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Old 09-29-2004, 12:48 PM
ccooper's Avatar
Unlicensed Mechanic
Join Date: Feb 2004
Location: Ladera Ranch, CA, US
Posts: 86
New Starter Installation - W126 / OM603 300SDL

I recently replaced the starter on my ’86 300SDL. I’m adding this thread because I didn’t find anything that specifically dealt with changing a starter on an OM603 engine in the archives. In a nutshell, this was an extremely easy project; the hardest part was holding the ~15 lb motor whilst on my back. This certainly applies to all engines in the W126 chassis and I presume this would apply to the ’87 W124s. It also appears to be the same for the M103 motors (at least the W126 300SE/SEL). The bottom line is that I took a job that previously cost me over $500 for my mechanic to do and turned it into a $200 project.

Just for some comparison between what is posted on the site about the older vehicles and the OM603xx. On the OM616 and OM617 motors, the starter is located on the passenger side, underneath the exhaust manifold. On the turbocharged motors, it’s clear that it’s not an easy prospect to get to the starter. From what I read, it seems like at best, there is a need to jack the car up and be able to turn the wheels to get the starter motor away from the car. From there, it seems that one needs to remove portions of the exhaust and other not fun things.

My experience was none of this. The starter is on the driver’s side, situated almost directly below the oil filter. I also noted the starter appears significantly smaller than on my 240D – maybe on the order of 10 lbs less and certainly much smaller (I had the “heavy duty” motor on it). The procedure in the factory manual shows four steps – which is about all it takes. Here is what I did:
  • I drove the car up on ramps – I’m in earthquake country and I avoid using jack stands unless I absolutely have to. Of course, I did this on level concrete, left the car in park, set the parking brake and chocked the rear wheels.
  • I disconnected the negative lead from the battery (I don’t like sparks!) This is VERY important since potentially lethal amounts of current pass through the starter.
  • From the solenoid (the small cylinder attached to the starter), I removed the large cable (direct from the battery) and the wire from the ignition. I was sure not to tear the rubber boot as I was pulling it back to expose the connections to the starter.
    ** Note: This is a good opportunity to add a relay – more at the bottom.
    From underneath the car, I unbolted the two 17mm bolts. A couple of words about this:
    • I used a deep 17mm socket in lieu of an extension. I could see no need to have the 18-24” extension that may be needed on other cars.
    • I disconnected the spring from the accelerator linkage since the linkage got in the way. I had to push the linkage up a little to get the socket in. (Make sure you connect the spring when you are done!
    • The two bolts have to go quite a way before being undone. My suggestion is to unscrew them in roughly equal increments, so one is not all the way in while the other is out. I was ABSOLUTELY SURE to support the starter with my free hand. Since I needed to reuse the bolts, I put them in an easily accessible place.
  • Once I had the starter freed from the car, it was a little tricky to get it out – especially given that a 15 lb weight feels much heavier when on one’s back. I ultimately worked it out in the space between the Pitman arm, the drag link (“center tie rod”) and the guide arm. It’s a very tight squeeze, but it went out. (If the motor was any larger, it would not have worked like this). I noticed how the motor went out, because the replacement goes back in the exact same way – and there is only one way it will go in. I found it does not go through the top without disconnecting many lines, which I was too lazy to do.
  • Make sure your replacement motor matches the old one. If it’s a Bosch, the motor itself will say “Made in Spain” while the solenoid will say “Made in Germany.” The starter motor should have the same part number, while the solenoid may differ – it did on mine, but worked nonetheless. Check that the exposed gear spins in the same direction on both.
  • Putting the new (or rebuilt) motor was not too difficult. There really is only one way it can fit in to the engine block, but you may have to work it a little to get it in. Ultimately, the edge of the starter MUST be flush with the edge of the pocket on the engine and you should be able to put the bolts through in and start tightening them. I held the motor while I started threading the bolts. I noticed the bolts went in much more easily while I was holding the motor flush against the engine block
  • From there, as they say in the Factory and Haynes manuals, installation is the reverse of removal. Make sure you clip the spring for the accelerator pedal, or you may find a little surprise when you take your foot off the accelerator.
  • I used this opportunity to clean all the electric terminals – I’d hate to spend another $200 to replace a starter motor when the only issue was corroded connections!
As a bonus, I added a relay to the wire coming from the ignition switch (the small one). Bosch makes a kit for this and my word of advice is to make sure there is a 25 amp fuse between the battery and the relay. The way it works is that rather than have the current to actuate the solenoid (and therefore the starter) go through my ignition switch and Outer Mongolia, the relay acts like a remote switch, giving the current far less distance to travel, and much less resistance. This significantly reduces the chance of the starter motor unexpectedly not starting. The only thing the wiring from the ignition switch has to do is to switch on the relay. This is also a great upgrade for headlights. Since I’ve written enough, and since others have covered the topic, I’ll leave it at that.

Casey Cooper
"From a long line of Mercedes ownership"

'86 300SDL 250K miles (Gone, but not forgotten); best diesel I have ever driven, too bad about the Achilles heel.

'81 240D 370K miles (Sold to my brother after 9 years and 150K miles of reliable driving!)

[Five other Mercedes in family clipped for less length.]
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Old 09-29-2004, 01:13 PM
Hatterasguy's Avatar
Join Date: Nov 2002
Location: Milford, CT
Posts: 19,307
Good thread! I will print it and save it, thanks.
2020 GMC Sierra Denali
2007 Tiara 3200
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Old 09-29-2004, 07:46 PM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
The W124 is MUCH tighter. Took two of us to do this job on the 300D (stuck in the driveway, alas, with totally locked up starter) because I wasn't strong enough to get the top bolt loose. VERY little space, we ended up with me holding the socket on and my brother using his foot on the breaker bar to get it out. Only enough room for a short extension, as I remember on the top bolt.

Else very easy. You may need to replace the connection on the keyswitch wire as the new starter have a different termnial than the old ones, clip the old one off and crimp the new one on. PITA because the wire is short.

Otherwise easier than the 220D.


1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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