AC, battery, alternator theory/question!

[gtex]

I was feeling pretty good about the car recently, after getting the A/C 0-rings replaced and getting the 998 recall sorted out. but then driving home last night, I stopped for beer, left the fog lights on with the car off for about 5 minutes and the car would not start.
For the better part of the last week I was in Mexico and the car was on a trickle charger. So, the battery should have been in good shape. It had no issue getting me towork, and starting the car after 10 hours at work. (Note, it's hot in Texas)

Something I've noticed since I replaced the compressor o-rings for the AC, the system appears to be holding the charge, and it blows cold most of the time. But, occasionally it's as if the compressor shuts off and it blows warm for a bit. this may be happening more often at lower engine speeds (i.e. 5th or 6th gear on the highway).

is it possible when the engine speed drops that the alternator output drops then the compressor clutch can't stay engaged? or maybe the compressor is difficult to turn now and the belt is slipping, allowing the alternator not to spin therefore running the battery down?

battery is only about a year old.

any ideas appreciated,

greg

 

[Bugman Jeff]

It's probably just a coincidence that you just started using the AC and started having charging problems at the same time. It has been reported a few times that the alternator doesn't work well at lower RPMs. By trickle charging it for a week, you brought the battery up to full charge. As you've been driving it, it's been slowly discharging and it finally got down to the point where it won't start the car anymore.

It's easy to test your charging system at idle. Start the car and measure the voltage at the jumper posts in the engine compartment. Then turn on all your electrical stuff(ac, headlights, etc) and see how far the voltage drops. Raise the RPM to typical highway RPM, and see if the voltage climbs to an acceptable level.

 

[coupe]

As stated, you'll have to measure some voltages to even begin to diagnose if there's a problem.

 

[gtex]

my battery is shagged again. hoepfully it's at least got some warranty left. Even on the trickle charge, when it says it's charged, it will not turn the motor over. Must have lost a cell. I'll replace the battery next week and beging looking for the killer next week.