At 03:23 PM 8/16/99 -0500, Rick Pleczko wrote:
>The fuel guage on my '57 MGA operates in binary mode, i.e. it either reads
full or empty irrespective of what is actually in the tank.
Refer to the simplified color coded wiring diagram here:
http://www.ntsource.com/~barneymg/mgtech/electric/circ_f1.htm
If the gauge reads Full until you're down to the last gallon or two of
fuel, then drops like a rock to read empty when it is, or maybe even stops
at 1/4 tank when it's really empty, then you may have a bad ground at the
fuel tank. If the gauge constantly reads empty, then you may have bad
power or ground connections at the gauge. If it alternates between these
two modes you may have both problems, and the power or ground at the gauge
may be intermittent. It may not be intuitively obvious that the fuel gauge
needs to be grounded.
The circuit is quite simple. The fuel gauge has three electrical
connections, power, ground, and the wire to the sender unit. The sender
unit has two electrical connections, the sender wire and ground through the
tank to the chassis of the car for earth return to the battery. It is
fairly easy to temporarily install separate jumper wires to replace all of
these wires in the car, even without dismounting either the gauge or the
sender unit. If you get it to work properly this way, then begin
reinstalling the original wires one at a time until the problem reappears,
and you will have found the bad circuit.
For reference, the sender unit should read zero ohms resistance when the
tank is empty, and about 70 ohms resistance when the tank is full, and
should be aproximately linear in resistance in between. As such you may
expect about 52 ohms with 3/4 tamk of fuel, 35 ohms with 1/2 tank of fuel,
18 ohms with 1/4 tank. The resistance change is instantaneous with motion
of the sender unit, so if you shake the car to slosh the fuel around in the
tank you should see the resistance change accordingly. This may be hard to
recognize with a digital ohm meter, but an analog ohm meter will fluctutate
with the fuel sloshing.
And then just to add to the confusion, 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". End result there is that if the sender wire is
disconnected, or the sender unit is open circuit, or the tank is totally
insulated from chassis ground, then the gauge will read empty.
The tank is mounted in metal straps with thin strips of rubber packing.
The fuel filler pipe is connected to the tank with a rubber hose. As such,
it appears that the original setup may only be grounded through the steel
fuel pipe. If your car has a replacement fuel pump connected with rubber
hoses, the tank may not be grounded at all. Even with the original setup,
the ground connection of the steel fuel line to the chassis of the cer is
very iffy, especially if everything was painted during restoration, or if
the metal line and mounting brackets are quite corroded. Safe thing to do
here is to add a grounding wire at the tank, This can be connected to one
of the sender unit mounting screws and to the bracket on the frame located
within a hand's span of the sender unit.
Good luck,
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
|