Hi,
I have decided to go through the ignition system on my 71 250 Coupe and so far, I have found: incorrect part number on coil, 6 ohm ballast resistor gets hot when ignition switch turned on, and a bare broken or cut wire (black) on the terminal connector to the switchgear (ignition module). I have been driving the car very little because it runs so rough. I replaced the points, condenser, coil, fuse in #4, and it runs rougher than ever. What I really need to start off with is a diagram of the wiring to the switchgear terminal. Can anybody help?

Chris,

If you have the engine manual for the 4.5, it's the same circuit.

Send me an email (cth@mbz.org) and I'll send you the PDF from the CD.

-CTH

__________________
- cth350@gmail.com
http://www.111lakeside.com/

Just what I wanted to see! Thanks a million. I will keep you posted when I screw it up.

Chris,

Once you have it running well, find a mechanic with an engine scope and check it there. You will most likely need to twiddle with your carbs (a favorite topic on the forum) and you can't get them in a great state of tune without first resolving ALL ignition issues, vacuum and valve timing. Also, have at least half a tank of gas present.

-CTH

PS. Note the vacuum lines for the advance and retard on the ignition. There's some fancy emissions circuitry there that can go south. I have to dig up the right digitized stuff for that. It's around here someplace.

__________________
- cth350@gmail.com
http://www.111lakeside.com/

This is the car that I picked up in Fredericksburg, VA in January and drove home to Tucson. It has been aptly described by others on the forum who had looked at it as a "donor" car. Perfect for me, as I have been using it as an educational project. I figure that the more that is wrong with it, the more I will learn. Frankly I was surprised that it made it all the way across the country! So far I have completely disassembled the interior, took the fenders off, and cut out all the rust. I replaced the rusted metal with heavy gauge aluminum panels that I cut and molded by hand. I was sore for about two weeks. This covered all of the floor and a better part of the firewall. Found some carpet on ifloor.com and went to the Home Depot to match the color and numbers, then ordered online for about 70% off the retail price. It looks stock. Following the "bachlauter" thread on carbs, I cleaned mine out and checked specs. I had it running pretty good considering it needs a camshaft. Then one day something burned up and ignition went to hell. So here I am happier than a pig in s#%@. I figure that if I am going to sell vintage Mercedes on eBay, I better know what I am talking about. It all started with getting on the web and reading: http://www.mindspring.com/~todgam/merz.htm

OK, so I have replaced the coil, points, rotor, cap, and checked the wiring. There are two wires in the harness to the switchgear taht are not on the diagrams; a black one and a brown one with a pigtail that was connected to the green/yellow distributor(condenser) wire. What are these? Also, I am not sure if I really ahve the right Bosch part numbers for the parts I replaced. Anyone have this info? Still runs like crap.

The brown wire should be a ground. To confirm that, check that green lead from the distributor... Is it a coaxial wire? The inner wire will carry the signal from the points, the outer wire will be ground.

The black wire is probably a pigtail connection to a diagnostic socket. Use it to pick up the signal from that inner lead of the green wire. The diagnostic terminal is often cut off, since bad solder joints in it, plus a little bit of water lead to a poor running car.

As usually, I am working from "memory", since I'm answering this from work. (In other words, check carefully).

Lastly, the condenser is just a radio noise suppressor. Feel free to remove it.

-CTH

__________________
- cth350@gmail.com
http://www.111lakeside.com/

Okay. I'll ask the obvious question first. Is the car hooked up to run "normal" point type ignition? By that, I mean is there a wire from the distributor going to the coil?
I have the diagnostic diagram from the training manual, but it only applies to the transitorized system.
Regards,
Randy D.

Hi Randy,
The only wire from the coil to distributor is the cable from terminals 4 (coil) to 4 (distributor). I am pretty sure this system was not bypassed. CTH sent me the specs from the V8 manual and the coil checks out fine. The green/yellow wire is not coaxial, but a single wire. It is connected to the green/yellow wire to the switchgear at terminal 7. The switchgear is Bosch number 0 227 051 015. Distributor number is 0 231 187 002 (aluminum housing). Condenser is 1 237 330 202. Points are 1 237 013 145. Coil is blue transistorized Bosch.
The other wires from the switchgear connect with the wires to the resistors; terminal 15 to 0.4 ohm and terminal 16 to 0.6 ohm. There are two more wires in the same harness. One is black, and one is brown. They are both connected to the empty screw on terminal 7, with the brown having a pigtail that connects to the green/yellow wire at terminal 7. I cannot figure where these go. There is another single black wire that is connected to the brown and black at terminal 7. There is also a two-prong female connector down there that isn't doing anything. Looks like front side marker which I don't have (fenders replaced). All of this is conveniently placed under the battery tray for easy access!

Hi,
That model should not have a wire from the distributor to the coil. The trigger circuit(from the points)should go to a plastic/phenolic block and the negative side of the coil was connected to ground(brown wire). It was common for folks to setup the system to by-pass the ignition box when it failed due to the replacement cost. That would explain the difference between the coils(it had to be changed to a standard type). The problem is that the "condenser" for that system was never meant to be a condenser. It's a noise suppressor. Send me a private e-mail and I'll give you the diagram. It would make more sense to convert to a "pointless" system like PerTronix and use a standard coil.
Regards,
Randy D.
WDurrance@aol.com

Randy,

The 250C had the coil-to-ground setup? I thought that was only in the 3.5s. I sent Chris the pages from the v8 manual, which rather conveniently layout all 3 versions of the system.

-CTH

__________________
- cth350@gmail.com
http://www.111lakeside.com/

Hi,
I thought the 70 and 71 models all had 1st version Breaker-Controlled Transistor ignition. Now you've got me thinking...? I sent him the diagnostic diagram from the M-B Training Manual for 1st version TCI (like my 3.5 used to have). But that really doesn't matter. If there is a wire from the coil terminal #1 to the distributor, it has been converted to standard coil ignition. It would be a shame to go through all of this to find that the TCI box is bad. I suggested he convert to a PerTronix trigger and keep a standard coil.
Regards,
Randy D.

Once Chris has the wires connected in something resembling the original spec, diagnosing a switch gear problem should be straight forward.

Chris, I guess it boils down to which is faster and easier (and time is money, even for the homebrew mechanic)

- Getting the wires back to spec and discovering that the switch gear is fried. Replacing it for 20-100$ (depending on your choice of junk yards).

- Chucking the switch gear and investing 100+$ in an alternative ignition.

I think that covers it. Randy, by the way, is the best guy I know on the subject, or so the mercedes vets list sure seems to think so.

If the switch gear is fried and you REALLY enjoy tinkering, it is fixable, should the problem turn out to be the power transistor. Like about 2 weeks ago, I read a tech note about it. I think it was up on http://www.mbz.org.

-CTH

__________________
- cth350@gmail.com
http://www.111lakeside.com/

Based on my "intense" working knowledge of this system the original on all 70-71 250C's was the Type 1 unit. I have the MB troubleshooting guide but it doesn't say which type was for what year. Nevertheless, Randy makes good sense (as always). I replaced my original Type 1 unit with a breakerless system. I used the Crane XR700 although I first tried to get a Pertronix but was told by local parts shops then Pertronix themselves that they had no unit for my application. This was over three years ago. Anyhow, after the conversion the positive results were immediate and readily apparent. Then again, if you want to keep the car original put the ignition back to the way it was designed.

Chris, if you do this and find your unit is fried, I have two units. One taken from my car when I converted and one taken from a junk car with a blown engine that was obviously running when it blew. The unit was connected in the system. Cost me $10.00 (I pulled the part).

If you get to your unit (under the battery behind the headlight) you might want to make sure that the only problem is not corrosion from years of sitting in residual battery vapors! When I pulled my unit, I had some very green and brittle wires down there and the wire from the unit to the distributor was just about eaten through.

Dan

__________________
Dan Taylor/ Tulsa, OK MBCA
'84 300D/'90 Jaguar XJ6/XJ40

Hi,
I want you to know that I am sitting in the Public Library in Downtown San Diego writing this. I will be travelling this weekend. I couldn't wait to hear what the outcome was. If there was an out-of-car test for the switchgear, things would be pretty easy, as the rest of the system is pretty straight forward. Also, the Crane system is an immediate fix taht I am thinking about. Who cares if it is not original? I am not building a show car. My priority though is to LEARN as much as I can about the original systems of the car, so it would be nice if I could get it running to specs before I modify it. Even if it does not run perfect, at least I will understand the system (maybe). Iam very interested in the switchgear rebuild info, and will dig into this when I get home on Monday. Thanks to all.