not my hyper active replacement unit.  The gauge is just held in from behind by a thumb screw.

Testing the gauge with resistors.  It reads "Full" perfectly at 70 ohms.

I've read of people adjusting the total resistance by running other resistors in parallel to the higher reading unit, but I decided I need to try to fix this by calibration first.  There are adjustment posts on the back of the gauge that move magnets to adjust the readings.  So, I drove to the gas station with my son, filled the tank to Full, and pulled the gauge unit out of the dash to adjust.  I first adjusted the magnet on the full side to make the gauge read right on F.  Then I shorted out with a wire between the ground and center screw to simulate being out of gas with a 0 ohm reading from the sending unit, and adjusted to read slightly below E (just to be safe - I figured it's better to have more gas left on E than to run out before it gets to the line).  I did one more round of minor adjustments between those settings in case moving the E magnet screwed up the F reading (it had slightly I think), but then I was done.  5 maybe 10 minutes at the gas station with a screwdriver/socket and some spare wire.

Loosening the post to move the "F" side magnet.  1/4" socket.

Rebuilt gauges in good working condition cost about $100.  You can't buy replacements that match the original style.  This repair cost $.50 plus the value of some spare wire, and required a screwdriver, some wire strippers and a standard socket wrench.