'92 300SE

Some will remember I've fought numerous battles with the A/C in my car. Biggest problem of late was the tendency of the A/C to dump most all the airflow to the windshield as the cabin temp approaches the set-point on the control.

In cooler weather, like in the early AM, this results in a windshield that "mists" up on the outside, requiring constant use of the windshield wipers. Curiously, this tendency is only on the driver's side. Cranking the temp down colder will cause the air to move back to the dash vents, but then I get too cold and have to dial it back up again - and the cycle repeats.

Digging through AllData, I found a TSB from MB indicating that this was a known problem on these cars, and that MB came out with different software for the controllers - the solution back then was to replace the control with a newer model that had the upgraded software.

There was a series of part #'s suggested, depending on what year model the car was and whether or not it had rear A/C.

I got the list of part #'s and found an exact match for the part # that was most recent in the list, which seemed to be one that would work for both rear and non-rear A/C vehicles.

Installed it and viola! - no more misting windshield. Pulling DTC's from the unit reveals that I either misread, or AllData's documentation is wrong - but this unit was apparently from a car with rear A/C, because one of the codes indicates failure to communicate with the rear unit. I figure no big deal - let it complain - so long as I don't have that problem with the air going to the dash.

Latest issue - and I can't believe I'm complaining (actually, I can believe it) is that when it's really hot outside, the car gets too cold on the inside. I usually set the temps at 74/75. The other day I notice that the fan was running pretty fast and that I seemed a little cool. It was easily 90+ outside. A quick check of the sensor readings from the control revealed an in-car temp sensor value of 68F!!!

It did the same thing again yesterday - outside temp (on the instrument cluster) indicated 103F. Temps set at 74, inside cabin temp 70.

Curiously, when this happens, the outside temp sensor (from the control sensor readings) is usually 8 or more degrees higher than the instrument cluster reading. I find this hard to believe - the instrument cluster reading comes from the front of the car, less than a foot from the concrete. The control sensor is in the fresh-air duct up under the hood, where it pulls air from the base of the windshield. I've recently replaced all of the foam-rubber seals in that area, so I'm relatively certain that the fan is pulling only outside air - no underhood, superheated air.

I know that the A/C makes decisions based on a combination of values that include the in-car temp sensor, as well as the outside temp sensor. There is no sun-load sensor on this car.

Maybe a faulty outside temp sensor is "goading" the A/C into thinking that it needs to overcompensate to work against the outside temp?

I think I'll just replace that sensor - can't be that expensive, and see what happens.

Any other ideas?