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1987 Mercedes 300D charging gremlins

Heres a crazy-maker. I’m visiting my girlfriend in the South for the summer and I used the opportunity to buy an old diesel Mercedes. Its already left us stranded several times, but I’m almost done getting the cobwebs out of it and getting a proper old Mercedes.

I just replaced the alternator, after I noticed the battery being drained a few times here and there. The new (rebuilt) alternator works …sort of.

This is the sequence of events. I start the car, and everything appears to be working fine. The battery light briefly flashes when I start the car (along with all of the warning lights), so I know that the bulb is good.
Several miles down the road, the lights start to dim, then the turn signals stop working, and then the radio goes. So the battery isnt being charged… But then! As soon as the lights go totally out, the alternator kicks in, and everything comes to life. This happens EVERY time I drive the car, so I’ve found it best to turn everything on (lights, rear defrost, fan, etc.) to drain the battery as fast as possible so the alternator will kick in and charge the battery for the next start.

Once the alternator starts charging, it seems to charge indefinitely. The wires coming off the alternator are in reasonably good shape, I just polished up the grounding cable off of the battery, and I’m still out of luck.

My girlfriend already thinks I’m crazy for buying the junker, the auto parts guys think I’m crazy, and I’m starting to agree with all of them!

Your girlfriend is a keeper.

How’s the accessory drive belt?

In fine shape . . . good tension . . . not glazed . . . ?

What shape is the battery in?

How old?

Group 49, I believe?

The battery is the foundation of anything electrical in the car

A good alternator won’t do a proper job without a good battery

If the alternator is spinning, it should be charging. The only thing to keep it from charging is the voltage regulator not getting a reference voltage. I’m sure you checked the main charge line to the battery, but have you checked the line from the ignition switch to the regulator? This line energizes the field windings and provides the reference voltage.

It sounds like the alternator has an internal problem and you may want to exchange it under warranty. That model had various issues to deal with.

Could be a low quality rebuild

For european cars, I prefer Bosch remanned alternators, not store brand remanned

I haven’t checked the line from the ignition switch to the regulator. I didnt know it existed, actually. I’ll try a new alternator and battery and see if that gets me anywhere. And see where that wire to the ignition switch leads. That’l be an interesting wire to track down!

Accessory drive belt is a little worn and loose, but not bad. I can see the alternator spinning, and tapping on it doesnt help. Admittedly, I didnt put the woodruff key in when I tightened the pully on, but it doesnt really fit the symptoms I’ve been having.

If there is an external regulator used in this model that could be causing the issue here

Have you replaced all the fuses? The contacts can corrode, causing problems, and it’s a cheap fix. Benz electricals are plenty troublesome, so going over everything is necessary.

Now you know why the previous owner sold it. You’ll want to join a few Benz forums for advice, they last a long time, so there are lots of folks keeping them going (maybe that’s why they last a long time…).

The regulator is bolted to the back of the alternator