W108 Ignition conversion

[dubsix3]

Hello,
I have read about all I can find on replacing the points to a Pertronix or similar ignition system and have a question.
After tracing out the wiring and matching them up to the electrical schematics I have found a wire that needs to be addressed.
What happens to the green wire leaving the #7 terminal on the switchgear unit going to the RPM controlled switch?
Any help would be much appreciated.

[cth350]

Try this thread:

http://www.peachparts.com/shopforum/vintage-mercedes/317515-failed-pertronix-install.html

And it in turn links to an older thread.

The advice there for you is on post #11 on the 2nd thread. Though I suggest you read both threads in their entirety before attaching that wire.

HTH -CTH

[dubsix3]

Thanks for the direction CTH350, but post #11 refers to running a vehicle with points, if I read that right.
My distributor looks to be a vacuum retard unit, activated by the RPM relay, which looks to be driven by the green wire that is connected to the switchgear box on terminal 7.
With this wire lifted, it appears that the engine vacuum is felt on the diaphram at all times.
Perhaps that is ok? This is where I am lost.
Thanks to any that chime in.

[Palolo]

Here is the content of post #11 of the second thread:

Originally Posted by ctaylor738

To get it running with points, you need to disconnect the lead from the box and run 12V off the ignition switch to the + side of the coil. You can use a jumper from ignition switch side of the first ballast resistor. Leave the green wire on the - side of the coil, and it should start.

Is that the same green wire?

Wouldn't that unit be made redundant and be rendered useless by the Pertronix? Hence green wire not needed? The SL113 ignition diagrams have the green wire from terminal seven going to the distributor (condenser/points?) as seen here: Pagoda SL Group Technical Manual :: Electrical / TransistorIgnition

[dubsix3]

Those are great schematics. I need to learn to search better around here.
What those drawings do not show is that the switchgear has a lead going from point 7 to a terminal strip, which then branches in two directions. One lead goes to the points, as shown. The other lead goes to the coil of the RPM relay, not shown because it is not part of the ignition system... directly.
It triggers this RPM module which drives the two way valve that affects the vacuum line going to the distributor "retard" pot.
From what I can tell, it inhibits retard at idle. Once the RPMs start to rise, the engine vacuum is allowed to run the pot as needed.
My understanding is that it is an emissions thing and not really needed.
Before I do the conversion, I am experimenting with leaving that wire unhooked and it does not seem to change anything noticeably.

[Palolo]

Ahhh. I see.

I will bow out at this point, as my car is no longer using the original vacuum-controlled distributor, or the (vacuum) retarded points ignition system, pun intended.

What year is your 108, and which motor?

It was my understanding (depending on the year; it "reversed" or "was reversed" at some point) that the vacuum, at idle only, induced a retard and when above idle, the vacuum was cut off, allowing the distributor to advance mechanically dependent on rpm. So if you have a car of the similar era as mine (71), the vacuum only came in to play at idle. Above idle, the vacuum circuit was cut off by that switch, is what I believe happens.

(Edited for accuracy: When I installed a 1-2-3 ignition, I took the vacuum switch out of the equation, and the vacuum line now goes directly to the distributor)

[dubsix3]

Mine is a '70 280SE with the M130 six cylinder.
The vacuum deal is what I am trying to figure out. I thought it was there at idle and cut off once the acceleration happened, letting the mechanical advance take over. This way, once you let go of the pedal, it would be re-activated, letting the "retard" pull the timing back quicker than just the mechanical advance "slowing" back down.
Your 1-2-3 system uses it's own distributor or some sort of ignition box for control?
I enjoy this conversation, thanks for the info/input.

[Michael C]

What year is your 108? Which distributor do you have? 051, 062?

[Michael C]

Oops, just saw your post on the year. 1970 would make you dist. a 061 or 062. Vacuum retard should be happening until the rev counter (2nd black box) reads about 2200rpm. at this pont the switch gear communicates to the vacuum relay (the little acorn looking thing) to shut off vacuum. At idle 600 or so RPM you should be around 6 to 8 degrees "after" TDC. As you rev the engine, some place around 2200-2400 RPM you should feel the vacuum shut off and will feel a nice little bump in timing. At 3000 RPM you should be at 30 degrees "before" TDC. Check these settings and let us know where you land. (This is all part of the US smog stuff)

For 1970 you should have 2 black boxes. I think they are down low in the engine compartment behind the left blinker. Also note, just to make sure all things are correct. Your vacuum line hooks up after the intake butterfly on your car. (Earlier cars before).

Also... question for Palolo. How much total advance are you getting from your 123? I also have one on one of my cars but kept the vacuum switchover.

[Palolo]

Not to derail this thread topic, but lower down in my response is a link to a recent 123 ignition thread... But my answer:

33 degrees, total, with advance curve "F". I am now getting from 8*ATDC at idle to 28*BTDC (+/-) @ 3000rpm, and for the kind of grandpa driving I do in this car, that suits me just fine. I do give it the odd Italian Tune Up on an on ramp, but typically just cruise serenely around town, not in a hurry to get anywhere.

My initial reading was closer to 25/26, but some additional timing adjustments and a better timing light (digital) helped.

I chased my tail when I first installed it, but after I got settled and systematic, it was easy. There's an older 1-2-3 ignition thread that I posted in, 123 ignition, and which had others who also use that distributor. For me, it was "get this distributor and kill two birds (points and emissions boxes) with one stone". Others may not see the value gained vs $ spent, but I appreciate the technology contained within and see how it benefits my car overall.

Cheers,

Pat.

[Michael C]

Palolo,
That's Awesome! I have my 123 hooked up with all the smog garbage. Do you remember what letter setting you have it set to? Is your vacuum before the butterfly?

[dubsix3]

I ended up using the Hot Spark kit and removed the switchgear box altogether. I kept the .4 ohm resistor to drop the voltage at the coil + from 13.9V to 13.4V and to have a termination point for the ignition wire to the coil +.
The timing was set to 8 ATDC at idle (680ish). Leaving the RPM green wire disconnected yielded the following results:
2200-2400 RPM the timing fought to advance but the retard pot pulled it right back to about 8 ATDC.
3000 RPM let the timing get to about 2-4 BTDC
I then hooked the green wire to the coil - like the tach signal:
Idle was the same
At about 2400-2500 RPM the vacuum retard cut out and the ignition advanced to 20 BTDC
3000 RPM let the timing get to 25 BTDC
Looks like all is good now and this appears to be the best way to hook up a distributor module to the retard style distributor ignition.
I'd like to add that I also switched out to the blue Bosch coil (3.7 ohm) because the original coil is a .6 ohm and not useable with the Hot Spark module.