I have only the factory installed alarm system in the car. I have no owner installed accessories in the car. All original equipment.

The battery terminals are clean. They have no corrosion on the outside or on the inside. I used one of those circular battery wire brushes.

I do have a spare voltage regulator. But shouldn't the defective regulator show the alternator test?

Any thoughts? Could it be the wiring harness? I see the positive wiring harness retails for about C$320, part number 126 540 25 30, which I would prefer avoiding. Maybe the juice is not getting from the alternator to the battery? I guess I could test it - check voltage at battery before starting, then start, check voltage at battery and at the B+ terminal of alternator. If B+ and positive battery terminal read different voltages, then the cable between the two is bad, especially if the battery is lower voltage than the voltage at the alternator b+. Right?

__________________
1989 420 SEL