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Why did the compressor lock up?

By Richard Hawkins, MACS contributor

All too often when a compressor lock-up occurs, the compressor itself is blamed for the lock-up. While it is possible for a manufacturing defect to cause a lock-up, there is usually a problem related to the service work or a problem with the system that is the culprit.

I have had a multitude of tech calls with shops over the years about this subject and usually someone is requesting a warranty because the compressor is allegedly defective. When deciding to write about this subject, I recalled a conversation I’d had involving a Lincoln Town Car limo. 

2009 Lincoln Limo

This limo had been through about 5 or 6 locked up compressors in a period of about 2 months. The limo company’s manager was at the parts counter of a facility that both sold A/C parts and had a top-notch A/C shop. He was complaining about the compressors being “junk” because another lock up had occurred.  It just so happened that the lead technician had come to the parts counter to get a part and he happened to hear the conversation.

The problem

He spoke with the limo company manager and got some more information about what had transpired and then told him directly: “You don’t have a compressor problem. You have another problem with that system and until you diagnose it and fix it, compressor lock ups are going to continue”. The limo company manager was “taken aback” a bit and replied in a bit of an forceful way:  “What do you mean I don’t have a compressor problem?  If the compressor is locked-up, then there is a problem, and it needs to be replaced. “

The technician explained to him that yes, the compressor was locked up, but the fact that this had happened so many times in such a short period of time indicated that another problem with the system was causing the issue.  

At that point in time the limo company manager became a bit more cooperative, and the technician was able to get some additional information about how the system was serviced and didn’t detect any issues with any of that.  He was also able to speak to the driver of the limo who had experienced the last two lock ups and get information about when they occurred.  In both cases he had been driving back from a higher elevation in the mountains where temperatures were milder, and the compressors had locked up when going through a valley.

The technician convinced the limo company manager to bring the limo over to the shop after they installed another compressor for some diagnostics. A couple of days later when the limo arrived at the shop, the technician took the vehicle for a long test drive.  

Initially everything seemed to be working OK. The system was blowing nice cold air from the vents, and it was cycling.  However, after a while, a problem surfaced.  The air coming from the vents was much colder than normal and the system was not cycling. The technician suspected a problem with the clutch cycling switch or the clutch relay. To better determine what might be going on, he got two test lights and made up some long test wires. He then connected one of the test wires to the wire powering the compressor clutch and connected the other test wire to the wire running from the clutch cycling switch to the clutch relay.

There is nothing “high tech” about test lights and test wires. They can be very useful when diagnosing electrical problems in circuits such as those powering a clutch and controlling a clutch relay. They should not be used on low amperage circuits with electronic components in them.

He ran both wires back into the passenger compartment and connected them separately to each of the test lights and grounded them. Now it was time to go for another test drive. This test drive went like the previous one. Everything was working fine initially. The compressor was cycling and when it did, both test lights would turn off.

As soon as the compressor turned back on, both test lights would light back up. After about the same amount of time as on the previous test drive, the technician detected that the compressor was not cycling. The air from the vents had gotten extremely cold and the compressor was running continuously.

However, instead of both test lights being lighted, the one connected to the test wire running to the compressor clutch was on. The one connected to the test wire running to the wire connecting the cycling switch to the clutch relay was off. That indicated the clutch cycling switch was working but there was an issue with the clutch relay.

Check back in next week and we will explain further what was causing the compressors to lock up.

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