Fw: MFTS symptoms

Phil Rose pjrose at frontiernet.net
Thu Jul 5 22:49:30 EDT 2007

I assume by now you've found the location of the front temp sensor. 
If that's the bad one (i.e., reading too low), just replace it with a 
known good sensor... or (as described below) simply substitute a 
resistor of about 1.9 Kohm  in its place, which that will make the 
Climate Controller "think" it's warm enough outside to safely operate 
the A/C compressor.


At 3:57 PM -0400 7/5/07, Andy Schor wrote:

>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
>>  >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 
>>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 & Judy Rose           Rochester, NY  *      * 
>>mailto:pjrose at frontiernet.net       *
>>No virus found in this incoming message.
>>Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 
>>269.9.10/876 - Release Date: 6/28/2007 10:56 AM

*  Phil & Judy Rose           Rochester, NY  *      
*        mailto:pjrose at frontiernet.net       *

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