A/C Compressors: Extending Their Service Life

December 16, 2008 | By | Reply More


What can we say about the A/C compressor? Believe it or not, it is one of the hardest-working components on the vehicle. Its toughest job is to generate cooling system pressure that provides air conditioning, yet spin at a multitude of speeds ranging from 750 to 7,000 rpm. Think about it:  If you turn the A/C on at highway speed, it has to go from zero to several thousand rpm instantly. So, it needs to have proper lubrication as well as the correct refrigerant charge. Mechanically, it needs to have sufficient grip from the belt that drives it.

What am I working on?

Register Testing

There is nothing like having the right tool. This meter allows you to run two thermocouples at the same time. You can then watch outside ambient temperature and compare it to the inner air register temperature to get a precise result of how effective the A/C system is.

Over the years, BMW has used several manufacturers’ compressors, and different construction types. Within the last two decades they have primarily been Behr, Bosch, Nippondenso (Denso in America) and Seiki-Seiko. In the last decade the company has settled on Nippondenso and Seiki-Seiko. Chances are if you see a Seiki-Seiko compressor, you are probably dealing with the rotary vane type. If you see a Nippondenso compressor, you are dealing with a different animal. These compressors are “swash plate” designs, otherwise known as variable displacement compressors. These vary their output depending on high- and low-side pressures and this needs to be taken into account when using system pressures to diagnose a problem. The Seiki-Seiko compressor is a straightforward compressor design, so conventional diagnostics apply.

Most of the BMWs you will see have IHKA, which is computerized climate control. These systems will have the variable displacement compressor. Since the compressor can vary its output depending on load, it does not need to cycle on and off as much so fuel mileage and emissions are better controlled. How does this swash plate work? Well, the swash plate has all five pistons attached to it. By tilting the plate at more of an angle the piston stroke is increased and more output is produced. If the angle of the plate is reduced, then the piston stroke is also reduced, thus lowering output. What controls the swash plate position? A control valve built into the compressor receives low-side pressure. If the low-side pressure is high, this indicates a high load on the system, and it positions the valve to allow high-side pressure to move the swash plate to maximum stroke (i.e. high output). Once the low-side pressure drops, the control valve is moved to block any high-side pressure from the swash plate and the plate returns to its minimum stroke (i.e. low output). Basically the high-side pressure moves the swash plate in or out and the control valve uses low pressure to determine how much high-side pressure makes it to the swash plate.

The diagnosis


A quick look at the compressor helps identify what you


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Category: BMW TechDrive, Cooling Systems

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