Hello All,

1993 940 b230f. 176,000 miles.
I had a 2 year old battery get strangely weak from 12.4 (engine off) to 13.5~8 (idle) but voltage drop to 13.1 with lights, fan, radio, etc. on at idle. Increase in RPM did not bring up voltage beyond 13.1 Anyway I figured I would start with a new battery and check/replace the brushes on the alternator first.
I got the replacement from J&N off the faq page. I suspect my brushes are worn but not completely dead..From what I've read, the Denso alternators are very robust compared to the Bosch ones and the bearings and diodes rarely have problems in these units.
Since I am working on it , is there a good way to diagnose the actual voltage regulator without taking the alternator to a tester ? (meaning on the car)
The faq page says its easy to replace the voltage regulator but then another tip on the faq says aftermarket voltage regulators can cause problems. The OEM voltage regulator is $200+ and the aftermarket is $40,
How do I know if my original one is functioning properly ? dimming lights, buzz in the radio, these don't seem to be very clear diagnosis..

Any help is appreciated