the mutli-MALfunction sensor strikes again!

Phil Rose pjrose at
Thu Feb 14 22:31:25 EST 2002

I've now become quite proficient at removing the connector from the
multifunction thermo sensor.

1. Clean and dry contacts, reconnect sensor, test drive=LOW boost
(but no longer any coolant leak)

2. Disconnect sensor again, test drive=FULL boost (verifying initial
diagnosis pointing to MFTS)

3. Clean contacts some more, more DeoxIT, reconnect sensor, test
drive=LOW boost. :(

Then I realized that with this sensor, a loss of electrical contact
would not cause the problem with boost that I was seeing. It must be
a problem internal to the sensor.

4. Disconnect once more, use DVM to measure resistance from the
sensor terminals (to ground) and find suspicious low resistance (on
the order of 1 or 2 megohms) from terminals R (to overheat warning)
and C (to A/C). I have a spare multifunction sensor, and testing
those same terminals show an open circuit with respect to ground (as
expected for a good sensor).

5. Install spare sensor, test drive= Full boost. Life is good again.

As Scott M. says--expect no more than a few years service from these
3-pin sensors--and indeed mine had been installed just about 3 yrs
ago. The 3-pin version doesn't tend to fail in the obvious manner of
its 4-pin predecessor --i.e., a dead temp gauge. Instead, it will
fail by giving a spurious engine overheat signal to the ECU (but in
my car this did not consistently produce that warning-light message
and chime) and can quietly rob you of boost and/or AC.


Phil Rose				Rochester, NY
'91 200q	(130 Kmiles, Lago blue)
'91 200q   (57 Kmiles, Tornado red)
	mailto:pjrose at

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