Question about M52 idler pulley and tensioner, and crank sensor

[Chilezen]

I'm putting together an engine that I took apart a while ago, and I've lost track of something. I'm not sure of my engine should have an idler pulley.

It's an M52 Z3 block, and I have an S52 oil filter housing. It came with a mechanical tensioner, and I don't think it had an idler between the alternator and water pump. But as I'm searching vendors to replace old parts, like a new tensioner, I see the options for a hydraulic one. And then there's the idler.

Does it matter? It looks like the belt length is the same whether or not there is an idler pulley, right? Does the oil filter housing (metal vs plastic cap) make a difference? Some people on other forums have noticed some engines do, some don't. I've read that the hydraulic tensioner is a factory upgrade from the mechanical. But does that mean I should add the idler too? Because I could just get the mechanical one and be done with it.


I also wanted to ask about the crank sensor in the block: there's a factory recall for the old style, and I'm not sure what that one looked like. The crank sensor I have on this engine has the plug on the sensor, not a pigtail to a plug. But the description says it has 4 terminals. My sensor has 3. It says I may need an adapter to draw the extra voltage from another plug. Do I need to buy this too?

Thanks!

[mjweimer]

Maybe I can help. My project also used a M52 Z3 engine but I kept the plastic lid oil filter housing. I converted from the spring version to the hydraulic version and neither set-up used an idler pulley; both would use the same belt since the tensioner pulley ends up in the same spot.

With spring tensioner:



With hydraulic tensioner:




On the crank position sensor, if you have a non-pigtail sensor, your donor had the recall performed and the retrofit harness installed. The new harness grabs power from the injector rail (vanos solenoid) and sends +12V to the sensor rather than the original +5V. See service bulletin SIB 12 10 99




Unrelated to your questions but good to mention if someone looks at this thread in the future: keeping the plastic lid oil filter housing created an issue with the return hose fitting on the PS pump interfering with the E34 pan. A change to the M50/S5x oil filter housing and PS pump/bracket would have solved the issue but I chose to extend the PS pump bracket and run a different belt.


Cheers.

[Northern]

I don't know what the determining factor is for whether an engine received an idler pulley or not.
It makes sense that it's there: it adds some amount of wrap angle for the belt on the alternator pulley, which is already small and has only like ~80 degrees of wrap, but why did some come with and some without?

[mjweimer]

I don't know what the determining factor is for whether an engine received an idler pulley or not.
It makes sense that it's there: it adds some amount of wrap angle for the belt on the alternator pulley, which is already small and has only like ~80 degrees of wrap, but why did some come with and some without?

Belt length and overall tension on the belt to keep it from slipping are the two primary factors. From what I understand through friends who do this kind of design work on large diesels, there is a constant balance of tension vs. pressure on the accessory bearings (H20 pump, alt, etc.) that comes into play when trying to also keep the belt from slipping. That is where the pulley wrap etc, also figure into the equation. It is interesting and something we all take for granted.

[Chilezen]

Thanks guys!
What I've gathered from here and elsewhere, is that there isn't a determining factor, at least not a known one, as to why some do and don't have an idler. I agree, it makes sense that it should have it. But at this point, if it didn't come with it and I don't have the extra long bolt, then I shouldn't add it.

Plus, there's a centered and offset style; which one would be the correct one? Does the alternator have an edge that determines a centered/offset idler?

Is there a preference for a hydraulic tensioner over a mechanical?
You have a hydraulic tensioner with no idler, and no problems?

That's a good note about the power steering pump. I saw another thread in which you discuss the issue and how you solved it (Link for others to see). I wonder how mine will fit, being the plastic-cap pump on a metal-cap bracket. I dunno.

As for the crank sensor, I took a closer look at the harness, and yes, the factory adapter has been added already. I saw that service bulletin prior to asking, just wanted to make sure.

[Northern]

I have one car with M52, m50 filter housing, M50 PS pump, idler, mechanical tensioner.
Works fine to redline of 7200

Second car has M52 with all M52 fixings, idler, mechanical tensioner.

Neither engine came with an idler (could've been deleted).

Idler also happens to be placed where the belt's longest free length is, so it might reduce vibration in the belt?