No heat condition, but a hot heater core!

Phil Rose pjrose at
Sun Feb 5 20:48:09 EST 2006

At 5:21 PM -0500 2/5/06, Kneale Brownson wrote:
>Sounds like the motor that runs the flaps that control whether fresh air
>goes over the heater core is not working.  It lives behind a cover on the
>driver side of the heater core box.  I think it's several Phillips head
>screws to get the cover off.  If I remember correctly,  it's much easier to
>get to the motor if you remove the wiper motor assembly.  I'd start by
>checking the wiring that comes out of the space behind that box and plugs
>into a harness near where the mounting for the wiper assembly is.
>At 03:30 PM 2/5/2006 -0600, Brian Larson wrote:
>>I am having an issue with the 91 200q I just picked up.  And it is cold here
>  >right now which complicates the issue (10F or less in the mornings).

Kneale's on the right track,  but I think you should first do an 
"easy" diagnosis for a flap motor (potentiometer) malfunction, before 
you start pulling connections and removing parts. That simply is a 
matter of reading the Climate Control code values for two diagnostic 
channels while the car is warmed up and the heater is ON.

Below is an excerpt from an old post of mine--describing how to 
interpret the Climate Control (CC) channels 08 and 09 codes,  and it 
also mentions reading CC channel 01 as an additional diagnostic tool 
(to determine if there's a  potentiometer/flap malfunction). I assume 
you know how to read the CC diagnostic codes--if not post back here 
and someone will explain:

During normal operation. an equilibrium air
temperature eventually is (should be) reached--i.e., when the inside 
temp-sensor agrees with
the CC temperature setting. At that point the heater control flap 
(determining the mix of fresh and heated air)
has been moved to some particular "equilibrium" position, and
consequently the (feedback) potentiometer wiper sits at a particular
position on the variable resistor. With use over the course of years, a small
range of equilibrium flap positions tends to be used--over and
over--and hence that same region/area of the potentiometer surface is 
rubbed and
worn and becomes unreliable. [Please note that the potentiometer
might also have similar problems because of dirt rather than wear.] So,
for some range of flap positions, there can be considerable electrical "noise"
(or drop-outs) in the resistance value fed back to the climate
controller. Because of this, the CC thinks the flap position is in 
error, so it causes the
motor to hunt around (in ever-increasing circles) for the "right"
value--eventually giving up and parking it at its extreme position,
which, unfortunately, gives you lots of cold air.

You can observe this happening on the CC
channel 08, which displays the  feedback-potentiometer signal. 
Normally the channel
08 value should closely match the "target" value (which is 
temperature-dependent, of course, and is displayed in CC
channel 09). When this agreement can't be achieved (due to the 
potentiometer "noise" described in the previous paragraph), the error 
condition will also show up as a
reading of "07" (instead of "00") in the output of CC channel 01. 
You can get a good
idea of when the flap/potentiometer position reaches an unstable
region (i.e., bad spot on the potentiometer) because the 08 channel output
suddenly begins varying wildly and eventually settles in at a value of "255".
Then you know you're gonna get cold.

Phil Rose				Rochester, NY USA
'91 200q	(156K, Lago blue)
'91 200q   (70K, Tornado red)			mailto:pjrose at

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