At 08:40 PM 10/10/98 -0400, Steven R Schultz wrote:
>....
>I know the tank currently has about three gallons in it. I haven't
removed the sending unit yet. I checked the tank grounding by taking a
test light and touching the sending unit wire stud with the ignition on and
touching the light probe to the tank. It is grounded.
This can be a little deceiving, because the ground connection is not
necessarily just connected (0 ohms) or not connected (infinite ohms). It
is common for this ground connection between the tank and the chassis to be
a high resistance connection. When you have a low wattage bulb with a high
resistance, a little extra resistance in the circuit just makes the bulb
glow a little dimmer, but it still lights.
When I had trouble with my gauge the tank ground connection was reading
about 30 ohms resistance, so the current would flow and the gauge would
work, but the gauge wouldn't go all the way to the empty mark when the tank
was empty.
>I touched the sending unit wire from the gauge to the chassis and the
gauge went to full (not to empty).
Uh, just a guess here, but you may have the wires on the back of the gauge
reversed. It should go to the empty mark when you ground the signal wire.
>.... multi-meter .... Grounded one end of the multi-meter to the chassis
and touched the other end to the electric stud on the sending unit. With
the ignition off and the gauge wire off I got a reading of 90.5
Measuring the combined resistance of the sending unit and the tank ground
connection here. Looks like a high resistance ground connection for the
tank. The reading should go from about 70 ohms when full to 0 ohms when
empty, never higher than 70 ohms. With 3 gallons of fuel in the tank the
reading should be around 20-25 ohms. If you check the resistance between
the tank and the frame, I think you will find at least 50 ohns resistance
there, and it should be 0. If so, it needs a ground wire for the tank.
>Grounded one end of the multi-meter to the chassis and touched the wire to
the gauge while it was disconnected . With the ignition off I got 63.8.
Measuring here the combined resistance of the gauge and the ground
connection of the gauge to the dash and chassis.
>Connected the sending unit wire and touched the multi-meter to the
electric stud and grounded to the chassis. With the ignition off I got 34.5
Doesn't tell you much here, as you have two high resistance ground circuits
in parallel, but the numbers make sense.
1 1 1
---- + ---- = ---
90.5 63.8 X
and solving fox X = 37.4, pretty close to you measurement.
>Are any of these what you were suggesting?
Yep, still looks like a high resistance ground connection for the tank.
>Should I get the 0-70 with the ignition on, wire connected and ground the
other end of the multi-meter?
NO !!! Don't try to measure the resistance with the ignition on. The
supplied voltage could blow out your ohm meter. If it actually worked you
might be measuring negative resistance. No help that way.
Add a ground wire from the tank to the chassis. If the gauge goes the
wrong way, try reversing the wires on the back of the gauge. One terminal
on the gauge carries a single wire that goes to the tank sending unit. The
other terminal on the gauge should have three wires, one being power from
the switched fuse, one going to the heater blower switch, and the other
going to the turn signal switch.
Point a browse to this address for diagram of the MGA fuel gauge circuit.
http://www.ntsource.com/~barneymg/mgtech/electric/circ_f1.htm
Barney Gaylord
1958 MGA with an attitude
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