cpu control of blower fan

[wicked94pgt]

Does the climate control computer tell the blower fan when to activate based on the temp at the heater core? Will it keep the fan speed very low until the core is actually hot enough, then ramp up the fan speed?

I've noticed this in the past when the coolant level was low. I would creep along on back roads and then get on the freeway and the fan would start blowing strong. I assumed that the higher revs would allow the water pump to push enough coolant up into the core to produce heat, despite the low coolant level, and that would trigger the computer to turn the fan on/up.

Is that what's really going on? I'm experiencing a much milder form of this now. The fan often speeds up a lot going from sitting at idle to moving. Does that mean I'm not getting enough coolant through the core at idle?

As always I'm going to try bleeding again but I don't think that's the issue. I'm afraid my water pump may have been compromised when I overheated last summer and can't quite push enough coolant at idle. Or, a piece of my expansion tank that shattered got stuck somewhere and is restricting flow.

Could the heater valve also be a culprit?

Other than this minor "symptom" the heat works great and produces LOTS of heat once the car is warmed up.

1996 BMW 328i
2003 BMW F650CS - R.I.P. 8/25/2004
2005 Suzuki SV650S - 3/26/2005

[Steve G]

Your '96 ventilation controls are very different from my '95.

I think you may have auto fan, which usually works as you describe.

But isn't there a manual over-ride? Can't you just turn up the fan and see how much heat you get?

[wicked94pgt]

I'm just wondering why the fan speed is varying with engine/vehicle speed. I'm suspicious that the core is still not getting enough coolant through it idle and that's why the computer keeps the fan speed low.

You're right that you can manually override the auto control and set the fan speed constant. Even if my computer was doing what I suspect in auto mode, I think you're right that in manual override it should stay constant regardless of core temp. Maybe it is blowing much cooler air at idle and I just don't know it with the fan barely moving.

That should have been obvious, duh. Thanks for pointing that out.

1996 BMW 328i
2003 BMW F650CS - R.I.P. 8/25/2004
2005 Suzuki SV650S - 3/26/2005