72 250 is running rich

[rgnprof]

Peter,

The plus wire on my coil goes directly to the lower of the two relays (?), the negative wire from the coil goes to ground.

Ryan

[cth350]

The dwell should be about 41 or so. Also, there are three different wiring patterns for the electronic ignition in the early models that had the systems. Make sure you've got the right pattern wired in (usually, it's all wired in fine).

The easiest mistakes to make are getting the balast resistors in the right order (iginition current from the key goes though both, but the starter wiring bypasses the bigger one) and also getting the coil in backwards.

Both of these mistakes will leave you with a working, but weaker igition.

-CTH

[rgnprof]

I haven't fooled with any of the ignition stuff, as far as I know the wires and resistors are factory - it all looks like original equipment to me.

My Mercedes manual says 30-36 for the dwell angle on my car, with the transistorized ignition.

Ryan

[psfred]

Ryan:

You have the early system. Connect the red (or green) lead from the dwell meter to the point wire to ground, OR the red (or green) lead to the switchbox side of the 0.4 ohm resistor and the black lead to the switchbox side of the 0.6 ohm resistor.

Connecting the dwell meter to the coil will give you the transistor switching time, not the point dwell.

Peter

[rgnprof]

Peter,

I'm not really folowing your description. I don't know what you mean by "point wire to ground"?

My meter has a green and black connection and I'm not sure - from your description - where to hook them. Are the ohm resistors that you refer to located right under the coil - rectangle shaped, about 3/4 in by 3 inches? The top one has a blue center, while the bottom one is white. If so, which is the .4 and which is the .6? Also, when you say switchbox, are you talking about the box under the battery? Thanks.

Ryan

[DANTRCAV]

Ryan:

Another perspective.

As Chuck said, Charlie is right about the transistorized system - but - it is over 30 years old. Now, my perspective (or history) with my '71 250C.

First thing I did was rebuild the Zeinth carbs, replace dizzy cap, points, rotor, plugs, plug wires, fuel pump and filters. Car ran OK but had all of the things you are talking about plus surged at highway speeds, long cold and hot starts and mileage was 10-11. Checked the coil and it was not within specs so I replaced it with a new Bosch blue coil. Car ran better - surged less at highway speeds but still not good. Replaced the Zeinth carbs with Webers and had them jetted locally by a Weber person. Much better but still not as I would have thought it to be. Mileage went to 12-13, surge at highway speeds was gone. Still rich, hesitation at start up, once in a while a backfire, rough idle now and then, hot starts still took a while, etc. Threw out the Bosch plugs and went with NGK (advice from Weber guy who just happened to be Japanese - yes we laughed about that). Made a difference. Better cold start, better idle. Still not what I wanted.

I wired the ignition around the transistor box and the hot start probem got better but overall running was off so I went back to the transistor box. Then I put the Crane XR700 and a Crane coil on and viola!! Cold start was fine, hot start (restart) problem went away, idle smoothed out, hesitation on startup was gone and the over-rich condition went away (except at idle). I opened the plug gap to .040. Mileage went to 14-15. I got 16.5 on two trips to Springfield. Weber guy told me that the rich idle with the dual Webers on my setup was just the "nature of the beast" so I never worried about it.

Now, to be totally fair here, I did scrap all of my "smog" stuff, plumbed the vacuum advance direct to the front Weber, disconnected the retard vacuum and set the initial ignition advance to 10 degrees Before. The car ran in that configuration for 5 1/2 years. Oh, the Webers did not use the fuel return so that line was capped.

My opinion is (or was) - and I could be way off base here - that the original ignition system was 30 years old and was not up to what it should have been. Plus I had to "assume" there was wear-and-tear in the dizzy also and that the Crane optical unit offset some/most/any of that wear-and-tear. I know I could have had the dizzy rebuilt but since I had no desire to maintain a totally stock MB and the thought of never having to either mess with points or ignition timing again pushed me to the Crane. For me it was a decision that I will never regret except that I should have done it when I did the Webers.

Your desires and situation may be different - but I did want you do know what I did with my 250C to solve my problems and irritations.

Good Luck.

Dan

[psfred]

Green clip to the wire from the points, black clip to ground. Wire from the points goes to a terminal block on the fender, then to the transistor switchbox (under the battery on the W115, I believe).

Don't remember which resistor is which on the W115, but using the point lead will work.

If you don't hook it up this way, it reads funny. I got quite a surprise when I hooked mine to the wrong end of hte 0.6 ohm resistor on the W108 -- read something like 60 degrees of dwell, impossible on a V8.

Peter

[rgnprof]

OK, I am attaching a couple of pictures that - I hope - will help clarify the connections. In the first picture you'll see the coil and the two resistors, one on top of the other, and in the second picture you can see the green wire which is coming from the distributor. This wire has been tapped into sometime along the way and this small black wire winds around to the passenger firewall where it goes throguh a hole and into the cab. I think it goes to an old, and not used, aftermarket installed cruise control. The green wire connects to the "box" located under the battery.

In reading the Mercedes manual, it clearly indicates that I have the transistorized ignition (blue coil) but then it gives the dwell meter connections for two different type of transistorized igntions (Germanium - TSZ and Silicon - TSZ), and if I'm reading it right the Germanium system coil is brown and the Silicon system coil is black (not blue)! l It also discusses SUN testers with and w/o changeover and Bosch testers. I'm still not sure where to test the dwell. Sorry if I'm dense, but I appreciate the replies.

Ryan

[rgnprof]

Here is the second picture.

Ryan

[psfred]

Connect the green lead from the dwell meter to the green wire -- it should go to a junction on the fender (it does on the 280, anyway). Black lead of meter goes to ground.

The "extra" wire is probably for a tachometer. Since it is added on, I would cut it off and seal the end with some electrical tape -- if that wire is grounding anywhere, even with high resistance, it will cause the ignition to miss.

Peter

[rgnprof]

Peter,

The green wire goes to the box under the battery. I hooked it up as you described and nothing. No reading at all.

Dan,

Thanks for yor reply and suggestion - I'm not far from buying a Crane! I'm something of a purist, but not obsessively so!

Ryan

[ctaylor738]

Given where you are at this point, it might make sense to go with a .3 mm gap, and re-adjust the timing.

Also, use a pair of insulated pliers to pull a wire off a plug, hold it close, and see if you get a nice blue spark arcing to the plug or to ground. That is a pretty good shade tree test of the ignition system.

[Oreo]

Sorry to interrupt.....but I lost you guys at the spark plug gaps. I have the Crane 3000 with Bosch electronic coil bypassing the ballast, so I'm sure I have the juice but what is the purpose of going with a wider gap as opposed to factory spec? Better economy or smoother idle?

Oreo
123 with 115 engine

[DANTRCAV]

Oreo:

A wider spark plug gap will (or should) give a more complete burn of the fuel and show an increas in power and also mileage. In addition, you should also see a smoother running engine longer (longer times between having to clean the plugs). This is due to the wider gap producing a fatter and hotter flame kernel (spark).

However, that being said, it must be remembered that increasing the plug gap also increases the total resistance in the ignition system which ups the voltage required from the coil to fire the plugs. If you have a low-voltage coil (under 20,000 volts???) or a weak coil or a "less than stellar" ignition system, then there may not be enough volts available to properly fire the plugs. You will then get a miserable running engine with many misfires. Usually when going to an after marked ignition system (Crane, Petronix, Jacobs, HEI, etc) you can open the gap as the system produces more voltage. I believe the coil I had with my Crane system is in the 45,000 to 48,000 volts range.

Ryan:

If you are interested, I have a copy of the original Mercedes Ignition tune-up and diagnostic guide for the four (4) types of igintion systemes used on the pre 80 cars. I will be happy to send you a copy if you would like. Let me know.

Dan

[rgnprof]

I'm a little confused. I originally thought Chuck was talking about the points gap - which per the manual, is set with a feeler gauge between .3mm and .4mm.

Chuck, were you talking spark plugs or points?

I still can't figure out the proper connections fro measuring dwell - if I hook green wire from meter to positive of coil and black wire to ground, I get a meter reading of 34 degrees. If I hook green lead to the switchbox side of the 0.4 ohm resistor and the black lead to the switchbox side of the 0.6 ohm resistor (per an earlier post by Peter), I get a meter reading of 40 degrees.

Dan,

I would like a copy - do you still have my address from before? If not, email me at ryan.newell@oc.edu.
Thanks.

Ryan