Question about the belt tenioner
 

I have a problem with a loud squeal in my 300 TDT. I have checked the AC pump and it is fine. The alt and w pump are quiet and everything else that spins. BUT there seems to be a noise from the idler pulley. If I remove the belt there is no noise from the pulley. As soon as I put the belt on it will squeal. Is it possible the tensioner is not on the belt straight? Do these things give problems? I read somewhere the wrong belt will make a noise. The tensioner does not look straight on the belt but I couldn't say for sure. Any suggestions?

Different car, but...
 

Mine is a 1991 300d, so it's different, I guess. Mine has the belt tensioner and idler arm pulley.

Sometimes the belt tensioner (the thing that looks like a little shock absorber) loses its ability to pull the belt properly. Might have to replace that. If it's OEM, might last for 5 years (other people would probably know better than me), if it's a lesser brand, it might last for 2 years.

jeff

well the belt seems to be tight. I was thinking that the mount would be bad somehow but who knows, it is driving me crazy though

It can be the bearing in the pulley.

The tensioner also has a bearing at the base of the swinging-arm where it mounts to the engine's front cover, and that often wears and allows the pulley to ride at an angle.

That's regarding the TD Turbo, T124, not sure what a TDT is (sorry Jimmy) or whether it is a T123, year and proper model will help.

I had that with my 87 300TD. I wound up replacing the belt itself. Believe it or not, the squeal went, even though the new belt was the same as the old.

I also replaced the tensioner, but the problem with mine was that the shock on the belt tensioner had worn out, and the tensioner was rocking slightly. I wore out two belts inside a year before I realized it was the tensioner.

I am going to replace the tensioner and see what happens. The belt is new not long ago.

Alt belt?
If so, you may need a new alt, if the belt is tight and still makes a squeak, probably the bearing.

The alt does not squeak when the belt is removed and that was off not too long ago.

On my '87 190D the tensioner main bearing became worn, causing the tensioner to run at an angle to the belt. The bearing in the pulley itself became worn, causing some noise and again a slight angularity. I replaced the tensioner about a year ago, there is already noticeable play in its bearing.

It's not under load, it probably won't squeak.
Make sure where the sound is coming from. Replacing the tensioner, I'm afraid would be a waste of funds.

I am pretty sure it is the tensioner, I can make it squeak by hand.

OK, but I don't know of anything in the tensioner that would squeak.
When I had a squeal in that area, I replaced the alt, the tensioner worked, does yours keep tension?
Rebuilt alt solved my problem.

This is a situation where the OP fails to state the year of the vehicle, and has therefore confused you by his omittance.

For the record, he has a 1987 300TD with the 603 engine.

87 300TDT as stated in the original post, yes it keeps tension but it is askew against the belt and I believe that is what is causing the noise.

If it's askew, likely the tensioner arm bearing at the front cover although it could be just the bearing in the wheel, check both.