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Old 06-20-2017, 06:20 AM
PlatinumV8 PlatinumV8 is offline
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Location: USA
 
Join Date: Apr 2017
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Mein Auto: 7 Series
Post Warm air from AC vents

It's that time of year again and I thought I'd share a hint with those of you wrestling with the 7 series warm air from the vents at stop lights and bumper to bumper summer construction traffic.

The DME (computer) algorithms were optimized for rich people commuting from estates to their offices and back, at high speed on German autobahn highways. The AC clutch is disengaged at low idle rpms and no matter what other systems you think may be causing your warm air with the AC called for from the cabin by occupants, the first thing you need to satisfy is testing with engine rpm above warmed up idle.

Many see cold AC on the cold start and then complain that while driving it blows warm at lights and gridlock. Hold idle above 600 and with all other systems operational, the AC will blow cold again within 30 seconds. To verify AC clutch engagement, clip a test light to the exposed wire section of the clutch power wire where it attaches to the compressor (BMW left that section bare of insulation for a reason) then ground the test light to the chassis and when the light glows the DME has had all its program requirements met and commands AC clutch engagement.

Many have added freon, changed pressure switches and paid for new compressors when it was simply the design of the car confusing traditional AC diagnosis. Cold start programming bumps the rpm to speed up catalytic converter heating for better emissions, satisfying the rpm requirement for AC compressor function. Once the car is warmed up, blip the throttle and you will watch the test light go out. Exactly the circumstances when crawling along in pedestrian (poor people) traffic in the Americas.

After you satisfy the above 600 rpm requirement and still have warm vents, move on to verifying that the auxiliary cooling fan in front of the radiator is functional. That can be tested by shorting the two leads of the lower radiator hose coolant sensor. Shorting the wires fools the DME into seeing a catastrophic overheat condition and commands the aux fan to full speed. It will sound like a jet engine and should only be run at that speed for a second or two.

If the aux fan is working, move next to refrigerant levels and pressure switch issues with a professional shop so that the freon can be recovered and replaced, which actually will save you money as well as protect the environment.

Good luck and stay cool everyone!
 

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