Bad alternator or bad voltage regulator?
 

On my 1996 850, with the AC on, the AC compressor short cycles on and off when the engine is under a load such as going uphill. My mechanic tested the charging system, and found that the alternator is not putting out enough charge when under a load to run the AC compressor. Can I rectify the problem by installing a new voltage regulator as some sites have suggested, or might I be looking at an alternator or both. The car runs beautifully with the AC off, but maybe after 20 years, the alternator is due for replacement.

not buying this - if the charge indicator lamp is off, then the alternator is probably not the problem. Going up hills does not put any additional load on the compressor.

If you're having a problem with the output being less than optimal replacing the regulator will likely cure the problem.

I'm not sure I buy the the alternator as being the cause of your short cycling . . . But

You can also take it to Autozone, Oreilly, ... and have a free charging system test done to try and confirm it's the alternator if you're in the US. You don't have a location in your profile

If you have a volt meter you can do your own stress test (there's a sticky on one of the boards -850 or V70 on self testing) - essentially you measure battery voltage engine off. measure voltage across the battery with engine at idle - should be 14+V. turn on headlights, rear window defrost, blower motor on high, AC/DC on stereo and test voltage. Should stay over mid 13s. Rev engine to 2500 RPM - any changes? the results will clue you to battery vs regulator vs alternator.

How badly does the car run with a/c on?

The performance of the car uphill, with the AC on, isn't terrible but is noticeable. I guess I'm tired of hearing the short cycling of the compressor, always wondering when the compressor will quit due to the stress it is under. I've taken to shutting the AC off when I'm running uphill! Like the suggestions about getting over to AutoZone for a check.

Does the alternator fitted have a clutch type pulley? or a solid pulley.
If so and it is now beginning to fail it affects the voltage output especially under greater loads on the system and the charge warning light does not come on.
I found out the expensive/hard way in March this year on a trip.

I have a 850 turbo and when I go up hill, I downshift to 4th gear to get more torque and I usually just go 60mph.