You sent me a bad compressor. It does nothing when I turn it on.

This is something we hear from time to time, especially when dealing with DIY'ers or with people working on older cars. This usually stems from a lack of understanding of how these systems are energized OR a particular nuance about a specific vehicle.

About a month ago I received a call from a customer to whom I sold a BMW Behr Bosch wing cell (rotary vane) type compressor. The complaint was "My mechanic installed it and it worked fine for about an hour and then it quit."

"OK, what is the compressor doing?"

"Nothing."

"Nothing at all? Does the clutch engage when you energize it?"

"No."

"OK, tell him to disconnect it from the system and jump it direct from the battery."

"He tried that. It doesn't work."

"OK, tell him to test it with an OHM Meter. If the coil is open it is bad. If it reads between 3 and 5 ohms it is good. Also, tell him to check the in line diode and make sure there is continuity through it."

After a day another call came with the news that it still doesn't work.

"OK, send it back and we will check it. If there is a problem we will replace it."

The compressor was returned, and, as I suspected, there was nothing wrong with it. To be on the safe side, we replaced the coil because the potential for erratic or intermittent failure, though rare, is possible and erring on the side of caution is the preferred choice.

A week passed and a second call came in.

"It still doesn't work."

"Then something is wrong with your system or with the way your mechanic is proceeding."

"Would you please speak to my mechanic for me."

"Be happy to. Have him call me."

Louis called me about ten minutes later and assured me the compressor had a good electrical connection with adequate voltage. I in turn assured him the compressor was thoroughly tested and good.

"Well, Louis, when you have power to an otherwise good component and it does not function my first thought is that it is not reaching ground. Are you sure it is grounded?"

"There is only one wire on this coil."

"OK, so the ground is through the body. Does the bracket have bushings on it?"

After a pregnant pause the answer was "Yes."

"Is there a ground strap on the compressor or the bracket."

"No."

"Well that is your problem."

"But, but ...."

"Louis, I am sure that is your problem. Ground the compressor and it will work."

Some older vehicles, most notably Volkswagen's and BMW's, used two piece brackets which incorporated small rubber bushings between the bracket pieces. The bracket bolts passed through the center of these bushings which effectively isolated the bracket and compressor, preventing them from reaching ground. To overcome this a ground strap was used. Obviously, in this instance the ground strap was either missing or damaged.

There are many other reasons, too numerous to mention here, why an otherwise good compressor won't turn on. For more information on this topic please visit our FAQ's at:

http://polarbearinc.com/faqs.cfm

I hope this helps some of you who are working with or restoring older vehicles.

Thanks.

Joe