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A Change in Temperature

By Steve Schaeber, MACS manager of service training

Throughout the year, MACS receives a steady flow of tech support calls and emails from members. Some are, (admittedly) rather simple, asking questions about how much refrigerant goes into a certain vehicle or which oil is the right one to use for another. Others get a bit more complicated, such as why should an accumulator be changed when only the condenser was damaged in an accident, or how to source parts for a vehicle located halfway around the world. These are all good questions, but it’s the challenging technical queries we like best, particularly when they stump even the best A/C repair shops. 

We had one of these puzzles back in December.  It involved a Mercedes model not sold in the U.S. No worries though; we have dealt with equally unfamiliar systems and managed to figure them out. After all, A/C is A/C all around the world, and just because a particular model is unfamiliar to us doesn’t mean there’s a secret to how it works. 

The Mercedes’ service history included multiple replacement compressors, which is never a good problem to deal with, particularly for a knowledgeable shop with experienced technicians (we’d more expect a situation like this from a less experienced crowd). 

Multiple replacements usually indicate skipped steps along the way during a previous repair attempt. For example, a leak that got away, debris left in a system or a flush not thoroughly performed. 

We don’t think those rookie mistakes apply here, though. On their 3rd attempt, this shop replaced the compressor (because the old one was seized), and just to make sure, they changed out the condenser, dryer and TXV for a second time. 

So really, the only parts left were the lines, hoses and evaporator. This is when we start scratching our heads!

There are only two reasons why a compressor would seize, mysteriously or otherwise. They are: lack of lubrication and lack of temperature control (overheating). The first one makes perfect sense; if the oil is low or there’s not enough oil returning to the compressor to keep it lubricated, one of its internal components will fail. But a compressor can meet a similar fate if it’s not kept cool during operation. There could be enough oil, but if there’s not enough cooling refrigerant flowing back to the compressor (to maintain a certain operating temperature), the compressor’s oil can become “cooked” and lose its effectiveness. 

That is what was happening with this Benz. We suggested that the shop use temperature testing to find the cause of the seize. 

Temperature Testing

Using a contact thermometer, they quickly discovered 50° on the suction line at the expansion valve, and down at the compressor it read 96°. Along any length of hose or tube, there should be very little change in temperature. The problem HAD to be somewhere in that line. By moving the probes closer together, slowly, along its length, we pinpointed the problem to a rubber hose section. 

The technician said his biggest surprise was the lack of any outward visible sign of damage present on the hose. However, when he cut the hose open to look inside, it was clear the inner liner had become partially detached from the outer hose material. We call this “hose delamination.” Depending on how the condition happens, and on which part of a hose, it can wreak havoc for diagnosis. 

Even the best of us get stumped a time or two, so when it happens, don’t let it get you down. And – if your shop runs into a tricky A/C problem, remember your association is here to help.

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