Hi,

I just purchased thye 74' 450SL thst does not want to start. The mechanic thought that it had a bad ign module ($400 ??) and now he said he is getting a signal to the points but the car still would not start. He said he is not familiar w/ the 74' ign system - he is a master tech at the local MBZ dealership w/ 30+ yrs of experience.

I brought the car back home so that I can work on it. Where should I start ?

Your help would be greatly appreciated !!!

Anyone

Does the car get a spark? If not bypass the points and module for a quick test.
1) Disconect the negative terminal on the coil.
2) Find an old spark plug and break off the side electrode to creat a huge gap. Or get a KD test spark plug.
3) Conect the spark plug to the output wire from the coil and ground the plug at a safe location (away from fuel)
4) Put the ignition key to the on position.
5) Verify at least 8 volts are on the positive side of the coil.
6) Take a jumper wire and attach it to the negative terminal on the coil.
7) Strike the other end of the wire quickly to ground in such a way to make and break the circute just like the points would do.

8) No spark (You need a coil) But I would suggest you first try this on a car with points that you know works just to get the hang of it.

John Roncallo

Thank you very much....I will give it a try this weekend. I also heard that 74' has the secondary ign system. Is that true ?....also heard that 74' was the only year that MBZ used this ign sys more like an old Porsche.

BEnz used that ignition from 1968 to 1976, shouldn't be a surprise to ANY experienced MB tech.

First thing to do is check the condition of the two ignition resistors. If corrded or cracked, you won't have spark!

If the box is bad, get a Pertronix ignition system, costs about $70 and coverts the car to breakerless ignition. Will eliminate all sorts of trouble.

As far as troubleshooting goes, the very first thing I would do is replace the points -- they cost all of $8 and are easy, this will let you know if they are the problem (they corrode badly sometimes). You can rig the igntion up "simple" -- that is, bypass the ignition module -- instructions are somewhere on this site, do a search (I don't have the manual available at the moment).

Verify that the coil works, and that you indeed have voltage to all the bits.

I would seriously consider the Pertronix (or Crane optical) -- won't be long before there will be no more ignition modules around...

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!