Hello all hope someone can guide me to a solution.

First post, have always found what I was looking for on the boards until now.

I've got a '92 240. It is having intermittent charging problems. I've checked the boards and tested many solutions, but haven't found anything specific to my problem.

Here it is.

I've taken my alternator and battery in to have them tested, both passed at two different stores.

With a voltameter my battery says 12.63 fully charged. I turn the car on and the the voltameter jumps to 14, hovers between 14 - 13's, so I know my alternator is charging.

Here's what I can't figure out.
I put a load on the charging system while checking meter, first the lights, everything's good. Radio, all good. But when I turn the heater on the readings on the meter drop and continue to decline, and with the heater on I get a horrid squealing noise I can't seem to locate. I turn the heater off and the meter jumps up and the alternator is charging again.
Could it be a bad internal regulator? Is a bad internal regulator something that would be missed by a bench test?

Any suggestions?

Look forward to hearing anything.

Thank you,
f
Kilian