I grabbed a W123 wagon a few weeks back and have just started to go through it and work the kinks out.
Has the typical no dash lights etc that I am correcting but overall pretty nice car, 193K miles.

The issue.
The tumbler is good, the key turns freely with no issues.
When I got it when you would crank it you could hear the starter still engaging after it cranked, I could bump the switch back and it would correct the issue. After driving it a few days I felt the spring in the original switch was shot so I ordered a new Mercedes Starter switch. Removed everything and on the bench it is obvious the spring was not kicking the key back to the run position. So I swapped the switch out and on the bench it was very clear the spring action was evident.
But when I installed it back in the car the same issue is happening, starter stays engaged after it starts, the key is back in the run position but I still have to bump the key back to kill the starter from continuing to try and turn.

What else could it be? I will get under the hood with a meter when the rain clears up but I assume I am getting voltage at the starter even though the spring has kicked the key back.
I guess the solenoid could be going bad on the starter but wanted to ask before I dig into it.

Thanks

Not sure on your terminology. The Ignition Switch is at the Bottom of the Steering Colum Lock. Then there is the Steering Colum Lock and then there is the Lock Tumbler.

I thought the springs that returned the Key were inside of the Steering Colum Lock assembly.

In post 19 at the below link I cut apart part of the steering column lock and you can see the springs.
Your Lock Tumbler turns part 1 and it rotates inside of the squarish hole. Part one also has a Detent Ball on it. It is hard to tell from this particular view but the casting around the detent is cracked on mine.
A crack in that detent might cause what you are speaking of a failure to return.

http://www.peachparts.com/shopforum/diesel-discussion/388556-w123-key-tumbler-not-issue.html

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84 300D, 82 Volvo 244Gl Diesel

See post 4 for some other broken insides of the steering colum lock.

http://www.peachparts.com/shopforum/diesel-discussion/397414-post-ignition-swith-install-gremlins.html

It could be a issue inside of the Starter itself (some times gunk) or on US vehicles I have twice had burs on the flywheel teeth delay the release of the starter gear from the flywheel.

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84 300D, 82 Volvo 244Gl Diesel

Yes check for voltage to the solenoid to determine if it is mechanical within the solenoid/starter or if there is an electrical signal causing the issue.

Had this on an early '78 240D. It was the cut on the key causing the ignition tumbler to not return completely to open the cranking circuit in the ignition switch. You could also try removing the ignition tumbler and using a screwdriver to turn the ignition switch to rule out a tumbler problem.

Good luck!!!

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

Yeah I need to check voltage.

On mine the spring action was in the switch itself. That's why I thought it was the switch as the key did not kick back to the run position with the old switch, it is very apparent it kicks back now.

I tested the stalk without a switch in it and it all moves freely. I don't mind the cost of the new switch as it feels brand new now and it was only $40.

After I posted this I went and cranked it again but without tapping the key. I now think it had nothing to do with me tapping the key as the starter stops at about the same time I would tap the key, maybe a second.

I had a solenoid do this on a gm car before.

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Ignition switch is faulty and contacts in side are still sticking in after starter is engaged

Ignition switch is brand new from Mercedes.

Same in that the spring return was in the switch but the key caused the tumbler to bind so the switch didn't return. It was tired and the spring return was probably weak. Check the voltage like you said. Please post the end result, I'm curious.

Good luck!!!

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

Had a second in between conf calls. The solenoid is showing voltage after it starts and the key has been released. I will pull the tumbler later and try it without it in place and see if it still sticks. I sorta want to buy a new tumbler with key from the dealer anyway so that was going to be a part I replaced.
I will run a wire to the battery from the solenoid and see if hand starting it offers the same symptoms.

And one more update. Running a jumper wire from the solenoid to the battery, Key set to run in the car, when I jumped it at the battery it cranked exactly as it has from the key, Cranks right up but starter spins for a second or two after it is running and I had the wire in my hand so guess I am ordering a Starter.

For giggles, leave the key OFF. Unhook the vacuum line to the IP so that the engine will run with the key off. Try your jumper wire test again. That would pretty definitively rule out the switch or column lock if you get the continuous crank situation.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Good Idea same result. Cranked right up spun for another second afterwards.

I ordered a Bosch Starter after the last test. I have no issue having a new starter on one of these either. It really has been the only thing I have found wrong with it so far. Well AC not working is the other.


Picture of said car with the 300d behind it.

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For those playing along at home wanted to update.

In this case it was the starter. I thought for sure it was in the switch hence why I ordered a new switch for it. It needed it as the spring back action was not there before.

I will have the original starter rebuilt and keep it on the shelf for back up or if I find a coupe and throw it on.

Although expensive, I don't mind swapping these types of parts on cars I just get so I have piece of mind on condition.

Starter wasn't bad to replace, just had to cobble up a tool to get that top bolt loose. Didn't remove anything but the starter, in and out in 45 minutes.

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How refreshing!!!

A Mercedes related problem...
Members sharing constructive advice...
Diagnostics...
A solution...
A posting of the repair...
No personal attacks...
Closure of the thread, it's like back in the day.

AWESOME!!!

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

FWIW,

This isn't a terribly difficult job, taking the old starter apart and cleaning / lubricating it and soldering in new brushes....

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better