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  #1  
Old 07-04-2015, 04:57 AM
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Unhappy OM617 Flywheel missing teeth ... cause?

I just rebuilt the starter on my '84 300D and when I went to check for broken teeth on the flywheel I found some missing - so I am trying to figure out why.
What I found interesting is that the missing teeth correspond to the top of the compression stroke on at least one of my cylinders. I advanced the cam 2 degrees and injection timing 3 degrees years ago. I am wondering if anyone else has had broken teeth possibly caused by advancing the cam or IP.


Last edited by kestreltom; 07-04-2015 at 05:09 AM.
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  #2  
Old 07-04-2015, 05:32 AM
t walgamuth's Avatar
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I cannot see any reason for advancing the cam or IP to break teeth off the flywheel. If you have broken teeth there I would suspect a misalignment of the starter. or possibly a starter with the wrong number of teeth on its gear.
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Old 07-04-2015, 09:37 AM
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How long have you had the car? Do you have any trouble with the starter slipping during cranking? The only thing I can imagine would damage the flywheel is the starter... Aside from an errant screwdriver impact...
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Old 07-04-2015, 11:18 AM
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Unless the motor was kicking back against the starter, your adjustments didn't cause the failure. And even if it was kicking back, the starter drive typically fails first by splitting in half.

Now for a detailed description on how a starter works.

When an engine stops, it is typicaly against compression and the flywheel will roll back a bit. This leaves a small range of flywheel teeth that are at the starter engagement point.

When a starter is commanded to operate, a sequence of events must occur to prevent grinding. For a solenoid operated starter, there are 2 coils on the same bobbin. One is a high current strong pull in and a low current weaker hold in coil. ( Have a look at a good wiring diagram that shows the internal workings of a starter and the below will make more sense. )

The current path for the pull in is from the battery , ignition switch, small cranking terminal on the starter, pull in winding. The other side of the pull in winding goes to the starter motor. What this does is allow the starter motor to slowly rotate as the drive is moving towards the ring gear. The hold in winding is also on the cranking terminal and the other side to ground.

Once the drive bottoms out, it triggers a set of contacts that apply full power to the starter motor bypassing the pull in winding. Since the hold in winding is across the cranking terminal and ground, it keeps the drive pulled in.

When the tips of the starter drive wear, the timing between pull in slow rotation and full power rotating becomes incorrect causing the starter drive to rotate at full power and impact the flywheel teeth. This will fatigue the teeth causing them to break.

There are a couple of other things that will cause teeth to shed. Cranking, letting off the key then recranking before the engine and starter have come to rest, this is a sure way to damage parts. A slipping starter drive one way clutch is hard on teeth too.

The other is that the flywheel was plain fatigued and needed replaced. What started out as a fuel tank cleaning on my IH backhoe ( German Neuss engine ) turned into a ring gear, engine bearing and seal replacement. ( Pulled tank to clean, easy access to starter, pull starter to inspect drive, found broken teeth on the flywheel, pulled motor, gave high wear items a freshining. )
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  #5  
Old 07-04-2015, 12:21 PM
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Quote:
Originally Posted by 97 SL320 View Post
Unless the motor was kicking back against the starter, your adjustments didn't cause the failure. And even if it was kicking back, the starter drive typically fails first by splitting in half.

Now for a detailed description on how a starter works.

When an engine stops, it is typicaly against compression and the flywheel will roll back a bit. This leaves a small range of flywheel teeth that are at the starter engagement point.

When a starter is commanded to operate, a sequence of events must occur to prevent grinding. For a solenoid operated starter, there are 2 coils on the same bobbin. One is a high current strong pull in and a low current weaker hold in coil. ( Have a look at a good wiring diagram that shows the internal workings of a starter and the below will make more sense. )

The current path for the pull in is from the battery , ignition switch, small cranking terminal on the starter, pull in winding. The other side of the pull in winding goes to the starter motor. What this does is allow the starter motor to slowly rotate as the drive is moving towards the ring gear. The hold in winding is also on the cranking terminal and the other side to ground.

Once the drive bottoms out, it triggers a set of contacts that apply full power to the starter motor bypassing the pull in winding. Since the hold in winding is across the cranking terminal and ground, it keeps the drive pulled in.

When the tips of the starter drive wear, the timing between pull in slow rotation and full power rotating becomes incorrect causing the starter drive to rotate at full power and impact the flywheel teeth. This will fatigue the teeth causing them to break.

There are a couple of other things that will cause teeth to shed. Cranking, letting off the key then recranking before the engine and starter have come to rest, this is a sure way to damage parts. A slipping starter drive one way clutch is hard on teeth too.

The other is that the flywheel was plain fatigued and needed replaced. What started out as a fuel tank cleaning on my IH backhoe ( German Neuss engine ) turned into a ring gear, engine bearing and seal replacement. ( Pulled tank to clean, easy access to starter, pull starter to inspect drive, found broken teeth on the flywheel, pulled motor, gave high wear items a freshining. )

Your explanation of the solenoid is pretty nice - it also explains of the "no click, no action" type failures experienced by starters.
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  #6  
Old 07-04-2015, 12:29 PM
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Walgamuth: The starter and flywheel are original, so the number of teeth has to be correct. The transmission was replaced about 3 years ago, and the alignment could be an issue, but I saw no problems when pulling it out to indicate that.

Vstech: I have been driving the car for 4 years now, and the starter has been making an occaisional grinding-screeching "slipping" sound for the last 2 years. After the first time it made that sound it wouldn't start and I called a tow truck. The driver suggested trying to turn the crank a bit to see if the starter would engage and his trick worked. I used an adjustable wrench on the power steering pump nut to nudge the crank. That wrench stays in the car all the time now...

SL320: Wow... that was detailed. Thanks!! - I have hunch that you are correct about the timing between the solenoid pull-in and the motor full power cycle being off. When you think about, there are several wear points that could affect this timing. The drive gear teeth are indeed worn, but the nylon solenoid plunger link, the yoke pivot, the slip ring, etc. are all worn. I wonder if it would be possible to shim or otherwise adjust one of these components to cause the gear teeth to engage earlier... Don't get me wrong, I am perfectly willing to order a rebuilt starter, but I can't help wondering.
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  #7  
Old 07-04-2015, 06:33 PM
t walgamuth's Avatar
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the starter has been making an occaisional grinding-screeching "slipping" sound for the last 2 years

This was your signal to act right away.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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  #8  
Old 07-04-2015, 07:49 PM
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Quote:
Originally Posted by t walgamuth View Post
This was your signal to act right away.
I sometimes can drive for days without the problem occurring. Not sure the problem has been made worse for the driving. The very first occurrence mandated a new flywheel and starter anyway. I have it up on jacks right now, so it's do or die time...
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  #9  
Old 07-04-2015, 09:40 PM
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  #10  
Old 07-04-2015, 09:42 PM
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Shimming where the starter bolts to the engine would make things worse from a timing standpoint. Shimming the solenoid would help delay full power.

A your welcome to those that appreciate the detail. There is way more going on in seemingly simple systems that it appears. I've been in and around the automotive business for 40 years and always have looked at things from an engineering perspective.
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  #11  
Old 07-04-2015, 10:01 PM
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Quote:
Originally Posted by Zulfiqar View Post
Your explanation of the solenoid is pretty nice - it also explains of the "no click, no action" type failures experienced by starters.
To expand on this: As you figured, no click no action can be caused by the starter motor portion not having continuity. When this occurs the pull in windings don't do anything and the hold in windings are too weak to pull the drive / solenoid contacts.

Sometimes you will get a partial / weak click usually with a hot engine. This is caused by an otherwise good starter not getting enough amps through the starter trigger wire. A very common fix ( and many times a factory design ) is to use a Ford style remote solenoid ( relay ) to trigger the starter. This is also called a "hot start kit". The Ford solenoid takes much less current than a starter trigger.

If you get a single click and no crank ( with a good battery ) , this is usually the large battery power wire at the solenoid or the solenoid contacts failing.

If you get a violent chatter when trying to crank with a good battery and cabling, the hold in coil in the solenoid is bad causing the drive to pop in and out of engagement when the pull in coil is switched out of the circuit.
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Old 07-05-2015, 05:33 AM
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oi... that's it!

Quote:
Originally Posted by 97 SL320 View Post
To expand on this:...

Sometimes you will get a partial / weak click usually with a hot engine. This is caused by an otherwise good starter not getting enough amps through the starter trigger wire. A very common fix ( and many times a factory design ) is to use a Ford style remote solenoid ( relay ) to trigger the starter. This is also called a "hot start kit". The Ford solenoid takes much less current than a starter trigger...
I get a weak click with a hot engine, followed by the screeching/grinding but no engagement (not always). I was going to mention the hot engine variable... I was wondering about the start trigger wire supplying enough amps to the pull-in windings. I have a dc solid state relay (ssr) that I can use to switch the current ( I measured 11 amps) and just use the starter trigger as a low level signal. That should improve the hot start issue.
To improve the pull-in / motor start timing issue I fabricated a .090" stainless shim that slips into the nylon link: https://photos.google.com/album/AF1QipM_A_7XykROfbuGEj9paTlH6sotdJFSe0FakD-r - I am hoping that this will not put undue stress on the nylon link...
I am also making a new 2/O cable from the front battery post (battery moved to trunk) to the starter to minimize voltage sag under load.
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  #13  
Old 07-05-2015, 08:23 AM
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A couple quick things.

Inrush to the starter trigger wire will be high, many amp clamps are heavily dampened so you might not be seeing the peak. ( RE: you might just be seeing the hold in coil. ) I haven't measured pull in so can't give you an exact # but do know it is reasonably high.

I use SSRs on an industrial basis but would not use in your situation, they are generally used when switching is frequent and a mechanical contactor ( relay ) would wear out rapidly. Electric resistance heating is a prime example.


When a SSR fails it usually fails on so your starter will remain engaged. In a heating app there is a mechanical contactor in series triggered by main power / high limit sensor. Even when turned on, the SSR has some voltage drop across it and you will loose a bit of voltage.

A SSR generally does not like inductive loads so a flyback diode across switched terminals will be needed. When a coil is energized and the power removed, the magnetic field collapses. This energy needs to go somewhere so the coil now becomes one side of a transformer giving a high voltage spike.

The pic does not work for me, google wants me to make an account.

With a remote bat it is OK to put the neg terminal to the body but be sure to bond the engine to the chassis. This is how MB does it from the factory.

Don't stray too far from factory parts in good condition as the timing will take care of its self. If the FW is missing teeth, change that before going any farther as the situation will degrade rapidly once another tooth is lost.
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Old 07-05-2015, 02:11 PM
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I am trying to paste the images directly into this post. We'll see if the moderator will tolerate this...



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Old 07-05-2015, 02:16 PM
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OK - hope that worked.

Thanks for your thoughts on the SSR. I forgot that they fail to a shorted state - not what I want.

You are right about fixing the flywheel, that has to be next.

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