At 08:35 PM 10/16/98 -0400, Steven R Schultz wrote:
>.... I removed the sending unit, measured the resistance .... It works
fine. I also installed a new ground .... to the base of the sending unit.
Unfortunately the gauge doesn't work. With three gallons it says 7/8 full.
.... How can I check the gauge? ....
One more thing to check in the wiring. There is a snap connector in the
wire from the gauge to the sending unit. This connector is located in the
engine compartment near the starter switch. Look for a green wire with a
black stripe going through a single pin connector. You may have a high
resistance connection at this point. High resistance gives a higher
reading on the gauge, and anything graeater than 0 resistance will read
higher than empty on the gauge.
Next, measure the resistance of the sending unit circuit at the gauge
connection. Disconnect the single sending unit wire (green with black
stripe) from the gauge, connect one lead of your ohm meter to this wire and
the other lead to chassis ground (the dash board or the case of the fuel
gauge). With a few gallons of fuel in the tank you should see about 20
ohms here. If you get a reading of 50 to 60 ohms, there's too much
resistance in the sending unit connecting wire, quite likely at the snap
connector in the engine compartment.
To check the operation of the gauge by itself, disconnect the sending unit
wire from the gauge. Check to be sure the gauge has 12 volts supplied on
the terminal with all the green wires attached (with the igniton turned
on). Check to be sure the ground connection is a good low resistance
earthing connection. Check the resistance from the ground lug on the fuel
gauge to a known good chassis ground, or better yet, to the ground
connection on the battery. The gauge won't work properly without a good
ground connection. Use an ohm meter, not a test light.
Then for the simple test, connect a jumper wire from the signal terminal on
the gauge to ground, and the gauge needle should go exactly to the "Empty"
mark. If it doesn't, the gauge would need adjusting. If it does, proceed
to the next paragraph.
Go to Radio Shack (or some similar source) and buy some cheap carbon
resistors. Nominal resistance values should be 68, 51, 36, and 18 ohms.
With the ignition switch on, and a good ground connection to the gauge,
connect each of the resistors in turn between the signal terminal on the
gauge and ground. The 68 ohm resistor should give you a reading of "Full"
on the gauge, 51 ohms for 3/4 scale, 36 ohms for 1/2 scale, 18 ohms for 1/4
scale, and 0 ohms (jumper wire) for "Empty" on the gauge. If the gauge
does not come close to these readings, it would need adjusting.
Bear in mind that the resistance change from the sending unit is not
exactly linear with the change of fuel level in the tank, and the markings
on the face of the gauge are also not linear (equally spaced), so the
readout on the gauge may not fall exactly on quartile marks when using
these resistors.
If after you have comfirmed a good 12 volt supply and a good ground
connection to the gauge, and the gauge still does not give the correct
readings with the specified resistors being used for the signal, then let
me know and I'll dig up some old e-mail correspondence explaining the
internal operation of the gauge and what you have to do to re-calibrate it.
Keep us informed. If this keeps up I may have to take my fuel gauge apart
to get a good picture to post on my web site.
Barney Gaylord
1958 MGA with an attitude
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