Ignition electrical short! 220D - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 09-28-2013, 06:33 PM Thread Starter
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Ignition electrical short! 220D

So today I turned the key on my '71 220D and when I got to the second position, the starter kicked in. (The starter switch is closed and besides, all the wires are disconnected!) I turn the key off, but the starter keeps cranking! I jump out and pull the (+) cable off the battery to get it to stop!

Talk about a gremlin... (I felt like Wm Shatner in Twilight Zone.)

On the starter's solenoid were 3 wires and I tracked the short to one that goes up to a DPDT relay on the firewall. No clue what it is -- it's not on the wiring diagram. 2 wires lead nowhere in the direction of the defunct a/c compressor, and the other is 2 brown wires (aren't those ground?) on one contact that run into the passenger compartment.

The diagram is easy to follow: battery --> key switch --> glow/starter switch --> [starter momentary push switch, also not on diagram] --> starter lockout switch (this is apparently not connected, because I can start it in gear ) --> starter. So why are there 3 wires on the solenoid to begin with? What is the relay with no purpose? And if nothing else, I disconnect wires and go with the minimalist approach to get it right.
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post #2 of 5 (permalink) Old 09-29-2013, 03:55 AM
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Sounds to me like you've got a pretty common "upgrade" for the diesel starting system; a mod that changes the starter switch, the glow plugs to the "fast start" type, the relay you speak of to handle the switching load, the push to start button (which needs that relay) and much quicker starts. There are several ways to wire this, like using the original position on the start switch instead of the button, but they present several problems as well. For a good explanation visit "mercedessource.com", who sells the mod I think you have. It's a GREAT site for purchasing all sorts of items, learning how to maintain your Benz and reading about maintaining and repair of your car by a fellow who is right on top of the knowledge heap. Check it out!!
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post #3 of 5 (permalink) Old 09-29-2013, 06:20 PM Thread Starter
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Definitely a Gremlin

It is a GREAT site. In fact, that's where the pencil plugs came from. But I had a mental lapse and left the ground wire so the current bypassed the new plugs. This popped (literally) the preglow resistor and burnt out (I think) the starter solenoid. I replaced the starter -- it was old anyway -- and welded the element back together. I unhooked all the wires on the starter glow pull switch and was working slowly and carefully to catch any other damage when this gremlin appeared.

Someone, in the guise of a repair, let a monkey do some wiring. The purple wire that I think should go to the starter button switch goes to the starter solenoid, and a new wire off the battery goes to the starter button (via a lamp cord... ) and then also to the starter solenoid. The third wire runs from the solenoid to a relay that controls the a/c compressor... ???

The frame itself was energized! So I unhooked the battery cable from the starter and the short went away. I put it back on, and the short didn't come back... can't explain that one...

I'm ready to rip out the monkey wiring and go with what I think is right: purple wire --> starter button switch --> solenoid.
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post #4 of 5 (permalink) Old 09-30-2013, 10:39 PM
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Be sure you're using solid core wires and gap your plugs. That simple step cures so many diesel ignition electrical problems...
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post #5 of 5 (permalink) Old 10-01-2013, 10:24 AM Thread Starter
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Will do! Thanks! I figured out that in the past, because the starter/glow pull switch didn't make good contact in the full out position to energize the starter, someone ran a wire straight from the battery to the starter button switch to the starter. The original wire bypassed the button switch, and they left it on the starter... And I still have no clue why a wire from the a/c compressor relay was attached. Basically, I took out all of the "repair" and moved the starter wire from the 3rd position on the pull switch to the 2nd, so it gets power as the glow plugs heat up. I got it started this morning, and it ran great! But it didn't want to stop... In messing with the pull switch, I got the fuel pump cable out of adjustment.
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