Ha,
Cheap laugh today, and another free fix.
Nine years ago I installed a non-stock fuel pump on my MGA, one that hooked
up to the fuel lines with rubber hoses. All was fine for a long time. A
couple of years ago my fuel gauge readout started to drift upward very
slowly and gradually over time, until recently it wouldn't budge from the
full mark until it was down to 3 gallons in the tank. Last week it decided
to stay permanently on the full mark, even when the tank was empty.
It turned out that the fuel tank wasn't grounded well enough. You know,
the tank that's mounted with rubber packing in the suspension bands, and a
rubber hose in the filler pipe, and now a rubber hose to the fuel pump.
The only ground connection for the fuel sender unit was through the steel
fuel line between the tank and the fuel pump, where that line is fastened
to the frame with a couple of "P" clips. Well the "P" clips were gradually
getting dirtier and rustier until with the resistance in the ground circuit
added to the resistance of the sender unit, the gauge was reading too high.
Easy enough to diagnose, when I put a jumper wire from the steel fuel line
on the tank to ground on the frame the gauge went to "E" like it should
(less than 1/2 gallon of fuel in the tank).
I took the sender unit out of the tank just long enough try it with a
ground jumper wire and to measure the resistance. The sender unit measures
70 ohms when the tank is full and 0 ohms when the tank is empty. With the
gauge wire hooked up and a jumper wire from the sender to ground everything
works dandy, with a nice smooth gauge motion from empty to full and back as
the sender arm is moved up and down. So I just put it back together and
added a ground wire from one mounting screw of the sender unit to the
ground screw right nearby on the frame.
For future reference, all you MGA folks out there should make a note
somewhere that the MGA fuel sender unit should read 70 ohms with a full
tank and 0 ohms when empty. If you take the wire off of the sender unit
and touch it to ground, the gauge should read exactly on the "E". With the
sender wire disconnected, the gauge will be pegged at the bottom end,
looking the same as when the ignition switch is turned off. Yeah, I know,
it didn't seem right to me either. But if you add resistance to the sender
wire, somewhere between 140 ohms and infinity the gauge does an about face
and goes back to "E".
Have a nice week end,
Barney Gaylord
1958 MGA with an attitude (and an accurate fuel gauge for a change)
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