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Old 12-14-2005, 12:26 AM
phidauex's Avatar
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Join Date: Oct 2004
Location: St. Louis, MO
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Broken ignition assembly? Repair it!

For about the last week I've been starting my '82 300TD by touching the bare ends of two wires in the cabin that I tapped to the terminal block on the passenger fender that connect to the starter. This crude process earned me odd looks from friends and coworkers, and relegated the wagon to 'emergency' use, and my girlfriend and I were forced to take guests around town in her Honda coupe out of shame.

The symptom was that the key would turn through its full range of motion, but the car wouldn't start. No solenoid click, no start. Accessories would come on however. Jumping the terminal block on the passenger fender would cause the starter to whirr to life, which damned either the neutral safety switch, or the ignition switch. I cleared the neutral safety switch by a process I call 'guessing', whereupon I assumed that because it happened with no warning, and no amount of jiggling or adjusting the shift lever, or trying in neutral, or whatever, would work.

I then ASSumed that the electrical portion of the assembly was at fault, and I ordered one from Fastlane (20 some bucks, not bad). However, when I finally pulled it out (after much hassle, that ignition assembly is really in there! This page helped a lot: http://garnet.acns.fsu.edu/~jjs5772/W123/ ), the shaft connecting the assembly to the electrical component came apart in pieces.

Sorry I don't have a picture, but basically, the shaft is a tube, about 7mm in diameter, with two 'wings' on the side that make it look a bit like a wire nut. The tube, and its wings, or splines, engage a hollow of the same shape on the electrical component. The wings had broken themselves off, and while one was jammed enough to rotate the electrical component some, IE, to turn on accessories, there wasn't enough purchase to make the final engagement.

It seemed I had wasted a few dollars on the switch, which wasn't actually broken, but I was now faced with having to spend over 100$ on a new assembly, which I did NOT want, as well as have my car out of commission for several days, since when the pieces fell off the shaft, I lost even rudimentary functionality.

So I repaired it, which is something I haven't heard of people doing... The broken off splines had pulled a bit of the shaft's metal out with them, leaving two notches in the tube. I used a file to make these notches more evenly shaped, and then used a small piece of metal (actually a piece of a broken drill bit I found on my workbench), shaved it down to be a 2mm by 4mm by 10mm rectangle, which fit nicely in the notches, and would replace the 'wings' and engage the switch mechanism. I used some JB weld to hold it in place, allowed it to cure, and then reassembled. It works!

If you try a repair like this, make sure your metal rod fits snugly against the notches in the shaft. The JB weld shouldn't be the part that actually gets pushed on, it should just be holding everything in place. Also, make sure that the rod is lined up in all three dimensions, it should be level, vertical, and most importantly, lined up directly over the line of the tabs, not advanced a few degrees. I made this mistake on my first positioning, and it prevents it from working properly.

Sorry, no pictures, my girlfriend's camera has gone missing for the moment.

peace,
sam

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"That f***in' biodiesel is makin' me hungry."

1982 300TD Astral Silver w/ 250k (BIO BNZ)
2001 Aprilia SR50 Corsa Red w/ 5.5k (>100 MPG)

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Old 12-14-2005, 12:27 AM
phidauex's Avatar
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Join Date: Oct 2004
Location: St. Louis, MO
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In retrospect, the MB design is dumb. Why use only two splines? Why not four? Or eight? A keyed spline drive with five splines, one larger than the rest, would make sure you could only install it one way, but provide enough mating surface to prevent this from happening, it seems like this is a very common failure for these assemblies.

Oh well, I wonder if they fixed it for later cars...

peace,
sam
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"That f***in' biodiesel is makin' me hungry."

1982 300TD Astral Silver w/ 250k (BIO BNZ)
2001 Aprilia SR50 Corsa Red w/ 5.5k (>100 MPG)

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  #3  
Old 12-14-2005, 06:56 AM
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Very interesting.....
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  #4  
Old 12-14-2005, 10:49 AM
phidauex's Avatar
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Join Date: Oct 2004
Location: St. Louis, MO
Posts: 806
Its still working this morning..

I give this type of repair the 'bathtub curve test' that gets applied to engineering decisions. If you took a bathtub, and cut it in half lengthwise so you were looking at a cross-section of it, you'd see a line that was high at one end, then rapidly sloping down then flat for a while, then rapidly sloping up.

That, as a graph, shows the failure rate for most devices, with the x axis being time, and the y axis being the number of devices failed after that much time. A lot fail right at the beginning, and a lot fail after a long time, but very few fail in between. Either they were defective, or they wear out, its rare that something inbetween happens.

So if my little hack lasts a week, I'll leave it alone until I happen upon a good replacement assembly for a price I'm satisfied with. Unfortunately, my car now has three 'little hacks', the plugged vacuum line in the CC system, the fuel line/ RTV sealant patch on the transmission cooler line, and now this.. I'm not a 'hackin' kind of guy, I like my car fixed properly, but desperate times require desperate measures.

peace,
sam

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"That f***in' biodiesel is makin' me hungry."

1982 300TD Astral Silver w/ 250k (BIO BNZ)
2001 Aprilia SR50 Corsa Red w/ 5.5k (>100 MPG)

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