Need M104 Belt Tensioner Help
 

Had a squeaking belt, no problem, spare on the shelf, same size.

Installed the belt, belt tensioner does nothing. Loosen the "nut", it just travels down the threaded shaft. Tighten it, it runs to the top of the threads and no belt tension.

Can someone help me to understand what this thing is supposed to be doing? The FSM isn't much help, I don't know if I have a broken tensioner and if so what in it would be broken? I have a funny feeling that something was already amiss when the water pump was changed.

Also, is the M103 tensioner the same? I could get a used M103 tensioner tomorrow, I'd like to get the car together before Monday.

Sorry, it's a '94 E 320.

Depending on the serial # of your car, it can take 2 different tensioners but from your description it sounds like you have the older style with the nut and adjusting rod. Looks like it is the same tensioner as on my 1991 300CE, part # is 1032000870 which looks like a M103 part. Before you can move the arm part of the tensioner (with the pulley), you have to release bolt that goes thru the center of the tensioner.

glenmore
1991 300CE
2000 C280

Does anyone have a photo of the back of the tensioner (early type)? I need to see the orientation of the flatted-shaft.

Looked at the stock photo from Fastlane, not sure that it is accurate for this use though.

TIA.

The center of the pulley should line up between imaginary lines drawn along the flats of the bushing. If the flats point off in some other direction, the tensioner is probably shot.

glenmore
1991 300CE
2000 C280

Sounds like the tensioner is shot. The tensioner consists of an idler pulley on a spring loaded arm. The adjusting bolt rotates the entire body of the tensioner, giving the ability to vary where the arm is in its arc, hence the tension applied to the belt. It doesn't work if you don't loosen the 10mm allen that locks the tensioner in place. Don't ask how I know this.

Most likely the spring has died, so the arm isn't applying any force no matter where you set the adjuster. Replacement isn't too bad - remove the radiator shroud, fan & clutch, belt, and pulleys from the water pump and PS pump. The tensioner has 3 bolts plus the aforementioned 10mm allen holding it in place. One of the bolts is nutted behind the ps pump and is a bit fiddly.

- JimY

Got it. The tensioner was relaxed/shot, still has spring force, but not clocked correctly anymore. I installed one from a handy M103 for now, the new belt works great.