The curious case of a 280SL that just wouldn't start

[ctaylor738]

A friend's early 280SL is just finishing a restoration.  Paint and engine were done.  At the trim shop, it got a new GAHH interior and top.  When he went to pick the car up at the trim shop, it started, ran briefly, died, and would not re-start.  So he had it towed home and asked Ray Schlicht and I to take a look.

We put a timing light on the coil wire and found no spark while cranking.  Looked inside the cast iron distributor and found a badly worn set of points.  Replaced the points and condenser, set dwell.  Still no spark.  Voltage to the coil was 10 V while cranking. Verified spark across points when engine cranked.  Still no spark.

Found that the owner had a new 123 distributor and a Bosch red coil that he intended to install.  On the theory that the coil was probably weak, we installed the 123 distributor and the new coil.  The engine fired briefly and died and refused to re-start.  Checked for spark again, nothing.  We went back and re-checked the installation of the 123, setting the crank to a couple of degrees before TDC and turning the 123 until we just got a green light, per the specs.  Still no spark.

Pulled the plugs out, observing that they were wet and sooty.  Noted the new and correct Mercedes plug wires.  Verified that all wires were tight in the cap.   Replaced the plugs with NGK non-resistance types.  Still no start, still no spark.

Almost not believing what we were seeing, we took a detour and checked for fuel at the CSV.  It was abundant.  We put the timing light on another car.  It was working fine.  We decided that resistance was futile and gave up for the day.

Today I returned with an idea of what the problem was, and I was correct.  After it was addressed, the timing light started flashing, and the car started instantly.  What do you think it was?  Answer tomorrow.

Hint - there's a clue in here somewhere.

Cheers,

CT

[Peter van Es]

Battery?

[badali]

Wires in wrong order??  I have done that before...

[Baybear]

Chuck you are such the gamester. I'm thinking plug wires are off as badadi said

[mnahon]

First coil was weak/dead. New coil needed to have ballast resistor removed.

[Jack Jones]

Bad coil wire.

[kampala]

Ground was disconnected due to paint job. 

[ja17]

Along the same lines, the ignition was incorrectly connected after the engine bay work.

[Atazman]

wrong coil installed with the 123 system or ignition switch

[Cees Klumper]

No spark then I would check the wiring of the ignition system, fuse and the engine ground.

[ctaylor738]

Hats-off to Jack Jones, who was very close with his "bad coil wire" answer.  Technically, the coil wire was not "bad."  It was new and working as designed.  The problem was that it had 22K ohms of resistance.  There was an additional 4K in the rotor, 2K in the center of the cap, and 2K in the plug wires.  So the coil was trying to overcome 30K ohms resistance to fire the plugs.

Tried to provide a clue by using "resistance" twice in my original post.

What probably happened was that since a set of plug wires from Mercedes does not include a coil wire (I am not making this up), the restoration shop, which was not a Mercedes specialist, probably just found a spare coil wire and stuck it in.  The car started and ran well enough to move around until it got to the upholstery shop.

Cheers,

CT

[DaveB]

resistance was futile

  very good

[Jack Jones]

Do I get a gold star

[Benz Dr.]

I see this a lot. Carbon core coil wire, wrong spark plugs, and wrong spark plug terminals. Engine will start and run but the more you idle it the less it runs. Eventually, it won't start at all.

 I use the minimum of resistance in my ignition systems. No resistors on the col wire - you want maximum voltage going to the rotor which normally is 5K ohms anyway, and 1k at the spark plugs which need to be non resistor. Until you get these simple things right you will waste a lot of time looking for problems that might not be there.

[Jack Jones]

I have learned that many times people search the most complex issues when more times then not it comes down to the basics. I once saw a top auto technician diagnose and replace an engine control computer to repair a severe hesitation. When the new ECU did not fix the problem he went back to the basics and found 1 cracked spark plug.