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Old 07-01-2019, 11:08 AM
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Join Date: Jul 2019
Location: NY metro area
Posts: 2
1968 200d. Om621

Hi. I would like some help diagnosing a problem. I turn key on. Pull out glow plug switch . Wait for salt shaker to glow,about 45 seconds. Pull switch to start, and all goes dead. Some smoking from resistor on firewall. Has anyone run across this before. If I have left out any details please let me know. Thanks. John B.
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Old 07-01-2019, 09:43 PM
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Join Date: Mar 2003
Posts: 419
Might be an internal preglow/starter switch failure or a short somewhere along the wiring.
Here are some docs how the system operates and what to check with DVOM (it is virtually the same for om636, om621 and earlier editions of om615 on w115 models):

What does "all goes dead " mean? Does the starter operate?
When the pull switch is in starting position both the glow plugs(although not via the salt shaker,it is bypassed in this position) and the starter are energized.
I'd disconnect the glow plug wire at the ballast resistor on the firewall and check whether the starter operates. Then disconnect the starter at terminal 50 and check whether the glow plugs operate properly with the pull switch in pre-glow and then in starting position...inspect the wiring,clean terminals...

Below is a stylized scheme of the setup from an earlier model, basics are the are the terminal #.

If the problem is in the pull switch unit you can always operate the starter motor and the glow plugs via discreetly mounted remote switches(ones that would support the current), leaving the pull/push knob to operate the IP start/stop lever only +a trouble-free line,if any left(the starter or the preglow one) until you find a replacement.
You may consider upgrading to the fast type, parallel wired glow plugs.
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1968 200d. Om621-glow-sys.jpg  
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Old 07-02-2019, 12:10 PM
cleeves's Avatar
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Join Date: Jun 2017
Posts: 88
Maybe a shorted glow plug more proximal to the salt shaker in the circuit. Happened to me, but the relayed saved the smoke. If the most proximal one is shorted you might be able to swap it with the outermost one and get going on 3 cylinders.

1976 Mercedes-Benz 300D, Colorado Beige / Mahogany
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Old 07-02-2019, 07:50 PM
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Join Date: Jul 2019
Location: NY metro area
Posts: 2
1968 200d John b

Thank you both for your quick replies. All goes dead meaning when I turn on key, pull out starter switch to ignite glow plugs, dash light dims for glow plugs to start, salt shaker glows after about 45 sec, I pull to full start position , started does not engage and all dash lights go out. I am new to any forums so please forgive my execution of this process. Thanks for your help. I will be working on the car this weekend and will try your suggestions. Thanks again John B.
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