Questionable Ignition Problem
 

Hello everyone, I have a bad problem and not sure what's the cause. I have a '79 300SD and it has a very strange ignition problem. What happens is that when you turn the key to the "on" position or III, it would crank, start the car and keep it cranking. You have to turn the key back just a little to stop the starter from cranking which is also the Acc (II) position, at which point the cars shuts off. So far, we've asked around and no one seems to have experienced this problem. Any information will be highly appreciated.

I think you either have a sticking solenoid ( which is just a big electrical relay which also pulls the teeth on the starter forward to engage the flywheel ) or something wrong in your key switch....
If it is the key switch... you might investigate to see if you could just cut that wire to that switch... and reroute it to a normally open , press on switch to use for the starter... old cars had this seperate switch for the starter ( my 54 f500 has it... and all my tractors )... but the nice thing is if you hide the button ( available at most auto parts places ) it becomes an antitheft device.
And that may be much less expensive than fixing or replaceing that switch....

Get a new keyswitch.

Peter

Thank You for the replies.
 

Leathermang, I think the problem isn't the solenoid because we've tried 3 different starters with the same results. What I think the problem may be is that there is a break in the line somewhere OR it could be the ignition switch. I guess I'll go to the autowrecking yard and pull a few ignition switches and give them a try.

Peter, is the key switch the ignition switch? Or is it the component in between the key tumbler and the ignition switch? I'll post a picture just in case.

Yes.

We diesel junkies tend to call it the keyswitch because we don't have an ignition, per se. And on the older MB diesels, the fuel and starter/glowplugs aren't operated off the key anyway, but by a knob on the other side of the dash.

You will need one off a diesel because it also has a vacuum switch for the fuel cutoff.

Peter

Thanks again Peter, I'll have to get that checked out then.

Alright, finally got some time on my hands, got 3 ignition switches, tried them, and it appears that the problem still exists. I'm assuming there's a short circuit somewhere, as I opened up a ignition switch and figured out how it works. On the picture, it shows those numbers and/or letters labelled next to each pin. What does those numbers stand for? By any chance the amount of amps that travel through them? :eek: With a good look on the inside part, the 2 pins with 15 are linked, and appears to touch a terminal in the middle when the key is turned to the preglow position. The one labelled P and P30 are the buzzer that indicates you left your key in the ignition. The two that says 30 on them seem to be linked to the middle terminals. I assume the 50 is for start and the R for accessories (power windows, radio, sunroof). Can anyone confirm this for me? I do not have any W116 repair literature. Thanks.

30 is battery, I think. Not sure of the rest, I will take a look later at what manuals I have to the W123 and see what I can find.

Shouldn't be all that hard to figure out which one is the starter wire.

Also, take a look under the cover for the connections to the starter. There are two or three (depending on model) wires that go with battery cable on the starter. If one of these is pulled back into the cover and doesn't get on the screw, it can short the starter solenoid on.

Otherwise, you are going to have to look for something that works off the starter circuit (AC relay, for instance) that is shorted so that the starter stays energized by a backfeed.

Look for "user modifications" in the wiring -- foolish people do all sorts of damage mucking about with automotive wiring.

Peter

I still haven't gotten it done :(
 

Found some new information about this stupid problem. If the battery is disconnected for at least 5 minutes (giving time for everything to discharge), then you hook up the battery, its normal. However, depending on how long you let it discharge for, it would start doing it again. What device "holds" a charge or triggers a switch to letting enough current through to crank the starter? My dad took a bunch of miscellaneous relays and "black boxes" from the junkyard last week. I wanna try and figure out more before i start "plugging and praying"

Hello jmak124
Your problem sounds like a magnetized relay; the field is weak and decays over time.
I just fought through this issue on a Jaguar and BMW, trying to explain the problem to a cheap skate mechanical illiterate in both cases was a major PIA….
Replace the relays and you should be golden.
Have a great day.

Hey thanks for the reply. Now you said replace the relays. Any particular one or all of them?

Since your dad took a bunch of miscellaneous relays
 

Hello jmak124
Since your dad took a bunch of miscellaneous relays and "black boxes" from the junkyard last week, try "plugging and praying".

I am sorry that I can not point to the exact relay.
I am down from four to only one computer, due to home remodel.
Sad to say, this machine only has Rolls-Royce, Bentley, Jaguar and BMW service information.
I hope one of the other members can assist you further.