Voltage Regulator Testing (off car)

[JamesDean]

Hello everyone,

I'm having some trouble with my 300SD's charging system and I've been running some tests.

Alternator, new as of 2006.
voltage reg, new as of 01/2010

No load, system output decent 13+ volts.

Load (headlamps+ac+blower on high), system drops to 12.5v or lower.

I dont think the alternator itself is bad. I put a DMM on the battery in AC mode and only got .03-.05 volts AC. Meaning my diodes are functioning properly.

I pulled the voltage regulator, and was testing the transistor on it.

You can test a transistor like a diode. You should get a reading one way, but if you flip the probes you should get nothing the other.

Theres three pins on the transistor: collector, base, emitter. You are after the B-C, B-E connections for testing.

I believe...

The collector is connected to the casing of the transistor, and goes into a brush.
The base is the one that runs to one of the mounting screws for ground.
The emitter is the remaining one that goes into a brush.

When I tested my B-C, B-E resistances I got readings both ways. This might be because there might be a resistor/capacitor/diode in the circuit path that I can't see. But, I think the VR is just bad.

I was hoping someone could corroborate readings with me.

I dont have a new-erish vr to test.