Its not just the heater. If I turn on my heater...I get about a 1.2v drop. Turn on my electric fan...another 1v drop. Headlights, wipers, sound system, another 1.5v drop. (headlights are LED, so I am guessing there shouldn't be much of a draw.)

Then it is either 1) Your alternator; 2) Your voltage regulator; or 3) The wiring connections between your battery, the grounds, and the alternator itself.

Consider where the volt meter is taking it's reading.
If the battery is in the trunk and the lights/fan pull from a point close to the reading, it may pull down to 10v or less. This can happen because of the wiring being a little undersized and nothing to do with the health of the battery or alternator.

Larry, note, I have taken measurements at the Alternator and the Battery...both are the same. The Volt Gauge in the car, while not nearly as accurate, is still pretty close to the same readings I am getting on my multimeter.

It's very easy to "jumper" a wire at the VR and test the full output of the alternator. You just have to be a little careful in revving the engine and watch the output voltage. I won't bother posting the details but whoever checks it over won't have any trouble pinpointing what's wrong.

So...as I started to remove the case from my power master alternator...some debris fell out. I didn't take the case all the way apart...but enough that I could shine a flashlight in there and realize the debris is from a brush that started crumbling. I stopped right there.

I'm a big fan of having alternators rebuilt, as opposed to buying new ones. If you feel comfortable doing it, take the negative cable off the battery, then take some pictures of your alternator, take the top bolt out, and the bottom one out, one of those two are on a sliding bracket of some sort, then take some pictures of the wires on the back of the alternator, remove the wires, slip the belt off, and take the alternator to the rebuild shop. Fifty bucks or less and you'll be all set. Then just reverse the procedure using your pictures. When you have it all put back together, lightly tap the negative cable to the battery first, and if you see no big shower of sparks, bolt it all up and fire it up.

I have rebuilt a few in my life. This one should be under warranty...but we will see.

It wasn't the voltage regulator or alternator. Going to add a ground to the salt and see if that helps.

That's all there is a to a charging circuit in these cars. Just the alternator, the voltage regulator, and the wires and connections in between. The problem is always in one of those three categories.

Grounded alternator to the frame....no dice. Still voltage drop if I turn on accessory. More pull of accessory...greater the drop.

How did you test the alternator and voltage regulator?

Checked volts at battery. Swapped in a new alt....same issye

How did you check the voltage regulator? You do have a separate voltage regulator, right?

I didn't. Both alternators had internal regs

Since you have two alternators, protocol dictates that you have the one that is out of your car bench tested at the local auto parts store. I know you're 99.9% sure that it's just fine, but a bench test there will confirm both the alternator and the internal regulator. You have to do it. Then when we swap that one back in, we'll search for the increased resistance in the main feed circuits by performing a voltage drop test while the system is under load. This is not magic. Houdini is not making electricity vanish before your eyes. Either the alternator/regulator is not putting it out, or something with resistance is eating it up (bad connection, "skinny" wire, partial short to ground, etc.).

One wire alternator?

Yes....one wire powermaster 150 amp

My 140 amp Powermaster wasn't keeping up with things, especially at idle. I rewired it for a three wire install and gained 1 1/2 volts with 2 volts at idle. Google this and you'll find that there's a ton of info out there on one-wire problems.

Resurrecting an old thread. I'll be the 1st to admit, I'm electrically illiterate.

I've got the same problems as allcarfan had. It appears I take a hit when switching on ANY accessory... fan switch... lights... wipers... heater... and with each accessory turned on... the voltage drops a little further. I can actually make the volt meter on the dash drop from 14 down to about 11 very easily. However... to this point I've just kind of lived with it... the cooling fan seems to have the least affect and doesn't affect operation of the car during the day... and I never use the heater... so... it's been a daytime driver only.

However... just this past weekend I was stuck out & about after dark, so I had to use my lights for the short trip home... and... when the voltage dropped... some of the other gauges began to act wacko too... for example the temperature gauge started slowly going up & then back down by about 10 degrees C... despite it being a cool night and no cooling system issues.

The multi-meter indicates at the battery... and also at the alternator... that the alternator is charging at about 14.55 volts max and the dash gauge reading about a 1/2 volt off at 14. With all the accessories running... the multi-meter might drop down to about 14.51... while the gauge on the dash will drop down below 11. If I'm getting the desired voltage at the battery at all times... with, or without, accessories... doesn't that rule out the alternator/voltage regulator?

Maybe I'm wrong (remember, I said I'm electrically illiterate)... but I'd almost bet it's a grounding issue... maybe corroded... maybe loose. I have VDO gauges... and I've read where VDO temperature gauges need to have a separate ground. However... before I start taking things apart... can anyone tell me where the ground(s) for the dash components on a BDR (#509 here) are typically located? Hoping to narrow down the search before starting to dig "in search of".

Any other advice appreciated.

allcarfan... it's been a year... what have you found on yours?