My brothers 87 300E (gasoline motor) 190,000 miles, started downshfting on the highway for no apparent reason when the AC is on. He just had the auto tranny serviced : fluid change & filter, also just had a brand new AC compressor put in,new serpentine belt, new fan blower (fan only blows on highest setting!)

What the hell is wrong with this thing? I thought the fluid may be the problem. The guy who put the parts in (a moolighting Benz tech) said just to shut the AC off. Whats up?

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Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)

Good Lord! Who's your mechanic? Larry, Curly, or Moe?

An old Groucho Marx dialog:
Patient: "Doctor, it hurts when I do this" (patient bends elbow)
Doctor: "Then, don't do that"!

Seriously, it sounds like the compressor sensor hasn't been connected. It's job is to adjust the idle whenever the compressor kicks on. It may be since the electronics hasn't determined that a compressor is present, when the clutch engages, it senses a load and determines that it is due to a change in driving conditions and downshifts to give you more power.

Just a theory. Wait for another expert to reply to your post...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

Where is that compressor sensor??

When Curley put the blower moter in he didnt change some switch that is located under the blower. Now he has to go back in there all over again. So the fan only works on the max setting. Do you think this is related to the compressor switch??

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Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)

I'm am absolutely not an expert on the A/C on these cars, so I'm just guessing here. I know your model is equipped with one.

I have to go back and look at my manual to see where that sensor is located. I'll do some research on the A/C system as well and see what I can dig up for you.

My real concern is that I hope your mechanic is doing this work with the battery disconnected. The climate control unit (the one on the console with all of the switches) is expensive, and is prone to failure anyway as your car ages. We don't want to aggravate the situation by having him work on your car while the electrics are hot, as it won't take much on his part to fry the unit, or anything else on your dash.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car