w124 e300 alternator bug
 

I have an '89 E300. The battery is recent and tests good. The alternator is getting +2vdc on the D+ terminal and has +13vdc on the B+. The dash battery light comes on when the engine is not running and shuts off when it is running. All per spec.

The problem is the voltage on the battery when the car is running is 12 to 13 vdc. Which means the battery is not charging. I've removed every fuse with no change in symptoms.

I took the car to a shop where they told me I needed a new alternator. I bought an aftermarket rebuilt and installed it, got identical symptoms (though I've not been back to the shop to retest the alternator). When I first installed the new alternator, it sounded different and the voltage was +14vdc, but that lasted less than a minute and since then it has always read battery voltage.

In reading other posts, I saw where people used a test light to start the alternator by putting +12vdc on the B+ terminal. This did not work for me.

I'm considering delicate adjustments with a hammer. Any better suggestions?

jked, if you still have the old alternator check the voltage regulator (attached to the back of the alternator with 2 screws) brushes. If the brushes are worn down to short nubs the alternator doesn't recharge the battery sufficiently, and the voltage regulator needs to be replaced. That's a much less expensive fix than replacing the alternator.

How much amperage is the vehicle drawing (clamp amp meter on battery cable). And how much amperage is the alternator putting out?

How about at 1000 rpm? Voltage / amperage?

I got higher amperage/voltage by further tightening (the already tight to me) belt.

Also do a voltage drop test from the battery center terminals to various points on the car - engine ground at alternator, + cable at alternator, +at fuse block terminals.

Search google.com for how to do voltage drop tests on a car. This will find loose or dirty electrical connections including poor main electrical grounds.

Also what is the problem? Is the battery not staying charged? A fully charged battery is 12.75 volts with the car off. Is it not around that voltage after you drive it each time? And is it the super large Mercedes spec battery?

The B post is the direct line from the battery, the D post is what excites the field winding to jump start the alternator.

Your alternator should charge at 13.6-14V in normal condition. Also make sure the bulb in the dashboard is a 3W bulb - if its not a 3W bulb the unit wont start.

It was the alternator. Again. Looks like the rebuilt was not very well rebuilt. What tipped me off was when the alternator light stopped lighting when the key was in 'run' position with the engine off. After driving, the light came back on, then off, etc, but voltage at the D+ terminal stayed +12vdc.

So off the auto parts store for a replacement alternator, some sandlot mechanic maneuvers in the parking lot to install it, and all is well.

There is a lesson here somewhere.

Thank you for your suggestions. They kept me from resorting to hammer time.

Most probably the regulator they installed was duff. Or the positive contact leaf spring for the regulator was crusty or bent out of shape.

My bet is that the regulator was a far eastern product and was made with sloppy workmanship.