My voltage at 1500 RPM ranges from 13.31 to 13.39 with no major electric consumers on (A/C etc.). Is that sufficient? Also, even though voltage is steady, can a regulator or alternator diode cause electrical problems(gremlins). I read a lot in a search but most of what I saw was not conclusive.

Hi Stephen
you should be pushing about 14v or higher at about 2500 rpm. If you still only get 13.5 or so - before you condemn the alternator or regulator - check your battery. Here's how. Let the car sit overnight to let the battery rest and the internal voltage to stabilize and eliminate "false readings". Then, the next morning - DON'T START THE CAR! Just lift the hood and read the voltage across the battery - It should be 12.5- 12.9 volts. If it is at 12v or lower - you have a bad cell that will suck the juice out of your alternator causing it to put out a lower voltage with the current needed for charging. If the voltage on the battery after an overnight rest is 12.5v or higher - then suspect your alternator or regulator. Just some time saving advice from someone who has been down that path several times.
maurice:p :p

Go to www.google.com and do a search on "testing alternator". There is a good check sheet for testing the alternator, battery, etc. If the mileage is not excessive on your 89 300E, I would suspect the regulator which is held in the back of the alternator by two screws. Costs about $40.00. Just did mine. If you need a new alternator go to http://autoexpress.safeshopper.com/125/cat125.htm?602
John O

Suspect voltage regulator as previously pointed out. 2 screws come out of regulator which is on back of alt. Easily accessible if you put front of car up on ramps first - pull large plastic encapsulation shroud underneath.

Mine was reading about 13.20 at idle, then one day out of the blue the idle fell off and the SRS/ABS light came on. This last situation is indicative of the brushes in the volt. reg. finally giving up.

New brushes are about 1/2" long. Pull your present ones and see how long they are. This is a VERY easy job. Takes longer to setup for it than it does to remove/install new voltage regulator(brushes).

After changing brushes, I see about 13.95 at idle first thing in the morning with no heavy current users active. After the car is fully warmed up, I see it around 13.60 - 13.70.

It was ok in the 13.35 range. It dipped to about 13.20. A few weeks later I ran into the problem described above.

Like I said - put your car on ramps and pull your brushes. It's a two min. job if that. Sooner or later(sooner), they're going to wittle down to the point where you'll experience problems mentioned above.

You might be way down the road when it happens. Fix it now.

So, do you think the difference between 13.35 and 13.70 is enough to set off the warning lights?

I've already pulled the regulator. The brushes look good. I shined them up and evened them with a half round file. They're making good contact. I'm more concerned about the components inside the regulator. And about those ramps, my car lives on ramps. It's there now for transmission trouble.