Spark Present but not when Cranking w111

[RossCormack]

Hope one of you gents can shed light on this problem. I am trying to test a 220 engine from a 1964 220SE w111. I'm using the electrics from another old Merc to supply the spark. So, I've basically wired up the starter and coil from the other car.

I find that I can see a spark on the plugs when I turn the key to position one then turn off - spark seen when I turn off, as expected. I see the spark when I turn the engine to line up the ignition distributor rotor to the corresponding lead to that plug.

What I don't understand is that when I crank the engine round by the starter motor, I see no spark. I would expect a spark every time the rotor comes round and then breaks away from its contact at the distributor. I tried cleaning the rotor and distributor cap but no change. Am I stupid to think I should see this spark? I just would like to see the engine fire. Any advice would be hugely appreciated, thanks in advance

 

[jaymanek]

Well it will be very brief are u having an assistant watch for sparks? Are u earthing the plug?are you sure distributor is ok and turning as engine turns....

 

[grober]

Almost certainly your car will have a ballast resistor ignition set up. This means that in the ignition switch start position the coil is supplied directly from the starter motor- in the run /or ignition on switch position its fed via a ballast resistor. My guess is that you have not wired up the starter correctly? here's a diagram its not the same car as yours but principle is the same.
Pagoda SL Group Technical Manual :: Electrical / IgnitionSystem

 

[grober]

AND YOU COULD TRY THIS SITE
https://translate.google.co.uk/tran...ttp://www.w111.net/html/forum.html&edit-text=

 

[guydewdney]

+1 on ballast resistor issue.

 

[RossCormack]

Thanks for the replies. I am sure the rotor is turning and the plug I'm looking at is earthed, I know it's a good earth since there is a spark when I turn the key off. What I did was clamp the plug to the side of the engine.

I will have another look at all the wiring today. In more detail, I am using a 1978 w116 SE to provide the ignition. The coil is wired in to that car and it has an ignition control module. I've removed the wire to the starter on that car and hooked that up to the starter on my my Fintail engine. Then added the live and earth wires needed to make the starter on the engine turn and to get the plugs to spark. The ignition key barrel is loose and I can easily see the plug and turn the key at the same time.