M52TU Z3 Drive Belt Conundrum - Help Please!

[staggerwing17]

Hello!
As I write this I have spent the better part of the afternoon putting back together the front of my M52TU motor, meaning pulleys, tensioners, idlers and belts. I thought I had put it all together properly but the belt appears too long to run as the diagrams and photos show. It is the same size as the old belt and the same brand and part number. And yes, it ran fine before!

The diagram in the Bentley manual shows an idler with a non-concentric hub that appears to mount to the top bolt of the alternator. The problem is, I have never had that bolt out, and I sure did not take it out for this overhaul. The replacement tensioner bolted right in no problem. I cannot see where the non-concentric idler would mount. Realoem shows the diagram at https://www.realoem.com/bmw/enUS/showparts?id=CH93-USA-03-1999-Z3-BMW-Z3_25&diagId=11_6226. Number 5 is called a deflection pulley. Here is the diagram:



Here is the overall setup right now:



Here is the replacement tensioner mounted and in its fully tensioned position:



And here is the belt installed and the slack available:





And here is a non-concentric pulley I received when I mis-ordered the AC tensioner:



And finally, here is the Bentley manual diagram:



So any of the following may be true:

• I am doing something colossaly stupid and the answer is right in front of me
• I have the wrong tensioner installed
• The diagram above is not correct for my motor
• The diagram at realoem is not correct for my specific motor

On top of all of this, I needed to clear space so I threw away a few old parts. That included the tensioner on pictured above. I know, bad idea, and it is killing me in more ways than one.

Does anyone have any idea what the issue is here? Please, if it is something embarrassingly obvious, let me know that! Otherwise I look forward to any responses.

 

[Lazzzydog]

The tensioner must be mounted incorrectly. You should need to hold it against the spring with a spanner on the lug to be able to refit the belt. Releasing the the tension then tightens the belt. Or maybe it’s the wrong belt?

 

[Rudyrov]

I did mine a few months back and found that Amazon sent the wrong belt, when I put the belts side by side and used something to stretch them at the same time the new belt was different. So I got a new belt. Otherwise your belt routing looks correct. I will include a picture but be ware that there is not much room once you have everything installed.
Also of note is that mine doesn't have the eccentric tensioner, My engine is a M52B28 single Vanos engine, 2.8L https://en.wikipedia.org/wiki/BMW_M52#M52B28.

 

[Rudyrov]

Check out the Pelican parts write up, they show both types of layouts (with & without Idler Pulley)
https://www.pelicanparts.com/BMW/te...lacement/12-BASICS-Drive_Belt_Replacement.htm

 

[staggerwing17]

The tensioner must be mounted incorrectly. You should need to hold it against the spring with a spanner on the lug to be able to refit the belt. Releasing the the tension then tightens the belt. Or maybe it’s the wrong belt?

First, thank you for responding so quickly! I have been staring at it and the tensioner seems to be correct. I remember pulling the belt out, and thinking that with a tensioner like that one, it couldn't really be any easier. If I put a 16mm socket on the ratchet handle and then onto the stud on the swinging portion of the tensioner, turn the handle clockwise, the pulley moves to the left. As it sits in my photos, it is as far tensioned as it will go. On top of that, there is so much slack in the belt, even with the tensioner at its limit, it is not even close to being able to take up all that slack. Sigh.

Now, is it the right belt? Well, it is the same as the belt I pulled out of it. Put side by side, the belts appear identical. And they are the same Conti belt. And the one I pulled out behaves exactly the same way - that is, lots of slack.

I think that based on your information I am going to pull that tensioner and make absolutely sure it is the correct one and mounted properly. I do very much appreciate your feedback as I am really perplexed! I will update post when resolved.

 

[staggerwing17]

I did mine a few months back and found that Amazon sent the wrong belt, when I put the belts side by side and used something to stretch them at the same time the new belt was different. So I got a new belt. Otherwise your belt routing looks correct. I will include a picture but be ware that there is not much room once you have everything installed.
Also of note is that mine doesn't have the eccentric tensioner, My engine is a M52B28 single Vanos engine, 2.8L https://en.wikipedia.org/wiki/BMW_M52#M52B28.

Hello! Thanks for sending those images! And yes, gotta resolve this before putting radiator hoses, fan shroud, fan and new radiator in. It is nice to have the space right now though! I am going to give the belt stretch a try just to see how they behave - thanks for the recommendation.

I have never seen the wiki for the BMW engines. Very useful and informative - bookmarked. My engine is the M52TUB25 with dual VANOS. It was a transitional engine so sometimes the engine does not match up with diagrams, etc. I still have a feeling I have overlooked something simple. Will tackle it tomorrow. Thank you!!

 

[staggerwing17]

Check out the Pelican parts write up, they show both types of layouts (with & without Idler Pulley)
https://www.pelicanparts.com/BMW/te...lacement/12-BASICS-Drive_Belt_Replacement.htm

Thanks for the link! That is helpful, especially the images. I will add that to my reference material. Thank you!!

 

[the Nefyn cat]

The tensioner seems to be working the wrong way, just had dealings with one of my M54s (much the same thing) and to release the tension it needed to be turned clockwise with the 16mm nut bit on the body. Your one looks as if it's applying tension the wrong way round, needing to be turned anti-clockwise to let the tension off.
The pulley should be pushing up towards the alternator pulley, not down towards the crankshaft pulley.

 

[Rudyrov]

Hi Nefyn Cat, that makes sense to me, the tensioner pulley pushes away from the water pump, towards the alternator.

 

[staggerwing17]

Hello!
As I write this I have spent the better part of the afternoon putting back together the front of my M52TU motor, meaning pulleys, tensioners, idlers and belts. I thought I had put it all together properly but the belt appears too long to run as the diagrams and photos show. It is the same size as the old belt and the same brand and part number. And yes, it ran fine before!

The diagram in the Bentley manual shows an idler with a non-concentric hub that appears to mount to the top bolt of the alternator. The problem is, I have never had that bolt out, and I sure did not take it out for this overhaul. The replacement tensioner bolted right in no problem. I cannot see where the non-concentric idler would mount. Realoem shows the diagram at https://www.realoem.com/bmw/enUS/showparts?id=CH93-USA-03-1999-Z3-BMW-Z3_25&diagId=11_6226. Number 5 is called a deflection pulley. Here is the diagram:

View attachment 119497

Here is the overall setup right now:

View attachment 119501

Here is the replacement tensioner mounted and in its fully tensioned position:

View attachment 119499

And here is the belt installed and the slack available:

View attachment 119505

View attachment 119503

And here is a non-concentric pulley I received when I mis-ordered the AC tensioner:

View attachment 119507

And finally, here is the Bentley manual diagram:

View attachment 119509

So any of the following may be true:

• I am doing something colossaly stupid and the answer is right in front of me
• I have the wrong tensioner installed
• The diagram above is not correct for my motor
• The diagram at realoem is not correct for my specific motor

On top of all of this, I needed to clear space so I threw away a few old parts. That included the tensioner on pictured above. I know, bad idea, and it is killing me in more ways than one.

Does anyone have any idea what the issue is here? Please, if it is something embarrassingly obvious, let me know that! Otherwise I look forward to any responses.

Hello! I figured it out! The short answer is that it is the wrong tensioner. Despite the fact that ebay shows it as a "98-06 BMW 323 325 328 330 525 528 530 X5 Z3 M3 A/C Serpentine Belt Tensioner" and the realoem picture looks like the part, it is still wrong.

The tensioner I ordered fit the location perfectly. There was some question about the tensioning direction and this one did increase tension as it was moved toward the alternator. However, even though it fit, it sat too far left to take up the slack in the belt, even when fully tensioned.




Here is the tensioner. Please note that one mounting ear is up higher to fit the protruding mount on the block:


Now, at the same time, I ordered what I thought was the correct hydraulic tensioner for the AC belt. The tensioner I ordered was this one:





Note the triangular shape of the positioning of the mounting holes. And here is the mounting point for that tensioner:



So when I received the new parts, I test fit them to make sure they were right. Keep in mind I had already screwed up by tossing the original parts. The drive belt tensioner fit perfectly, bolted right in. Alas, the AC belt tensioner did not. Different triangle. Sigh.

As my buddy and I were puzzling over the drive belt slack (late night, many lights in engine bay, cocktails), I noticed the 2 additional holes in addition to the two existing holes. Wait just a damned minute, that looks like the incorrect AC tensioner would fit there? And so it did! It moved the pulley to the right to take up the slack and then some. Alas, I had no properly sized bolts for the hydraulic mount so off to ACE Hardware this AM and I found correct bolts. Mounted it, used a long handled ratchet to loosen the tension to allow the belt to slip over it and released. It fits perfectly and tension is great! See below:



I made a bunch of mistakes here but was partially rescued by the fortuitous result of ordering the wrong tensioner for the AC belt. I threw removed parts away too soon (this really cost me because the correct mounting plate and arm for the AC tensioner plus the hydraulic tensioner and the correct pulley totaled over $200). So thank you for all of your help - it is invaluable having other perspectives so I am grateful for your thoughts!

 

[the Nefyn cat]

Good to see it back together.

 

[colb]

Good lesson learnt, never throw old parts away until the new parts are fitted in place, must admit my old parts are kept in the garage for years only getting cleared out when room forces a clean out. Still got some Morgan and MGB bits taking up space.