Batman 357

Any guesses would be a good title for this post. My 82 911 SC has over 200k on the clock and runs well. After lots of time and money the AC works, kind of... At stop lights with AC on the plug in amp gauge will read in the low 12's. The temp guage will be almost halfway between the middle and higher marks. After a bit the idle will oscillate and motor will die. If I try to keep revs up it will still oscillate. Battery is a high end 730 CCA and less than a year old. Alternator is about 2 years old. AC re do is 2 years old and included a rear fender well mounted condenser with electric fan. Vehicle also has an upgraded oil cooler with electric fan. According to a $ 30.00 Amazon guage the battery and charging system are good. I would like to be able to use the AC in town. Right now that just won't work.

My guess is the extra electrical demand is too much for the alternator. I don't know what amperage alternator was installed. Is there a way to tell the output. Will a 75 amp replacement fill the bill or do I need to kick for the 175. With the AC off the car is solid. Highway with AC on picks up the temp a bit but car is still good.

All thoughts appreciated...

Pavegeno928

I ran a full four-fan Griffith's a/c on my original alternator for more than a decade. I would note volts in the low 12s but everything kept working fine and the a/c kept me very cold even in the summer.

However, I upgraded to the Classic Retrofit alternator about two years ago and now the volts never dip below the high 13s. Here is the link for an 82: https://www.classicretrofit.com/en-u...75a-alternator

Batman 357

So alternator output was low. Ground strap was very thin. New alternator and much heaver strap helped voltage readings. Still won't run AC in traffic. Dies between shifts. Chugs and starts once higher gear engages. I can live without AC in traffic jams. I'll have to..

Pavegeno928

Sounds like time for a bigger alternator.

systemsc

Very likely your A/C compressor is loading (low RPM) the engine too much, which results in a lower voltage output from the alternator.
It's also possible that your fans are drawing too much current. A higher output alternator will not solve your problem.
Before wasting money on buying parts, do some troubleshooting measuring voltages and currents at various points in the electrical system.