Is there a way to tell if I need an alternator or the voltage regulator?
 

1996 E300D
Is there a way to tell if I need an alternator or the voltage regulator?

On your W210 the voltage regulator is the brush pack. If you meter the + - terminals with the engine running your voltage should be north of 13.6VDC.
The brush pack is the likely suspect and its two screws to remove and not an expensive part. When you seat the brushes back in compress the brushes with a piece of dental floss, insert then pull the floss out and put screws back in.

I just tested my alternator with my smart charger and it read "Alternator OK"
But voltage was down to 11.89. Which is what it was on the battery I just replaced.
I drove it yesterday. Drove it this morning. No check engine light.
I started the car about 6 times and immediately turned it off to check for warning lights.
Check engine light back on.
It was off this morning.

Fully charged batteries should have a voltage reading of +12.6 volts static (no engine running)
With the engine running the battery reading should be 13.6 - 14.5VDC
There is a warning light for alternator on the bottom of the cluster.
What code is present when the check engine light comes on? The check engine light is something else.

I have to read the codes.
I will check the voltage when it is running for higher than 13.6 - 14.5

When the car is running it reads 12.2 volts. This is about 10 minutes after I topped off the charge on the brand new battery.

I tested the alternator using my smart tester and it said ok, but obviously it is not charging

I don't have the code reader. BUt the last time I ran it it said glow plug relay or pre-glow timing relay.
BUt I tested that and it is fine as are my glow plugs.

Check the voltage through the CCU while driving. It should be above 14v. The CCU # will be 0.5v below the reading at the battery so add 0.5v to whatever number u see on the display. You likely need a VR.
http://www.mbcluster.com/Old_Website...%20Reading.jpg

the test to isolate those two pieces is called a full field test, on an old Bosch it requires the DF and D/B to be shorted.

It is quite easy on japanese and american alternators, a bit involved on german. I would suggest you stick these terms in google and search for it, that way you can learn the test and its results.

Your smart tester isn't very smart if it thinks 12V is a good charge voltage, or a good voltage of any kind. Might need a visit to the recycling can. New regulator as was said.

I have been thinking that years of partying have made my smart tester not so smart.

I've never done this.
Can I do it with the alternator in the car?

sorry bout that,

DF, B, D and W are pin designations on bosch alternators.

OK.
So to isolate the alternator for testing I need to put a jumper pins D and B to each other and what do I jumper DF to?
Can I do this on the car?
Do you have a visual?

Then I can put my meter on the positive and the negative terminal of the alternator and read the true output.

Here are the results of my readings and error codes. What do you think?
http://www.peachparts.com/shopforum/...rror-codes.jpg