Alternator Diode check check with meter from my notes
With your meter across the battery, on Volts DC, you should read 12.5 - 14VDC
If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure. If VAC is OK but the VDC is not correct, then the problem is with the voltage regulator.
In post nuber 3 in the below thread I made an Alternator test cord that somewhat duplicates the wiring to the Ignition sitch and Battery Charging light. I keep posting the pics but so far as I know no one else has made one.
'83 W123 battery drain problem
'83 W123 battery drain problem