no heat, help please
pjrose at frontiernet.net
Thu Nov 1 06:56:31 PDT 2007
At 11:47 AM -0700 10/31/07, Tom Egbert wrote:
> A/C system still works normally but no heat in the chilly morns.
>No codes from the A/C control head.
> Can I manually apply vacuum to the heater control valve and
>eliminate some possibilities?
It's possibly the heater control-flap (motor) that's not working
properly. Although you state that there are "no codes", look again to
verify that--especially look on CC channel 01, which--while the
heater is malfunctioning-- should give a reading of something
different than "00". You must read the diagnostic code while the car
is warmed up AND the heater is turned on (calling for heat, but
actually blowing cold air). It should read "07" in CC channel 01 if
there's a flap-position (potentiometer) error.
Also, you should first do an "easy" diagnosis for the flap motor
(potentiometer) function, 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).:
>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
'06 A3q 3.2 V6 (5K, silver)
'91 200q (156K, Lago blue)
Gone to Audi Valhalla:
'91 200q (Tornado red)
'89 100 (Bamboo gold)
More information about the 200q20v