1990 300D 2.5 Turbo
I had my alternator checked/rebuilt recently, and now I also replaced my voltage regulator in hopes of correcting my problem, but with no luck.

The battery read about 12.6V at off.
At cold idle it goes to 13.8 - 13.9V.
I turn on the exterior and all interior lights, plus the radio and the read at the battery continues to be level at 13.7 - 13.8V.

As soon as I turn the blower motor/heater on (whether on its own or with the above items), the reading begins to drop to 13.21...13.20...13.19...13.18...13.17...13.16...and goes down at a steady rate.

The alternator light never comes on, and I'm left stranded with a dead battery after driving around, needing a boost, and then driving without heat to let the battery charge.

Any ideas??? Thanks.