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  #16  
Old 07-05-2015, 05:42 PM
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Appropriately sized pictures are OK in a post.

On your starter I don't know what bottoms out first, the drive or the solenoid core. If the drive is what bottoms out first, there may or may not be enough solenoid core travel to trigger the starter motor to turn. If this occurs you will just get a click ( and that would be a good time to measure pull in coil amps, just don't hold the key in the crank position for more than a second or two as the winding will become overheated. )
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  #17  
Old 07-06-2015, 09:09 AM
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As it sits on the bench right now the starter operates perfectly with the shim installed. What I don't know yet is what it will do when re-installed on the vehicle. I am planning to re-install it today to check and see if engagement with the flywheel prevents the solenoid from pulling in the motor contacts. First, I am upgrading the start signal circuit using a 30A Mbz relay.

BTW... I checked the Mbz starter wiring diagram and surprised to see that there is no relay in the circuit. The driver's key switch handles the full load! I also noticed that the start signal current path is directed through the neutral safety switch on the transmission before it finally terminates at the starter. I checked my wires and noticed that there is worn insulation down near the transmission, as well as old and degraded fiberglass insulation pretty much everywhere under the hood. It looks as if installing a relay to "amplify" the start signal is good preventative medicine...

I also did the math to determine if upgrading to a 2/O starter cable would be worth the effort and found that it would reduce energy lost in the cable by about 20 watts, which isn't much for a motor drawing around 2kW - so decided to just clean the terminals, use a corrosion inhibitor, and leave it stock.

Last edited by kestreltom; 07-06-2015 at 09:33 AM.
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  #18  
Old 07-06-2015, 11:09 AM
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Yeah, the keyswitch contacts are fairly robust, and the starter solenoid is a big relay, so not much load on the key...
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  #19  
Old 07-07-2015, 07:26 AM
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Talking problem fixed!

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Originally Posted by vstech View Post
Yeah, the keyswitch contacts are fairly robust, and the starter solenoid is a big relay, so not much load on the key...
I measured 11 amps on the hold-in coil and 43 amps on the pull-in coil. The pull-in only activates for a fraction of a second, but still draws a lot of current. I saw degradation of the start signal wiring circuit in 3 places that most likely caused problems with the speed and timing of the starter pull-in. This ultimately cost me 2 lost teeth on the flywheel.

The three problems with the start signal circuit path that I witnessed were:

1.) the wire bundle near the neutral safety switch on the transmission had lost some insulation due to friction & vibration. I do not think that it had shorted to ground before I discovered it.
2.) on the passenger side front fender well, the white start signal wire is bundled with the motor power wire and passes through a plastic wire support grommet about 10 inches from the starter motor. That wire had lost its insulation and some copper due to vibration within the bundle at the location of the grommet. It was not visible or apparent until I removed the wire bundle completely.
3.) the motor windings themselves complete the start signal circuit path and accumulated dirt & grime on the brushes & commutator created a higher-than-normal impedance in the ground path.

The start signal path is long, maybe as much as 10 ft. of wire going through 7 or 8 junctions. If any of the wire or junctions are degraded due to age, oxidation, grime, vibration, etc. then the signal path will have a total impedance that is probably out-of-spec according the the original design. The higher-than-normal siganl path impedance caused a slow and/or late engagement of the starter pinion gear which ended up breaking 2 teeth on the flywheel.

What I did to fix the problem:
- I rebuilt the starter motor. The commutator was in good condition and only needed to be reconditioned on a lathe using a file and a scotch-brite pad. The brushes were in good condition. The motor was cleaned, lubed, and reassembled with a .090" shim installed in the solenoid plunger nylon link to advance the pinion gear engagement timing (see photos in previous post)
- I installed a 50 amp relay in the start signal circuit to "amplify" the signal close to the starter motor.
- Since I removed the starter wire bundle, I ended up replacing the #2 motor power cable with high quality 1/O welding cable.

How it performs now:
The starter engages and spins the motor with authority now. The 2 missing teeth do not seem to affect anything unless the motor comes to a stop with the gap lined up with the starter gear, which happens now and then because the gap is located just shy of TDC on the cylinder with best compression. This is not the best solution - but it will hopefully get me by until I can find the time to go pull a good flywheel, get it match balanced to mine, and replace it.
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  #20  
Old 07-07-2015, 06:50 PM
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Yep, 43 A is a lot and why some use a "hot start" relay. What did you use to measure current and did that include the starter motor as installed on the engine or running free on the ground?

You have a good thought process going in order to solve problems.
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  #21  
Old 07-08-2015, 11:20 AM
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Talking

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Originally Posted by 97 SL320 View Post
Yep, 43 A is a lot and why some use a "hot start" relay. What did you use to measure current and did that include the starter motor as installed on the engine or running free on the ground?

You have a good thought process going in order to solve problems.
Thanks!
I used a Fleabay DC clamp-on ammeter.
I measured the solenoid pull-in coil by energizing it with 12V pos. connected to the small screw start signal terminal on the solenoid and 12V neg. connected to the copper braided link between the solenoid and the motor. The motor itself was not energized.
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  #22  
Old 07-09-2015, 12:40 PM
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The ford solenoid as an add on is cheap. For some reason many German cars in my experience seem to have a lack of a good strong trigger current. As they age in general this situation usually does not improve.
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  #23  
Old 07-10-2015, 03:18 PM
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Originally Posted by barry12345 View Post
The ford solenoid as an add on is cheap. For some reason many German cars in my experience seem to have a lack of a good strong trigger current. As they age in general this situation usually does not improve.
After I measured 43 amps going to just the pull-in coil, I reached into my box-O-relays and grabbed one of the beefier ones. It is a bit of a mystery as to out how the engineers at MB decided on such a tenuous start/trigger signal supply to this obviously hungry load, maybe they reasoned that the duration of the demand was so short that it didn't matter... or perhaps they were intending the overall impedance to act as a sort of current limiting "ballast" of sorts.
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  #24  
Old 07-10-2015, 03:32 PM
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Quote:
Originally Posted by kestreltom View Post
After I measured 43 amps going to just the pull-in coil, I reached into my box-O-relays and grabbed one of the beefier ones. It is a bit of a mystery as to out how the engineers at MB decided on such a tenuous start/trigger signal supply to this obviously hungry load, maybe they reasoned that the duration of the demand was so short that it didn't matter... or perhaps they were intending the overall impedance to act as a sort of current limiting "ballast" of sorts.
I question the 43 amps measurement of the pull in coil. You used a cheap ebay DC clamp on meter as I recall. I am not sure how accurate that meter can measure a signal that is a few micro seconds long. I know the hold coil is around 8 amps (measured in series with a digital ammeter), so my guess is the pull in coil is not much more than that. Maybe you can measure the resistance of the pull in and hold coil and calculate what the currents should be?
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  #25  
Old 07-10-2015, 09:05 PM
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Quote:
Originally Posted by funola View Post
I question the 43 amps measurement of the pull in coil. You used a cheap ebay DC clamp on meter as I recall. I am not sure how accurate that meter can measure a signal that is a few micro seconds long. I know the hold coil is around 8 amps (measured in series with a digital ammeter), so my guess is the pull in coil is not much more than that. Maybe you can measure the resistance of the pull in and hold coil and calculate what the currents should be?

See his post # 21.
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