I have an ambient temp gague installed in my dash. Lately I have been noticing that when I turn on the lights the gauge drifts upwards about 3 degrees C. I thought it had something to do with voltage draw so I started up the car an turned on the defrost, heater fan, heated seats and the gauge stayed perfectly still. But when I turned on just the parking lights the gauge drifted again: It appears that the illuination circuit for the gauge has something to do with the problem. Has anyone seen this or know what is causing it?

Thanks,

Matt

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'82 DL - 158k, '93 945 - 116k

Is the temperature sensor installed near those hot bulbs?
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Don Foster (near Cape Cod, MA)

I'll bet the power wire for the gauge is run to the back of the parking light fuse.
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Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)

"...bet the power wire for the gauge is run to the back of the parking light fuse..."

That could be it.

But isn't that fuse powered only when the parking lights are on?
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Don Foster (near Cape Cod, MA)

Do those gauge senders have their own ground return or do they reference the body at a bolt somewhere near where the headlights or parking lights are returned to ground?
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Art Benstein near Baltimore

Those temperature sensors are two-wire devices, and both wires connect to the readout instrument. Neither wire is ground -- at least not directly to the chassis (might be grounded inside the instrument).

This insures that the sensor is truly isolated from whatever happens under the hood.
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Don Foster (near Cape Cod, MA)

Sounds like they did a nice job with those. I think I'll start looking for one to make a nice Christmas present for the #1 daughter.

With respect to the isolated wiring, would it be plausible there's a fault to the mounting bracket in the sensor assembly? I used to have one of those radio shack remote thermometers before the volvo days, and I hung the thermistor from the inside of the bumper in front of the right tire in a very hostile winter environment. After a couple years the thing would go nuts in the wet. But it was a $6.98 thing.
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Art Benstein near Baltimore

Indeed,

best location for outside temp sensor is in the most outward of the front wheel arches, away from sunshine and hot air from the rad. Mine gives same temp as a calibrated temperature gauge: it is one of VDO!
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240 '88 B200K 260kkm virgo+all gauges,Phoenix Gold ampli, Focal door speakers, debadged, 6 rear red lights

Yes... the sensor for mine is tucked down behind the front bumper somewhere. But you never know what someone else did without thinking (or even WITH thinking)...
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Don Foster (near Cape Cod, MA)