Fw: Fw: MFTS symptoms

[Andy Schor]

----- 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?
>
> Thanks,
> Andy
> '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
>
>
>> >Phil,
>>>
>>>Thanks for forwarding the message.  I need to re-sub to the list with a 
>>>different address.
>>>
>>>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 
>>>sensor output.
>>
>>
>> Phil