On my 1981 240D, the starter usually works, (once in a while takes a few turns of the key before it engages). But twice recently, the starter was loudly spinning,(not engaged with the flywheel), after i stopped the car and turned it off! Whacking the solenoid made it stop, but once it spontaneously started spinning again after the whack treatment! Is this the solenoid, or a short somewhere?

Whatever it is, I’d look into a new starter. If this one breaks your flywheel you’ve got bigger issues. Tracing an intermittent short may be really tough. Could be the wiring to/from the key, or the starter itself.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

Pull the starter and take it apart. Something is keeping the solenoid on. It can be the solenoid, or it can be the linkages in the solenoid stuck.

If the shaft the starter Bendix/pinion slide on is gunked up enough that can also stick it.

The pinion gear has a Bendix that allows the gear itself to roll away and pull back from the teeth of the flywheel could not go back.

The starter solenoid is held on from the nose end of the gear end of the starer by 2 long skinny bolts with Phillips head slots on it. The heads on those quickly stripped out. I dilled out the heads and you need to be careful a when the drill bit hits the aluminum it pulls in and goes deeper.

I replaced the screws with some from an industrial hardware store.

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84 300D, 82 Volvo 244Gl Diesel

Ring gear and pinion damage notwithstanding, you're lucky the starter didn't burn up yet. They are not rated for a duty cycle any longer than ~1 minute of cranking (and most are not even rated for that).

Definitely get that starter off the car and either replace or rebuild it ASAP.

If whacking the solenoid stopped it, it's not going to be a wire from the key switch, but mechanical binding as Diesel911 describes.

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1993 300D 2.5L Turbo

Thank you. Yet another project!

My '78 240D did that, IIRC it was the starter. If the starter is old replacing it along with a new solenoid would be my first move.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

New starter on the way.

I have had a starter do this, and it melted the lead terminals of the battery posts before i could cut the cable... disconnect the battery as soon as possinle if this is happening.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!