spontaneous start 82SD - PeachParts Mercedes-Benz Forum

vstech · #1 03-01-2011, 01:47 PM

OK.
put key in, turn to "ignition/glow" position, and it starts. starter stays engaged until you turn key back to acc position...
it does not matter where the shifter is either...
WTF? I am unaware of a fail to start position on the ignition switch.
I suppose something is shorted somewhere. glow plug relay?
that would make sense. purple wire is backfeeding the starter solenoid... I'll check this.
really blew me away.
I of course rigged something to work around the issue, but I have to open the hood to start the car now...

tyl604 · #2 03-01-2011, 02:05 PM

I seem to remember that there is a cut-out switch which prevents the car from starting in any gear but Park (maybe Neutral too). Sounds like that switch is not working if it will start with the gears in any position. Forget what the switch is called. And my memory is from a gasser but I assume diesels have it too.

Stretch · #3 03-01-2011, 02:07 PM

Quote:

Originally Posted by tyl604

I seem to remember that there is a cut-out switch which prevents the car from starting in any gear but Park (maybe Neutral too). Sounds like that switch is not working if it will start with the gears in any position. Forget what the switch is called. And my memory is from a gasser but I assume diesels have it too.

They do indeed it is a plug on the transmission on my 722.118... but I guess you've got something more exotic (like a .3!) in an SD

But how that relates to a starter just starting is beyond my understanding at the moment - haven't started on the electrics properly yet - sorry

vstech · #4 03-01-2011, 02:09 PM

that's just it.
the NSS (Neutral Saftey Switch) is bypassed entirely in this case. The NSS prevents the START position from energizing the starter when not in park/neutral... the starter in this case is energizing BEFORE the key hits the start position. it's energizing as soon as the glow plugs are getting power...

moon161 · #5 03-01-2011, 02:19 PM

Stuck relay? You should be able to pop the top off and see it operate. Rear wiper intermittent relay from my 91 volvo 240D was a mechatronic li'l wonder. Had rotating eccentric that opened and closed the contact when energized. I think I still have that, along with the 'gotcher keys' dinger. Musta been a patent beater, most timing you'd think would be solid state by then.

toomany MBZ · #6 03-01-2011, 02:20 PM

Yeah, could be the NSS, but if it's not releasing the starter, maybe not just your issue.

Glow plug relay, I doubt it, that would eat up glow plugs.

Ignition switch, maybe.

kerry · #7 03-01-2011, 02:36 PM

Is it possible that someone put a wire in the junction box on the front right inner fender on the wrong post?

tbomachines · #8 03-01-2011, 02:37 PM

Is your ignition switch loose or anything? Perhaps the rotating assembly could have gotten rotated? I would suspect the wiring in the back of the ignition switch...unfortunately not a fun job and not easy to get to...

vstech · #9 03-01-2011, 04:24 PM

ill report what I find. no way it's the nss
& this happened today on my car, so no wiring errors
& I don't see the ignition either...

barry123400 · #10 03-01-2011, 04:26 PM

If you think the glowplug relay is backfeeding the starter solinoid relay. Just disconecting the glow plug relays power supply should tell you. I do not have a wiring schematic handy but your guess sounds good even if unusual.

andrewjtx · #11 03-01-2011, 04:39 PM

This may be a stupid question, but brainstorming none the less...

If fuel had leaked into one/more cylinders, could the glow plugs ignite the fuel and start the engine?

Does the starter actually engage?

barry123400 · #12 03-01-2011, 04:59 PM

As they say the only questions that are stupid are the ones not asked. What you are refering to basically is spontainious combustion. This has never been reported on one of these diesels to my knowledge and is not the case here.

Remember his starter does activate. Just somewhat uncontrollably before he wants it. Also I think it is remaining engaged as long as the engine is running.

This is hard if so as the speed obtained by the starter armature would be higher than designed for. Unless there is an overrun clutch built into a starter.

The most common self starting senario would be to park a 123 diesel on a steep hill with a manual transmission left in gear. If the vaccum system released the shut off on the injection pump. Plus the car started to roll down the hill it could easily self start.

vstech · #13 03-01-2011, 06:45 PM

only thing easy to disconnect on the glowplug relay is the service wires to the plugs, and the control wires to the relay. disconnecting power is VERY difficult. there is a 10mm nut right against the firewall, so battery disconnect is needed to unhook the power feed wire, but unplugging the control wires to the glow plug stopped the issue.
put in a good spare controller and all is right with the world.
it's the only thing electrically that made sense... had to be the glow plug controller. nothing else would energize the starter with the car IN GEAR... short of a mis wire on the junction block, and that is not the case here.
I'm glad I was right, and I'm glad it was easy to repair.

ngarover · #14 03-01-2011, 07:05 PM

SAme issue here on the 81, but only when the car was really cold, the solenoid on the starter was sticking, so it would just try and start and start and start, even if you turn off and pulled out the key.... I put a new starter in it and it great now.

junqueyardjim · #15 03-01-2011, 07:23 PM

I think the problem is a worn ignition switch, this time the back part with all the wiring and contacts has to be the problem.