I have a 78 300D, the battery started to lose charge after a drive. I assumed either the regulator or alternator. Took the old regulator out, and it was toast. Replaced it with a new one and all is well...

Immediately after installing a new regulator when slowing down to a stop, the alternator will make a screech and stop at idle. Pulling away from a stop it screeched even louder, then went out. That only has happened once so far. Just changed out the regulator this morning.

Any idea why the sudden screeching noise? It's never made this noise before the regulator swap.

I just went through this on the 1984 300DT. The alternator bearings are sending you a message; either take your alternator to a shop and have new bearings installed or buy a new bosch alternator.

When the warning signs were there, I blew it off and kept driving. WRONG. One day the alternator bearings FROZE UP SOLID, the BELTS howled and smoked like a cat being tortured!!! The car had to be towed home; believe me this was no fun. The towing company DID NOT bring a roll back as requested; it was getting late in the evening so I gave in and let them tow it home. Just call me STUPID and LAZY for not addressing this issue SOONER.

So.......time to replace either those alternator bearings or man up and buy a new bosch. I managed to locate a gently used 65 amp Bosch WITH the mounting bracket from a pick-n-pull for (are you ready?) $50!!!!! It has been working flawlessly since install......

Yeah that's what I figured. Very strange it decided to start making noise when the new regulator was installed this morning.


Looks like it's time for a new alternator.

Did you charge the battery after your regulator problem?
If not, then the alternator would be heavily loaded and if your belts are not properly tightened they will squeal.
Bearings usually get noisy and stay noisy long before they fail.

Yeah I had it on a charger.

The only noise I've ever heard an alternator make is that nasty screech. I've been told sometimes they can make a clicking or tapping noise, that will sound like an internal motor bearing failure... I've never heard one like that in person though.


Going to take off for lunch shortly, and see how things act....

This all started this morning after the regulator swap.

Worse case I have to buy an alternator, not the end of the world. Just weird timing.

Try pouring some water on the belts the next time it squeals.
If the sound goes away, you may just have glazed and/or loose belts.

If it is a bad alternator, consider the AL129X upgrade:
http://www.peachparts.com/shopforum/diesel-discussion/364297-diy-al129x-alternator-1978-w116-300sd.html

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

If anyone else is looking for tech info on the Alternator, check out the DIY links below.

Mercedes-Benz W123 Alternator Replacement | 300TD 1977-1985 | Pelican Parts DIY Maintenance Article

Mercedes-Benz W123 Voltage Regulator Replacement | 300TD 1977-1985 | Pelican Parts DIY Maintenance Article

Your battery was down with the bad regulator. When you installed the new regulator the alternator load increased substantially to bring the battery back up to where it should be.


The alternator belt slippage because of this is what I believe you are hearing. The best test is there is a small cooling fan in the front of the alternator just behind the pulley.


With the engine off of course if your can turn that fan by hand the belt or belts are far too loose. If ever far away from home and no tools and this occurs.


Get a bar of hand soap and rub it on the v section belt or belts. It will quiet them down and restore their traction until you can get the adjustment done. I believe it is the glycerin component of the soap that aids traction of the belt or belts.

With the car off, I can easily turn the fan. The pulley doesn't move at all

The '78 has the earlier type of "geared bolt"adjustment mechanism.

from my experience that type is a bit fiddly to get just right and is sensitive to having the correct size V belt. 13x1030
Pelican Parts - European Automotive Parts and Accessories - Porsche • BMW • Mercedes • Volkswagen • Audi • Saab • Volvo • MINI

when i drove my '78 i messed around for a while trying to get that belt right. new belt made all the difference. if yours is old and crispy probably want to go ahead and grab a new belt and go from there.

__________________
Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

The fan is held in place either with a key or by simply being torqued tightly against the shaft. Either way, if it turns by hand , it's wrong.

Engine off I can turn the fan by hand, and it doesn't move the pulleys.. With the engine running, the fan works? Some kind of clutch system?

When you say you can turn the fan, are you taking about the small fan on the alternator or the engine cooling fan?
Barry in post 8 is talking about the alternator fan.

Engine cooling fan.... Ha ha


The alternator fan, I can spin counter clock wise, and then belt doesn't move. No idea if that's normal or not?

I had to ask!

It is not normal. The nut on the shaft must either be loose or you are missing a spacer.