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You Knucklehead, you caused this compressor to lock up!

By Richard Hawkins, MACS Contributor

This is part five of a series of articles about compressor lockups. If you have not read them yet, they can be found here. Part 1, Part 2, Part 3, Part 4.

Last week’s article ended with the following: “As I was completing the job, it hit me what had happened and my first thought was:  You Knucklehead, you caused this compressor to lock up!”  

“Please check back in next week for an analysis of what had occurred.”

What did the Knucklehead (me) do to cause the compressor to lock up?  The short answer is I left that jumper wire (referenced in part 3) in place and ran the defroster for about 20 minutes.   When the defroster is operated, the A/C of course also operates.

In the defrost mode, the A/C operates in the fresh air mode.  With an ambient temperature of about 25° F, there is very little heat load.  Even when the defroster warmed the inside of the van up, that 25° F air was still being pulled in from outside.  The way an automotive HVAC system is constructed, the outside air reaches the evaporator core before reaching the heater core.  If the system is operating properly, the clutch cycling switch on the Astro van will cycle the compressor off when the low side pressure gets down to 26 PSI and turn it back on at 46 PSI.

With the jumper wire in place, the compressor would not cycle off.  As a result, the low side pressure dropped down very low.  Of course, the lower the pressure gets, the lower the temperature of the refrigerant and the oil that is mixed with it gets.  I don’t know how low the pressure and temperature got, but it was low enough to stop the flow of oil to the compressor.  This was proven because when I replaced the compressor, I tried to drain the oil from it, and none would drain out.  It was either trapped in the evaporator or accumulator or maybe some was trapped in both.  Please see picture #1.

Picture #1. If I had taken the locked-up compressor from my van apart, the inside would have looked very similar to the one in this picture.

Now let’s go back to part 3 and the trip to Raleigh:  It is very likely that the compressor ran much longer than 20 minutes with the defective cycling switch causing it to run continuously.  That was indicated by all the ice that had built up on the accumulator and the huge loss of air blowing from the vents because of the evaporator being iced up.  I can only speculate, but it is likely that the compressor ran continously for an hour or more.  If it ran continuously for an hour, that is three times as long as it ran with the jumper wire in place and in the defrost mode. The question needs to be asked:  Why didn’t the compressor lock up then?

The answer would be because of the difference in the ambient temperature.  It was in the upper 80s or low 90s on the trip to Raleigh and about 25° F when the defroster was running.  I was likely running the A/C in the recirculation mode (and not bringing in the hot ambient air). 

However the air inside the van which was probably somewhere around 70° F to 75° F provided a lot more heat load on the system.  As a result, the low side pressure didn’t get down anywhere near as low as it did when running in the defrost mode with the jumper wire in place.

There are two morals to this story: 1. Never leave a jumper wire in place to substitute as a clutch cycling switch.  2. If you are working on a cycling system, make sure it will cycle.

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