At 07:45 PM 10/4/98 -0400, Steven R Schultz wrote:
>.... '61 MGA gas gauge .... The wire is OK. With the car running the
gauge usually reads from half full to three quarters full. When the wire
is disconnected from the sending unit and ground the gauge goes to full
which I am told indicates that everything is OK. All of the above would
suggest that the sending unit in the gas tank is the culprit.
>
>How do I test the sending unit without taking it out of the car? Can I
use a multi-meter to test it when the talk is full and again when it is
close to empty?
Check with an ohm meter. The sending unit should read 70 ohms to ground
when the tank is full, and 0 ohms to ground when the tank is empty. So if
you ground the sender wire at the tank the gauge should go to the Empty
mark. If you have a bad earth connection for the fuel tank the gauge will
not go all the way to the Empty mark.
The tank is mounted in rubber insulated metal straps, and the filler pipe
is connected with a large rubber hose. Original setup allowed for the
sender to find ground through the metal pipes on the fuel pump. If the
fuel pump is currently hooked up with small rubber hoses, there's a very
good chance the tank is not grounded. Best solution is to run a ground
wire from one of the mounting screws of the sender unit to the grounding
lug on the frame which is about six inches away from it. If that doesn't
solve the problem, the sender unit would be bad.
>If I take it out of the car how do I test it? Can I ground it and move the
float up and down while attached to the sending unit wire and gauge?
Yes, but you have to have power to the gauge, and the housing of the gauge
itself also has to be grounded. By itself, the sender should be 70 ohms
with the arm full up and 0 ohms with arm full down, and a continuous smooth
transition of resistance through the full travel in between.
>Should I try to fix it myself, send it to a specialist to fix it or should
I buy a new one from one of the Mail order houses?
Check the resistance of the ground connection at the sender first, from the
tank to the frame, and add a ground wire if necessary. Then measure the
resistance of the sender unit. If it doesn't transition smoothly from 0 to
70 ohms, then you need a new sender unit. I'd bet on the ground connection
being bad. These things are pretty reliable. Mine's 40 years old and
still working fine.
>When I reinstall it should I use Permitted?
I had the sender unit out of mine to clean and paint the tank 20 years ago.
I put it back together with a THICK paper gasket, no sealant, and no leak.
just had it out again a couple months ago to measure the resistance, put
it back in with the SAME old paper gasket, still no sealant, and still no
leak. If you do need to use a sealant, be sure it is a type that is
specified for use with gasoline. Read the container, don't guess.
Barney Gaylord
1958 MGA with an attitude
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