Fuel Tank Sender 101

["Tim Lentz" <timlentz@domain.elided>]

>Gas gauge doesn't function on my 74 Spider. I've tested the sending unit
>with an ohm meter and it seems fine. I've switched the wires (pos and
>neg) from the unit

the unit?
if you are talking about the sender unit ... the two wires are not pos. and
neg.
One wire is for the "red, almost out of fuel" light (the white/black wire)
and the other wire (violet) is for the needle gauge (how much fuel do I
have).

The ground for the circuit(s) is a bit more complicated. The sender is secured
to the tank via small screw-head bolts. But between the sender and the tank is
a rubber gasket. To make matters worse ... the tank is screwed to the frame of
the car with sheet metal screws, but there again, a gasket separates the two.
So, the ground starts with the car's frame, travels only through the screws
securing the gas tank, then only through the bolts securing the sender, and
lastly the body of the sender itself is grounded. This is why many recommend a
"new" wire from a "good" ground to the sender (placed between the sender's
body and one of the securing bolts).

When the fuel in the tank reaches a low level, ground is shorted to the
white/black wire and that little red light on the gas gauge comes on.

The other circuit is a little more involved. Between ground and the violet
wire is a coil of tightly wound wire (a variable resistor). As the float on
the sender moves with the level of gas in the tank, the resistance of the coil
is changed by a contact point that moves along the coil. This changes the
voltage drop across the coiled wire thus manipulating the needle gauge at the
dashboard end.

So much for theory ...
Regardless of the condition of the sender unit - if you short the white/black
wire to a "good" ground the "empty" light should come on. Likewise if you
short the violet wire to ground the gauge should read "Full".

Troubleshoot backwards from there. Pull the sender and operate the float with
your hand while you watch your dash gauges (of course this means you will need
to supply a ground wire to the sender's body).

Tim Lentz
Tampa, FL
'74 Spider
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided