BIMMERMIKE

I've read Wally's Tips, researched Rennlist forum and now pleading for help. I can't control or vary heating or cooling, always full hot When I change the temp slider on HVAC head the arm on the mixing motor doesent change can't hear the motor turning when it was working.
Here's what I checked and replaced;

-outside sensor is good, correct ohm readings when varying temp with blow dryer
-I replaced bullit connectors with new spade(male/female) terminals
-checked for resistance at flap mixing motor pins 4 and 12 and good resistance numbers when changing temp slider at HVAC control head unit
-rebuild HVAC control head
-checked vacumm lines(at soleniods) to actuators all hold vacumm
-checked vacumm at check valve off brake booster
-replaced heater valve,also applied vacumm and heater valve arm changes with vacumm.
-inside temp sensor fan is working
-all solenoids at vacumm lines seem to work, I switched electrical connection and vacumm line of water valve and no change when sliding temp control on HVAC unit, water valve fully open.

Could the mixing flap motor be bad? How do I check?
Does anybody have a spare unit?


Any help would be greatly appreciated , this has been driving me crazy for long time. It worked intermittently up until till the fall of 2010.

Thanks


Replaced mixing flap motor and now all is working as it should.
CHECK YOUR MOTORS!

Thanks for all your help.

SeanR

Sounds like it is a leaking solenoid or more than one in the center console. If everything else tests out good, which it sounds like you have done, that is the only other thing I can think of.

Mrmerlin

remove the HVAC control head and open it,
look for a burned wire trace on the circuit board

BIMMERMIKE

The solenoids control vacumm to actuators(open /close)right? I can control :defrost, footwell, and center vent air by slider on HVAC head.
The white vacumm line and electrical connection was interchanged with another know working solenoid and no change. I can feel and hear clicking when changing slider

BIMMERMIKE

This is a rebuilt unit from 928 Intl receievd yesterday.
I replaced the orginal with an Ebay unit before finaly buying a rebuild.

Mrmerlin

Um OK,
check the setting motor on the left of the heater box when you move the temp selector does it move?.
check with the key in the run position

BIMMERMIKE

Just updated my origianl thread....no movement at all.
When it worked I could hear the motor and of course feel the arm movement with key in run or on positions.
I've read that mixing motors rarely go bad but I'm wondering if any way of checking operation.
With a bad motor, would that cause water valve to be open?

SeanR

There are also two micro switches inside the head unit that can have an effect on this. When you move the temp slider, there are contacts at the extreme of each. I've seen a few times, the one on the far left come out of the holder so it would not make contact with the slider.

BIMMERMIKE

Sean, the unit is a rebuild from 928 int, also with previous HVAC unit I carefully inspected the temp slider touches micro swithces.
I'm wondering if mixing flap motor has gone bad? Any way of checking with multi meter? IF mixing motor is dead would this cause the water valve to stay fully open?

Bill Ball

Mixing motors do go bad. There are a couple of transistors that go bad. There is a thread on this but I can't find it with search now. Anyway, if the motor does not move when you move the temp slider, that's the likely culprit.

BIMMERMIKE

Anybody with a known working mixing motor?
After spending multitude of hours checking and rechecking I started looking a possibilty of removing that motor.....any checks before removing?

StratfordShark

Yes a transistor can feel on circuit board in mixing motor - you've checked for correct resistance variation at the motor connector so very likely to be problem.

Motor easy to detach but uncomfortable wriggling head in and out of footwell - held to bracket by couple of bolts then easy to open. You could just desolder each transistor (think there are 3) and test off board. Either use dedicated transistor tester or multimeter on low resistance range to verify base to collector and base to emitter behave as diodes (conduct one way but not when voltage reversed).

I suppose motor could be bad, but check driver transistors first as likely fault lis there.

BIMMERMIKE

How difficult to remove motor? Once I have the motor out I can test as noted although you lost me at 'dedicated transistor tester or multimeter on low resistance range to verify base to collector and base to emitter behave as diodes (conduct one way but not when voltage reversed).' LOL

Perhaps I need to source a known working motor and connect for operation when moving temp slider!!

Imo000

Mike, or somone else,

If you or they can post a pic of the mixing motor, I can see if I have a spare one that we can bench test and then swap it out.

BIMMERMIKE

Here is a picture.........