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Intermittent battery drain
Posted to Technical Tips Forum on 3/3/2015 12 Replies

I had an interesting failure on this Merc Sable. The customer said that every few days her battery would be dead. I did the usual starting and charging system tests and checked for drains. After a half hour or so everything would power down and the drain was under 20ma.

I could not duplicate the problem. The second time I saw the car the customer asked if it could be an issue with her remote start system. It was an aftermarket system that was not working properly. We took a shot and disconnected it. Unfortunately about 6 days later I got the call again. Battery was totally dead. She had it jump started and drove it in.

This time I got lucky. I pulled the car in and left it running to check the charging system. I noticed right away that the A/C pipes were all frosted up. That seemed a little strange since it was only about 15 degrees out. I went inside the car to turn off the hvac and I saw that the switch was already in the off position. I rotated the switch back and forth and the compressor continued to run.

Finally, some much needed clues that helped to locate the intermittent drain. The problem was a damaged A/C compressor clutch relay. The cover was cracked like something heavy was dropped on it. When the cover cracked a small piece broke loose and was jamming the relay contacts. The relay power feed is hot all the time so when it jammed, the A/C clutch would stay energized even with the key off. This would obviously drain the battery pretty quickly. A new relay and all is well.

A/C clutch relay

Robert from Massachusetts

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car Vehicle Data

2003 Mercury Sable GS 3.0L

Engine3.0 L
Trans4-speed Automatic (Electronic)