More On Fuel Gauge

[jack@xxxxxxxxxxxxxxxxxxxx (Jack Hagerty)]

Last week there was a question about the erratic operation of a fuel gauge
in a '74 GTV. I responded that mine just started exhibiting the same
behavior so I pulled the sender from the tank and tested it. I used a
VOM (yes, the archaic type with a needle that moves...just like a fuel
gauge) and the resistance followed a nice smooth curve from about 5 ohms
(at the "Full" end) to several hundred (at the "Empty" end). There were
no breaks or discontinuities. I even opened it up for an inspection. The
wire was wrapped tightly to the board and the "finger" seemed to follow
it smoothly.

This lead me to suspect the guage itself and I mentioned how several
years ago it had acted in a similar manner when the needle assembly
had not been seated in it's bearings correctly. Tom Sahines, however,
reminded me of a simple test to check the basic operation of any 
resistive gauge: remove the wire from the sensor and ground it. This
should give you full scale on the guage. Ungrounding it should give
you minimum scale. I did this with my fuel guage and it seems to work
find. I banged the needle back and forth several times between the
hard stops and it moved quickly and smoothly. If there was any problems
with the coil/needle mounting, I think this should have uncovered it.

The question now is, what's going on? The sensor passes the operational
resistance test and the gauge (and wires) pass the full-scale operation
test. Is it possible that the sensor works in air, but not when submerged
in gasoline (or gasoline vapors)?

Any and all speculation invited.

- - Jack

+-----------------------------+---------------------------------------------+
| Jack Hagerty                |                                             | 
| jack@domain.elided   |                                             |
| Robotic Midwives, Ltd.      |  Black holes are where God divided by zero. |
| Livermore, CA		      |             - Anon.                         |
|                             |                                             |
| (925) 455-1143 (voice/fax)  |                                             |
+-----------------------------+---------------------------------------------+
|        ARA #97, NAR #55105, LUNAR #002 / TRA #3943, Aero-PAC #168         |
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