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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