w210 99 E320 climate control fan problem
 

The climate control fan on the 99 doesn't increase speed normally with the rising engine temp - from time to time. It starts blowing when the engine reaches operating temp, or whenever it wants to. So when it's calling for heat, the fan speed indicator on the display will start filling in normally as the fan increases in speed -- even though the fan isn't on. I can feel warm air coming from the vents, but that's just air drifting in over the heater core.

Sometimes it operates normally, right from the start - no problem.

My guess is that there is a temp sensor somewhere that's gone wonky.

Any thoughts?

Blower Resistor?
 

I have had to replace Blower Motor Resistors (or Regulators) on two of my cars, and i think there's a good chance you may need one as well.

I just did one last week. To verify my diagnosis, I accessed the blower motor (under the glove compartment) and put a voltmeter across its two inputs. I could then see that when the bars on the CCU display were climbing (calling for a faster blower speed) there was in fact no voltage on the motor's terminals. But then after a while all of a sudden there was, the blower was working, and I could manually adjust the speed up and down, and watch the voltage go up and down. If there had been voltage with no motor activity, I would have suspected the motor.

I suppose that my test didn't rule out an actual problem with the CCU, but the resistor is the usual culprit in these scenarios (and also the least expensive). A new resistor fixed my problem.

When my W210 had the problem a few years ago, I learned that MB had changed the design and required a new resistor and motor. I got an old resistor from a boneyard instead so that I wouldn't have to replace a perfectly good motor. This was before I discovered this forum - I think I read here once that Phil can get the old style resistor, in case you find yourself in the same situation.

Sounds like the resistor is my problem. Is it located in the line near the fan or on the fan?

I'm pretty sure that it is bolted right onto the fan, though I did that job over 5 years ago or so.