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

Bernie Benz b.benz at
Fri Jan 3 12:06:49 EST 2003

Hi Phil,

I have no BTDT experience with the heater flap motor or its feedback
potentiometer, but have cleaned and reworked other fb pots, such as throttle
position.  The pot needs to be opened, cleaned, inspected, and relubed.  If
the resistance trace is worn thru, one can alter the wiping radius of the
wiper to a good arc of resitance material.


> From: Phil Rose <pjrose at>
> Date: Fri, 3 Jan 2003 13:20:07 -0500
> To: 200q20V mailing list <200q20v at>
> 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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