MFTS trouble, redux

[Phil Rose]

About 6 weeks ago I posted about solving a low-boost problem by
replacing a defective Multifunction Thermoswitch. Well, folks, the
replacement switch--admittedly a used one--managed to last about 3 or
4 weeks before exhibiting the same symptoms: low boost, intermittant
overheat warnings (in the IC display)-- although the dashboard temp
gauge continues to function OK. The diagnosis of a bad MFTS can
include one or all of the following steps:

Step (1) Remove connector to MFTS, and does full boost return? If
"yes", it is pretty definitive for a problem with MFTS.

Step (2) Replace MFTS connector. Turn ignition ON; set CC to "auto"
and temp control to "Hi".  Activate diagnostic channel 17. What is
the reading? Should be a simple little dot. No? If it shows segment
7, the "hot light" (i.e., overheat), this also indicates a defective
MFTS. BTW, this also means your A/C will not work. :-(

Step (3) connect multimeter between MFTS pin "R" to ground (sensor
body). Should be open circuit. No? Short circuit reading or some
finite resistance (even 2 or 3 megohms) is evidence of malfunctioning
MFTS. The reading from pin "C" should also indicate "open circuit".

The new MFTS I've just installed is the commonly available "Behr"
brand. Anyone know of other brands? The MFTS  sensors that previously
have gone kaput  have been the Behr brand as well as the no-name
version supplied by VW/Audi (with the VW/Audi mark).

I don't know what one can do to avoid the fairly short lifetime these
sensors often seem to have. One work-around might be to install a
switch in the harness wire coming from MFTS pin "R". Thus, by opening
that switch, a false "overheat" condition can be bypassed (at least
temporarily). After all, we all closely monitor our temperature
gauges, don't we? Do we really need some stinkin' idiot light on the
dashboard?

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