1975 450sl, Petronix made me an electronic ignition that will not work right
 

I have had a full year of trying to get this car running. Initially an aftermarket circuit board broke=not fixable. Petronix said they did not have an electronic ignition, but they could make me one. After install, car would not run at all smoothly-one mechanic said it appeared the points regulating the injectors fired simultaneously with the spark plugs.....when there should have been a short time lag. I have contacted petronix (a couple days before I hope to get the tech online who did this work)----does ANYONE out there have a lead on an electronic ignition that will work on this '75..?!?!?!..(the ONLY year MB that Petronix does not make a stock system for) ---any help is appreciated, before I rip the last 3 hairs outta my head......lol

Since the petronix is likely to trigger at a different angle to the original points it is possible that they are no longer synchronised correctly with the trigger points for the fuel injection below. Since the injectors fire in pairs the timing probably wouldnt make that much difference though, worst case the vapour hangs around in the intake longer before the intake valve opens.
Have you tried re-instating the original points to see if it runs fine and if it does why not stick with them and face timing the car once a year rather than loosing another 3 strands?

I don't understand just what happened.

Which aftermarket circuit board broke?

Which part did Pertronix make for you?

Forget the Pertronix for now. Revert to conventional points. And, if it is the MB switchgear that is faulty, they can be purchased used for not much. That would get your ignition back where it was originally. A good starting point. Later if you want to replace the points, perhaps the Crane optical kit will do it for you - that is what other '75 owners seems to have done.

But, your problem may very well be with the injector trigger points. If you remove the distributor, you can set it up on the bench. Hook up an ohmmeter to each pair of outputs in turn (center is ground). Then rotate the rotor slowly by hand. Each pair should close for about 100-115deg. If the fiber rubbing blocks wear, this increases and in my case I had one that almost never closed. You could check this out if you can get a spare set of trigger points and test them.

Hi Brian

Like gary says the injectors fire into the plenum, they keep a cloud of fuel vapour in the intake manifold and probably only about 5% of the fuel from any given injector goes into that particular cylinder. I have a pertronix on mine and they are pretty good - you can get an allison system (now crane cams) for a bosch 8 cylinder but it needs some mods.

you might be better off though checking other things like the spark power - you need at least 12 to 15 KV for a decent spark and around 22Kv for a good spark. it will run on as low as 7 or 8 Kv just not very well, you may have a damaged or underpower coil that just isnt feeding enough voltage to the system. pull the rocker covers too and check that all is well with the valve timing and that it lines up with the dist position and also if you pulled the distributor that you have not installed it 1 tooth off so you timing is actually out even though the crank position is set right
cheers
Barri

yes, I am with Graham ,what part do you need , the ECU?

As far as what year Petronix makes stuff for, I don't see why a 75 would be different from a 74 if it's D-Jet electrical injection or 76 if it's mechanical injection. They probably dont have the data because its a "Cutoff date" and some 75's (mainly Grey market or Euro ones) are mechanical injection, whereas others (most US-spec ones) are D-Jet.

If ignition is making you pull your hair out and you switched because the stock box burned out you can always buy a condenser & run it the way they did before electronic ignition.

agree tomguy, you can bypass the thing

The '75 D-jet distributors are different and there is apparently a problem mounting the Pertronix. I think it was Radiotek who posted about this.

responding to all the help/suggestions
 

CraneCam sounds good, but what modifications does it require?
----I am told that I need a new distributor if I want to revert to points (THAT would be great, then I can just change those)....any clue where I can find a distributor for this 75 450sl?

I'd be on the lookout for one from a 71-74 4.5. That way, you can have your cake & eat it with the original setup plus the ease of upgrading parts if you wish with AM stuff.

Surprising you'd need a new one to revert to points but I suppose that one doesn't even have the top plate setup for points?

someone on this forum posted a picture of a box of distributors awhile back, perhaps he will show up here. Also on ebay from time to time

Thanks Tom/ Ron
 

I was told I needed a new distributor in order to get back to the original points system, but I must wonder why, since the cam is readily visible under the rotor,,,,,,I am really inclined to pick up some points and a condensor......until I either get some $ back from Pertronix, or they fix this problem with the electronic ignition,,,,but will look at ebay :) I am honestly rather comfortable with points if need be...just want this baby back out on the road before the snow flies for a nice fall color road trip, will keep in touch here. Here in the Twin CIties area so far - even the dealers/servicemen don't seem to know much about this car=and you guys do !
Glad I found you, and this site !

Tomguy
 

Yes the top plate is still there, Since I was relying on other mechanics (ok, I admit that was dumb - when they didn't know jack about this car) to fix this, I had not actually looked at the distributor innards.....like my Dad used to say - you want something done right, you have to do it yourself,,,,,,,lol ( se my previous response)

Tomguy
 

When I bought this, I was told it had mechanical injectors.....been so busy keeping up with work, I have not had time to get under the hood much...will get to that to dbl check (though not sure how to check if elec or mechanical)

Do the injectors each have a plug? Or does each have its own fuel line going to an octopus-type device?