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

Gary Anderson wanemardo at
Fri Jan 3 21:16:42 EST 2003

Phil, and Listers....
Seems several of us (including lurkers) are having this problem.
In short I have experienced your symptoms, and the opposite (too much
heating, or too much cooling) and occasionally its just right. (sounds like
a Goldilocks and those three Bears!).  Pulled the codes, and notice the same
reading you saw.

So, I took a look at the "spare heater assembly" (courtesy of my parted out
wreck), and here is what I found inside the left side cover.  There is the
suspect motor/pot assembly (~2"x 2.5"), where there is a small 3 wire pot
attached to the output shaft of the assembly.  Pins #5,4,3 in the 5 pin
connector connect to the pot, with #5 being common.  My cheapie multimeter
shows pins 5-4 with 5ohms at the cw end travel, and increasing to 1k ohms at
the mid point and beyond.  From the other end (ccw) of the pot, pins 5-3
ranges from 5 ohms to 1k at the mid point and beyond to the other (cw)end of
the pot.  The increasing value from 5 to 1k ohms is not linear, more like an
exponential resistance increasing rate (turning cw and ccw).  I could not
determine if there were any finite skips or breaks in the pot readings from
this unit.  I recall it would occasionally "act funny" on occasion before
the car  was no longer road worthy.

I have some digital pix if anyone wants to see the motor interior parts
(motor, gears and the suspect potentiometer), let me know offline, and I
will send'em to you.
Also looking for an answer from you good electronics guys!
Gary Anderson in Sammamish

Ps: Lotsa 200 parts in the garage, ready for new homes, body and motor gone

----- Original Message -----
From: "Phil Rose" <pjrose at>
To: "200q20V mailing list" <200q20v at>
Sent: Friday, January 03, 2003 10:20 AM
Subject: feedback (very lonnnng) on the heater flap feedback-potentiometer

> 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
> --
> Phil Rose Rochester, NY USA
> '91 200q (135 Kmiles, Lago blue)
> '91 200q   (58 Kmiles, Tornado red)
> mailto:pjrose at
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> 200q20v at

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