Fw: Fw: MFTS symptoms
walbum at realalbumart.com
Thu Jul 5 16:00:55 EDT 2007
----- Original Message -----
> Hi Phil (& Folks on the List),
> As long as my outside temp reading was above the ambient, the A/C was
> working fine. Today I made a 300 mile round trip (Pittsburgh > Cleveland
> > Pgh), the weather was mid 70s and humid. When I pushed "auto", the air
> from the vents was not 'conditioned'. I checked the outside temp and it
> read 18 degrees! I tried cycling the system off/on, turned the car
> off/on, to no avail. I tried it again a few times during the trip, at one
> point it actually said (-34F). So now I'm jonesing for the days when it
> was too high!!! I did pull over and check the connections of the sensor
> in the passenger side 'rain-tray'. As a side note, when the A/C was
> working (up until today), I could hear the compressor kick on. Today it
> seems as though the low temp reading was not signaling the compressor.
> Does that make sense? What should I try next?
> '91 Avant 289K miles
> ----- Original Message -----
> From: "Phil and Judy Rose" <pjrose at frontiernet.net>
> To: "Andy Schor" <walbum1 at verizon.net>
> Sent: Thursday, June 28, 2007 4:16 PM
> Subject: Re: Fw: MFTS symptoms
>>>Thanks for forwarding the message. I need to re-sub to the list with a
>>>I looked behind the grille for a temp sensor (I had a '90 CQ that had a
>>>wire with a sensor clipped to the grille from behind), couldn't find one.
>>>Then I came back in and checked the Bentley. I see an outside
>>>temperature switch in section 87.38, is this what I'm looking for? Or
>>>maybe there's one in the grille that is missing?
>> As I recall, the "front" sensor is located just to the inside (left) of
>> the headlight on driver's side (between headlight and intercooler). I'm
>> sure you know that you can also read (via your Climate Control
>> diagnostics channels 04 and 05) the output level codes --i.e., the
>> resistance value produced by each sensor (thermistor). These values can
>> be converted to actual temperature using a table in the Bentley manual.
>> Your outside temp reading (press "outside" button) is just the front
>> (grill) sensor's output after being automatically converted to (F or C)
>> degrees. Below is a copy of an old (really old!) post of mine that gives
>> a bit more detail about the temp sensor setup:
>> At 3:48 PM -0400 1/12/99, Phil Rose wrote:
>>>The outside temp is measured by two thermistor probes: one mounted up
>>>front--just behind the driver's side grill (cowl?) and another under the
>>>plastic plenum cover near the hvac blower assembly. Below I've copied a
>>>qlister's posting from my archives. I'll add a couple of comments:
>>>(1) A way to tell which thermistor is the defective one is to output
>>>channel 4 and channel 5 of the CC diagnostic display. These channels are
>>>the temp sensors' output--but resistance values, instead of degrees F.
>>>Since the CC outside temp display is programmed to use the _lower_ of the
>>>two when/if there are different readings, and since you're getting an
>>>outside temp reading that's too low, thus the lower temp is the
>>>*in*correct one. In other words, the bad sensor is the one that shows the
>>>_higher_ resistance value. As I recall, the grill-mounted sensor=channel
>>>5 and the sensor in the plenum=channel 4. The Bentley manual--you have
>>>one, right?--will give you a table of temp vs. resistance values.
>>>(2) Also, a stop-gap "cure" that I'm presently using on my '89 100 is to
>>>insert a resistor in place of the defective sensor. I chose one with a
>>>value of about 1.9 kohm, which acts like the thermistor at 52 deg F. If
>>>you do this, the display will show the outside temp according to the
>>>_other_ (working) sensor during the winter--assuming it's colder than 52
>>>F outside. In warmer weather, you'll be seeing an incorrect temperature
>>>(52 F) all the time, but at least your AC will be able to function. Using
>>>your bad sensor, you may not get air conditioning to work later in the
>>>year, because the system may think it's too cold outside to allow the AC
>>>compressor to run.
>>>Oh, one more thing: channel 01 of the CC diagnostics will display either
>>>a 4 or 5 to indicate if one of the two temp sensors has an open circuit.
>>>It doesn't seem to me that you have an open circuit--just a defective
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