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Discussion Starter #1
clutch bearing is rattling some, clutch magnet ingaged all the time. 186xxx on the car. was thinking of diy rebuild but with weather and no where inside to do it I was thinking of replacment.

found a place to get a rebuild one with core exchange but they say that the mag clutch must be working on the core. I read somewhere that if you can manually turn the mag clutch with your hand that the mag clutch is ok and the problem is somewhere else in the system.
can anybody comfirm this????? or is there another way to test it???

I tryed it and I can turn it by hand. so what does the group say????
 

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Registered 2011 SLK300
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I believe the clutch is like the one on the a/c compressor. (I don't have the clutch on mine so I can't go out to the garage to check it.) If it is the same, the clutch will spin 'cause it is connected to the s/c shaft. If it is rattling and not engaging, then it probably is the magnet which pulls it in to the belt pulley or the pulley bearing itself. That should not be a problem to diagnose and fix.

While the car is off, unplug the wire from the magnet & jumper that wire that comes from the magnet to the battery + terminal. The clutch should snap in audibly and visually. Now, try jumping it with the car running. Should do the same thing and get the s/c spinning and it should stop spinning when the +12v is removed from the wire.

If that is the case then look for the reason why there is +12v being applied to the clutch whenever the key is on. Any weird noises or other strange behavior will need further investigating. If the pulley bearing is bad, it should make noise all the time, but in a rare case it may only make noise if it is puled out of alignment by the magnet. Get a stethoscope and listen carefully to everything in that area. Hope this helps.
 

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clutch bearing is rattling some, clutch magnet ingaged all the time. 186xxx on the car. was thinking of diy rebuild but with weather and no where inside to do it I was thinking of replacment.

found a place to get a rebuild one with core exchange but they say that the mag clutch must be working on the core. I read somewhere that if you can manually turn the mag clutch with your hand that the mag clutch is ok and the problem is somewhere else in the system.
can anybody comfirm this????? or is there another way to test it???

I tryed it and I can turn it by hand. so what does the group say????

For the pre-2000 models Slk230 with SC clutch, open your hood, at idling, you can see the SC is not spinning, then rev the engine above 2500 rpm, your SC should come on. You can see see the clutch engaged and hear it too.
 

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Discussion Starter #5
For the pre-2000 models Slk230 with SC clutch, open your hood, at idling, you can see the SC is not spinning, then rev the engine above 2500 rpm, your SC should come on. You can see see the clutch engaged and hear it too.
I said the clutch is engaged all the time. the face of the pulley is spinning all the time. always on so I hunting ways to test it. I just found a thread where someone had to replace a mosfet in one of the control boxes to fix it. I'm hoping its something simple like that.
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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I have a 99 230 that was doing the same thing - SC constantly engaged. I googled this and compared to some working cars... And I decided to try the MOSFET replacement. It did stop the constant engagement - the SC doesn't engage at idle any more (which is good, and validates the problem is the motronic) but it doesn't seem to want to engage at all now (which is bad). The best I can get out of it is a few turns when I rev it up - nowhere near the positive engagement it is supposed to have.

So I'm thinking maybe I didn't solder the replacement MOSFET in well enough, or I damaged the new one during the replacement.

Interestingly, I measured battery voltage at the SC connector... I'm wondering if the motronic uses a PWM signal to engage it.

Also I've heard there is a reset procedure for the bypass valve - has anybody here used that procedure?
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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Answering my own question here -- the ECU provides the ground to the SC clutch - not the +12V. So the fact I can measure +12V at the clutch connector is irrelevant.
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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No joy still, and a bunch of dumb questions related to bypass valve and ME-SFI

I re-soldered the MOSFET but I'm still seeing the same result - one weak revolution then the SC disengages.

I found a procedure for "resetting the bypass valve", but it's unclear exactly how to do it. Supposedly this is in SSM - can any of you with SSM find this procedure? It would be nice to know 1) What position the valve should be in to start the procedure, 2) How far I'm supposed to manually turn the valve with my finger, 3) At what point I'm supposed to remove my finger, and 4) What I should expect to see if it works as expected.

(I'm also not sure how the reset procedure knows that I'm holding the valve with my finger -- is there some sensor that tells it this?)

Also, if anybody has a wiring diagram for the 99 ME-SFI - that would be most helpful too (so I can diagram what *all* the MOSFET's are for, not just the one MOSFET for the SC clutch).
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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OK - end of story - The SC is working great now. Two problems at once.

First, the MOSFET was indeed bad, that's why the SC clutch was constantly engaged. This was surprisingly easy to fix given my kindergarten soldering skills. I did employ ESD protection (if you try this repair, you should too -- cost about $15 USD).

However, after replacing the MOSFET, the SC clutch wouldn't stay engaged. It would engage briefly at the prescribed times but then stop after maybe one revolution.

Second, there was a broken wire to the bypass valve. Apparently the clutch is disengaged when this particular wire is broken. I do wonder if the Motronic is making this decision digitally, or if there's something analog making this happen.

I don't have an accurate 99 wiring diagram for this part (M16/7) so I don't know what this wire is.

The key to finding the broken wire was to disconnect the air hose from the bypass valve so you can see the valve, and then watch it during startup. Comparing it to a working unit was helpful... When revving the engine the butterfly valve should clearly open and close and basically be moving a good bit. In my case, no motion at all no matter what. Further inspection led to the broken wire.
 
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