Variable Compressors and Beyond

There is, as they say, nothing quite like being there.

When it comes to Mobile Air Conditioning
Society Worldwide tech sessions, nothing could be more accurate.

Among the many sessions offered earlier this year at the association’s annual conference, Dave Hobbs’ talk on
Variable Displacement Compressors (VDC) is a fine example of why.

While billed as a technical discussion on that component, the actual session was really better described as an open exchange of information.

There were 10,000 years of experience in the room and probably 9,999 opinions, said Dave Hobbs to kick off his interactive session.

Because he was uncertain of the level of technical proficiency that everyone was starting at, he began his discussion with a very brief rundown of the basics of the a/c system, taking less than a minute to outline what the high side and low side are, and things got lively with answers on what kind of throttling device was found on a VDC system.

Hobbs started with the basics of the high and low side and the a/c system, pointing out a component and asking whether it was on the high side of the system or the low side. It would seem basic, but looking around the room one was reminded that some attendees were there to expand their already considerable knowledge, with others there to begin their learning curve.

Responses were called out from the floor on this and other points, and then things got lively.

As Hobbs ran through the basics of VDC designs—piston, scroll and rotary vane—and which models were found on which applications, the discussion slowly shifted from a tech session to a workshop, with experienced techs bringing up points on troubleshooting and real-world situations.

Rotary vane compressors, for example, have a habit of “sticking” when left stationary for long periods of time, or if there is too much oil or the wrong oil in the system.

The prescribed fix is to bring the refrigerant charge to 50 per cent, bring the engine rpm up to 2,500 or 3,000 rpm, and cycle the clutch on and off. Essentially, you’re looking to shake the vanes free.

From the floor the point was made that it is wise to advise the consumer that this may be a short-term fix or a long-term fix. One attendee said he doesn’t do the 50 per cent charge and uses a full charge, but otherwise adheres to the same strategy to good effect.

True to Hobbs’ opening salvo about the experience in the room, the discussion broadened to many topics including hybrid maintenance practices, tools and why some systems cool their best just before they die.

Hybrid vehicles: Use a Class 3 meter, proper gloves and get out of the habit of checking a circuit with two hands. Instead, on a high voltage system, use an alligator clip on one probe and then probe with one hand, the other hanging free.

This prevents a circuit from running through your body and your heart should there be a short.

Check Your Refrigerant: Check your scale yearly. Ensure that you are putting in the amount of refrigerant you think you are. With smaller-capacity systems, accuracy is even more critical.

Watch Belt Tension: Belt slippage can kill a compressor. The slippage will create heat. Clutches have a grease seal and when it goes liquid, you can lose the grease from the system and then the bearing will fail not long afterward.

Prelube VDC Piston Compressors: The compressor has a wobble plate that runs on a brass bushing. When you see evidence of that brass bushing in the orifice, it is evidence that the compressor was not properly prelubed. It is critical to prelubricate the compressor.

There is a tool (Delphi CB10049, Carquest EQP67700) for turning the compressor that is in fact designed to be “Compatible with most industry V5-, H6- and V7-style compressors.” Because it allows circulation of the compressor lubricant, the tool helps prevent new compressor damage at start-up.

This discussion on rotating a compressor led to a discussion on how the operation of variable displacement compressors is still misunderstood, causing comebacks and unnecessary rework.

Techs, for example, are used to rotating a compressor by hand before installation, feeling for resistance, to give them confidence that the unit is good. On a standard fixed displacement piston unit, this is an easy, reliable check.

On scroll and vane variable displacement units, this method will yield nothing useful. Unless you can hand-rotate the unit at a few thousand rpm, that is.

Scroll and vane VDCs simply do not build pressure when rotated by hand, and there were many stories around the room of having to deal with technicians who had trouble understanding this.

“I had a technician call to complain that he couldn’t get the clutch to cycle, couldn’t get it to disengage,” offered a supplier in attendance. “Our representative explained to him that there was no clutch on the system. The technician said that clutchless compressors don’t work.”

And, at this, it was almost as if the room let out a
collective sigh.