300SD won't start unless terminal box is shorted

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Mercedes W126

The Mercedes-Benz W126 is a series of S-Class automobiles manufactured by Mercedes-Benz between 1979 and 1992. Premiering in September 1979 as the successor to the W116 line, the W126 was the second generation to officially bear that prestigious designation, an abbreviation for the German Sonderklasse or "special class." The W126 was initially offered with straight-six, V8, and a turbocharged diesel engine for the sedan. A C126 2-door coupé versions was introduced in September 1981.

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Jasen Friedle @vehiclefreak409

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300SD won't start unless terminal box is shorted

Hi everyone,

I picked up a ragged 1982 300SD over the weekend for a great price. However, the other morning I couldn't get it to start. The starter wasn't turning over, but the car had power and a good battery. I couldn't hear a starter solenoid click either. From my research online and owning a couple of W123s, I tried shorting the two outer terminals of the starter terminal box with the key in the run position and it immediately fired right up.

What part could be going bad? The starter is new from the prior owner, so the only other ideas I could think is that either the neutral safety switch is bad or the ignition switch on the rear of the ignition assembly is bad. Are there any other parts I might want to look at? If all else fails, I'll install a push button start in conjunction with the key, but preferably, I'd like to fix the problem. Thanks!

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Do the dashboard indicators light up when you turn the key? I forgot, was the SD a diesel?

mayer 의 2018년 3월 15일

Yes the indicator lights pop up. The glow plug light comes on for 10 seconds and then shuts off, and the battery light comes on until the car is started. And yes, it's the diesel. The starter sounds very healthy and the car starts up immediately when you short the terminal block.

Jasen Friedle 의 2018년 3월 15일

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tcagle53 @professorc

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Professor C's E-repair and Upgrade Service

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Sounds like you are on track with your diagnosis of neutral or start contacts in key switch. Check for voltage on the start lead when you key the start position (you will need an assistant unless you have very long test leads that will reach into the drivers position). If no voltage try jiggling the shift selector a bit while keying to start position. I would check for voltage at the park/neutral switch (key side of circuit or both if you are not sure) while keying to start position. Still no voltage? .... it is the key switch in all probability and that will be a pain to change out I expect. Maybe a discretely mounted dedicated start button is going to be much easier to do in that case.

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Turns out the neutral safety switch is bad. I'll get around to replacing it eventually, but I'll be mounting a push button start in conjunction with the key. Thanks!

Jasen Friedle 의 2018년 3월 16일

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Do the dashboard indicators light up when you turn the key? I forgot, was the SD a diesel? — mayer
Yes the indicator lights pop up. The glow plug light comes on for 10 seconds and then shuts off, and the battery light comes on until the car is started. And yes, it's the diesel. The starter sounds very healthy and the car starts up immediately when you short the terminal block. — Jasen Friedle
Turns out the neutral safety switch is bad. I'll get around to replacing it eventually, but I'll be mounting a push button start in conjunction with the key. Thanks! — Jasen Friedle

Mercedes W126

300SD won't start unless terminal box is shorted

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