feedback (very lonnnng) on the heater flap feedback-potentiometer

Phil Rose pjrose at
Fri Jan 3 13:20:07 EST 2003

Recently a number of us have had heater-control problems, and a
lister logically suggested to clean up the electrical connector to
the heater control-flap motor. So, I did that a couple of times
(using Caig Pro Gold); however in my case there was no improvement at
all. I then carried out the electrical diagnostics recommended in
Bentley, and the result was advice (from Bentley) to replace the
combination "motor with feedback potentiometer". By observing the
heater flap operation (while a helper changed temp settings in the
car) I could see that the motor seemed to respond OK. What I
hypothesized (and verified during a long trip in cold weather) is
that my particular problem seems to be with the motor's feedback
potentiometer--not the motor itself. So in the meantime (before
actual fixing/replacement is possible) how best to temporarily work
around the problem?

First of all, what's going on? During normal operation an equilibrium
temp eventually is achieved when the inside temp-sensor agrees with
the temperature setting. At that point the heater control flap
arrives at some corresponding "equilibrium" position, and
consequently the (feedback) potentiometer sits at a particular
position on the variable resistor. Over the course of years, a small
range of equilibrium flap positions tends to be used--over and
over--and hence that same range of the potentiometer is rubbed and
worn and becomes unreliable. [Please note that the potentiometer
might also cause similar problems because of "dirty" vs "clean"
sections--i.e., sort of the inverse of the "worn-out" situation.] So,
for some range of flap positions, there will be considerable "noise"
(or drop-outs) in the resistance value fed back to the climate
controller. The CC thinks the position is in error so it causes the
motor to hunt around (in ever-increasing circles) for the right
value--eventually parking it at its extreme-frustration position,
which is lots of cold air. You can observe this happening on CC
channel 08, which displays the  feedback-pot signal. Normally the ch
08 value should closely match the target value (displayed by CC
channel 09). When this isn't the case, the error shows up as a
reading of "07" (instead of "00") on 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
suddenly begins "hunting" wildly and eventually settles in at "255".
Then you know you're gonna get cold.

So, how to avoid freezing/boiling until something is fixed? When the
cabin suddenly starts getting very cold, people naturally react by
_in_creasing the temp setting by at least several degrees. Sure
enough the heat starts to come back--eventually getting much too hot.
So we then start to back off on the heat setting, and the complete
shut-off problem repeats. What I recently found is that taking the
opposite tack with the temp setting was more effective for me in
getting reliable heating--namely I decided to back off from my usual
temperature setting. In my car, the problems (at least when outside
temps were around 30-35F) seem to be with settings in the range of
68-72F. I found  that I could get good long-term heat by _lowering_
the temp setting from there (instead of increasing). Sure, it's
cooler than you'd like but for me it's better than freezing or
overheating, and IMO if you can maintain something near 65 it's not
too bad (some might want to wear gloves?) I could get reliable
heating at either 64, 65 and 66F, whereas there were definite
problems beginning with 67F and warmer. Naturally this also be
affected by the outside temperature, since the "equilibrium" position
of the flap (for any desired interior temp) also takes into account
the outside air source. Like I said, it's just a temporary
work-around. Something needs fixin'.

So, here's one for Bernie (but do you ever need the heater in
Nevada?)--If it's just a dirty potentiometer contact causing of the
problem, I wonder if it can be accessed for cleaning as an
alternative to replacing the motor assembly? I suspect not and that
I'll be needing a new motor.  Anyone BTDT (clean the potentiometer)?
Or replace only the potentiometer?


Phil Rose				Rochester, NY USA
'91 200q	(135 Kmiles, Lago blue)
'91 200q   (58 Kmiles, Tornado red)
	mailto:pjrose at

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