From under the hood, you can verify the operation of the voltage stabilizer that keeps the fuel and temp gauges accurate. For those with 1981 and later 240s, checking the voltage at the end of the yellow wire pulled from the temp sender in the head, it is simple. Expect 10 volts.

The mechanical stabilizer used in the earlier cars operates like the old turn signal relays, using a bi-metal strip heated by nichrome wire, and like those old relays, tends to fail in the closed-circuit (shorted) position. This will make the gauges, both fuel and temp error to the high side.

Checking with a test light (use a small one) will reveal the battery voltage is interrupted fairly rapidly. Because the gauges are thermally operated, the rapid fluctuations are not seen in needle movement. Checking with a digital voltmeter makes it look like the stabilizer voltage is anything but, when it is working properly.



Getting a clip lead on it...





--
Art Benstein near Baltimore

Did you hear about the kidnapping at school?
It’s fine, he woke up.