Kind readers,
My '79 240 4-speed has been off-line for the last 6 months while I tracked down water leaks and replaced steel.. a sordid tale that I won't bore you with here.
It stopped turning off with the key, and I tracked the leak to the ignition switch. I pulled the offending member (vacuum switchover valve), and tried to figure out how it works, and why it failed.
From the looks of it, it appears that a cam in the ignition switch ramps up and pushes in a pin that closes a (controlled) vacuum leak, allowing the vacuum to switch over to the pump shutoff valve.
But.. on closer inspection, it appears that the ramp is down with the key in the OFF position, and up when the key is in the RUN position. (?)
1. Has anyone experienced the ignition vacuum shutoff cam getting "out of time", or am I just looking at this incorrectly?

I did some more diagnostics, attempting to turn the key, and immobilize the cam wheel, to see if it was loose, or able to turn independently, and it seemed to be 'fixed' in position.
On the next startup, the engine was making a new sound, so I shut it off.. and the sound continued.. the starter was continuing to turn, but it wasn't engaged with the ring gear.. (WTH?) Had to pull the neg. battery cable to stop it..

First guess was that I had really hosed the ignition switch, and the start circuit was hot all the time now.. but more thinking points to a stuck starter solenoid.. looks like an original '79 starter, which is probably bone dry, hasn't been opened up and lubed in almost 30 years.
Thats as far as I got last night.. Next step is to disconnect the starter solenoid circuit and see if it still fires when I connect the ground lead.
I'm guessing corrosion and neglect, because the drive pinion isn't engaging either..
Any BTDTs would be appreciated.
Regards,
Mark

Sounds to me like two separate issues. There is a spring loaded switch at the base of the ignition tumbler that the tip of the key depresses, thus allowing the ignition switch (ignition lock housing) to turn and activate the starter. The vacuum shut off should be fairly easy to diagnose. Make a note of the two (usually brown) vacuum lines coming to the ignition and their respective positions. Pull them off and after starting the car and turning the key back to off, suck on each line until the car shuts off. If it doesn't, there's a leak somewhere between ignition and vacuum shut-off valve on the IP, or a bad vacuum shut-off valve (pretty easy and inexpensive to replace). If the car does shut off with mouth suction, you either have no vacuum to the switch (the other brown line) or a bad ignition lock housing (unlikely to fail in that way).

Sorry I haven't yet had the ignition switch apart to be able to say exactly what its mechanism is inside as to cams. And good luck with the starter, it is possible that issue also originates with the ign. lock housing. The 7-8 wires that come out the back of said switch are accessible by removing the black dust cap.

__________________
1985 300D Gretchen (Astral Silver) 220k
1983 240D 4-speed Evelyn (Orient Red) 203k


TANSTAAFL

Vacuum to the switch is fine, and when jumped to the Brn/Blu line, the engine shuts off.
A sharp rap with a metric hammer disengaged the contacts in the starter solenoid, so that's pulled out of the equation. I'll still pull it and rebuild it on G.P.s, but its no longer a possible 'symptom'.
On the other hand, it still wouldn't shut off after starting, and when I went under the hood (in the dark) to press the shutoff lever, I saw that the glow plug buss bar was glowing.
Disclaimer: I had turned it off at the ignition to see if the vacuum switch was working (after removing / greasing the seals / reinstalling).. it wasn't, the engine continued to run, so I turned the ignition back on, and the GP light lit for a few seconds and turned off. The ignition switch was still on when I went under the hood to kill it, and I saw that the GPs were still fired up.. a minute later. They went off with the key, so I'm assuming that the internal timing of the switch is still 'off'..
How long are the GPs supposed to stay 'hot'?
Mark

If they are newer glow plugs they have an afterglow feature that keeps them on for a longer period of time, perhaps 30-45 secs. Not sure exactly. Longer than that, or if you have older glow plugs, I'd say you have an issue with the glow plug relay, its fairly easy to test the glow plug wiring and the plugs themselves, both for resistance and voltage. That should narrow down the source of the problem if there is one. You'll need a multimeter. And do a search on this forum, its been awhile for me and I don't want to misinform you. I don't see how it could be an electrical fault in the ignition switch keeping the plugs on, but have no experience with that symptom that wasn't fixed in the glow plug circuitry.

If does sound as though the shut off cam inside the ignition lock housing is faulty, do you have any junkyards close-by? It's usually an easy to find item and much cheaper than the $150 or so a new one costs.

__________________
1985 300D Gretchen (Astral Silver) 220k
1983 240D 4-speed Evelyn (Orient Red) 203k


TANSTAAFL

So I ran out to the local 'yard' over lunch and scored what appears to be a nearly new ignition vacuum switch, and the car shuts off like its supposed to.
It should have been a tip-off to me that when I pulled the original vacuum switch for inspection, it came out in pieces.. I prolly could have epoxied the 'plug' back in, but it works..

The other issue still stumps me, but shouldn't happen in normal use.
When I shut off the car, and it continues to run due to bad switch, and I turn the ignition back on, the glow plug circuit fires and stays hot (literally). The bus bars glow until I kill power to the ignition.

Thanks for the input.
Mark