Blower Fan Not Working On Auto Setting, "3 dials" semi-climate control

[Roger]

Hello folks,

The 3-dial semi-climate control on my 1990 build XM is not working properly. The blower fan does not work on the auto fan speed setting. I can get any fan speed I want by turning the dial manually, but the auto setting gives me nothing. Also, it will deliver only ambient air, not heated air. The airconditioning compressor kicks in fine, so I can get cooled air. This "no auto fan, no warm air" problem was intermittment a couple of times earlier this year (it started working again after a couple of days), but has now been continuous for the last couple of weeks (ever since I did a bit of messing around when I had to replace the ignition module - it's a V6 12V auto). I've checked the fuses and they are fine. There is no corrosion under them.

Roger

PS I'm a regular contributor on aussiefrogs but thought there might be a bit more XM knowledge here.

[Zaphod]

Have you checked the cabin temp sensor in the roof (its a cylinder in the roof light console), this should have a fan on an electric motor in it. Not sure excatly how it controls the heater fan speed, but it defiably controls the temp and I wodul imagine it woudl also have some say in the auto fan setting

Stewart

[Peter.N.]

Hi Roger

There is a servo motor that controls the air input flap on the heater, this motor can fail, often intermittently. If its stuck in the cold position, it will give you cold air from the aircon but no hot. Its quite easy to get at, under the glove box.

This wouldn't account for the lack of auto operation though. My green estate does not work on auto, but it doesn't bother me in the least, in fact the SX hatchback has a purely mechanical set up which I prefer as it has a dual control system (seperate controls for each side) and is infinitely variable - doesn't stop regulating at 26C.

The lack of auto is probably a fault in the control panel, you should be able to get a secondhand one and change it over.

Peter.N.

[Roger]

Folks,

I finally fixed this problem this morning, by installing a new secondhand heater flap motor. Peter, you were right, that motor was the problem. Interestingly, the motor failure did cause the auto fan speed control to stop working. Apparently, if one of the sensors or actuators fails the control unit is programmed to disable the auto fan speed control. The flap motor eventually failed completely in the full hot position and it was starting to get uncomfortable with winter ending! So I had to fix it. Now I have auto air temperature regulation and auto fan speed regulation. Just like a new one. Except for all the other little problems.

Cheers,
Roger