Weird noise from brand new alternator and pulley
 

Hello everyone. First post here and hopefully everything goes well. I will try to describe exactly whats happening so please read entire post.

82 300D is the car. As stated I have a weird noise coming from the alternator. Now before you tell me its the pulley or the alternator, they're both brand new. Also, my previous pulley and alternator were doing the exact same thing! So I know its not the pulley or alternator being bad. The sound is basically like a squealing belt. Well i replaced the belt twice. I even loosened the belt so loose that im worried it could fly off. I have tried every tension from too tight to too loose to make sure its not the belt. Also, the alternator isnt charging at all. I only have 12.0v at the battery with a brand new alternator. Now here's the weird part. While the car is running and the alternator is making the sound, I can disconnect the battery and the sound stops!! Thats whats throwing me for a loop. I have cleaned every connection, replaced parts, put in a new battery, and still no luck. Please if anyone has come across this issue let me know. Also if someone can point me in the right direction to look for an issue please do. Thanks for any help you can provide

V Belt Squealing
 

It certainly sounds like the belt is too loose .

Look at the part #'s on the belt, it's important to use a 10MM belt, Pelican sold me two sets of 9MM belts in the proper length because I didn't check them before taking both cars apart I decided t use them and both required seriously tight adjustment before the squealing stopped .

Do you have a "Cricket?"
Many here check belt tension by feel. I am too neurotic for such practices...
This is from memory (so not reliable) but I think the FSM lists a belt tension of 35 to 40 pounds (? I can't recall the unit for deflection). A cricket will put you in that ball park with certainty.



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I mean, easy test is to spray water on the belt while it's running to see if the sound stops or changes significantly.

Drive Belt Noise
 

A better way is to spray some inexpen$ive hair spray directly on the inside of the belt as the car is running .

I might take the main wire off the alternator to see if it is charging into a dead short. As you have noise and no charging. Plus the other altenator you had was doing the same thing apparently.


I suspect the alternator could be noisy when overcharging. Or be too much of a load under that condition for the belt to work.


Was the alternator you removed tested before being replaced? On a test machine?


Just something to consider. Or not to overlook.

When belts are involved, I have followed the Factory Service Manual for decades:
1. Use belts that are the size specified.
2. Use a Cricket for adjustment (and follow the instructions included with said).

My shop manual calls for a 9.5mm belt for my '81 300SD (even though Peachparts lists it as 10.0mm). Wouldn't the 300D be a 9.5mm also?

Ah, HA ! .

Now you've got me wondering .

X2, it works. I don't spray I get a glass of water and pore it away from the belt on the alternator. Stand to the front of the Car.

For the not charging issue you should have battery voltage at both of the large slots electrical connector that plugs into the alternator.

You should have no voltage at the small slot till you turn the ignition switch on and it will be slightly less then battery voltage.

Then of course there is that never ending issue of having a good contacts at your engine to chassis ground strap and the battery to chassis ground.
It is even possible for where your alternator bracket bolts up to the engine to be so rusted it is no longer grounded well.

Once your engine has started pull off both the square and rectangular electrical connector at the Glow Plug Relay and see if the voltage at the battery changes.
If after that the voltage goes up your Glow Plug relay is stuck on or there is an issue with the ignition switch keeping the relay on..

Never, and I mean never, remove a cable from the battery or alternator while the car is running. Good way to burn a diode. If you're done this, then take the alternator to Autozone or whatever and have it tested. Maybe you're lucky.

The reason the belt squeaks only when the battery is charging is that the alternator is pushing a lot of current through the battery. High load results in more back EMF, which is a force which opposes the rotation of the alternator, so the belt system has to work harder. Either the battery has a shorted cell, there's an undersized pulley, the alternator has a bad diode, or it's oversized. It's also possible (but not likely) that there's a short elsewhere, so do 911's glow plug test. You said the alternator was new, but was it stock sized?

Field Testing Alternators & Generators
 

Actually, the damage is done when you reconnect the battery ~ I don't recommend it but if you disconnect the ground cable and tuck it down behind the battery where it cannot connect and the engine still runs, the alt/ge is good , go SHUT THE ENGINE OFF before re connecting the ground cable .

This was part of my GM factory training long, long ago .

They taught us lots of things you're not supposed to do because if you don't do them *exactly right* you'll damage things .

To clarify the above is done without the engine running.

Good grief. Mechanics do stuff that makes engineers cringe. And they're taught how to do this cr_p in school! Here's an article that explains why what you've just stated is exactly wrong. When the load is dumped, the alternator creates a voltage surge that's rapidly dissipated in smoke and heat. You've just described an excellent procedure for creating endless threads about mysterious problems with brand new alternators and electronic components.


https://en.wikipedia.org/wiki/Load_dump


We used to see this a lot in old Jags that used the Lucas "helmet" battery connectors. These were famous for coming loose, which would result in the premature death of a very expensive vintage alternator. You never ever want to disconnect the battery from a running engine.