Author Topic: Ignition miss  (Read 2278 times)


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Ignition miss
« on: May 07, 2020, 23:00:42 »
I have a 1969 280SE W111 Coupe, that was in need of a tune up, and here is what I found:
1.  WR7DC plugs gapped at .028
2.  Dwell set at 20 degrees (yes, 20)
3.  Timing set at 11 degrees BTDC.
4.  A 1000k screw in resistor at the distributor cap connection on every wire (Bosch resistor and Bosch wires).
5.  A 1000k resistor end at each spark plug end (Bosch).
6.  Stock OE MBZ 15K coil and ballast.
I replaced with:
1.  Bosch OE MBZ "Red Coil" and correct ballast from another 280 MBZ (used but good).
2.  Petronic pointless ingition set (wired as per instructions; left ballast in system)
3.  90 degree plug end connectors at the distributor and coil caps (unknown manufacturer probably not OE MBZ).
4.  New Bosch rotor and cap.
5.  New Bosch W9DC plugs gapped at .035.
The HT coil wire will arc right through the HT boot to the LT terminals, causing the car to eventually stop running.
Do I have too much spark?  Too much resistance downstream?  Or is the HT boot simply not doing the job?
Thanks, Galen


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Re: Ignition miss
« Reply #1 on: May 08, 2020, 01:56:42 »
Most likely to much resistance downstream. Check all your connections. Check all your components with an ohm meter. Try going back to .030" on your plug gap.
Joe Alexander
Blacklick, Ohio
1969 Dark Olive 280SL
2002 ML55 AMG (tow vehicle)
2002 SLK32 AMG (350 hp)
1982 300TD Wagon turbo 4spd.
1963 404 Mercedes Unimog (Swedish Army)
1989 flu419 Mercedes Unimog (US Army)
1998 E430
1974 450SLC Rally
1965 220SE Finback


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Re: Ignition miss
« Reply #2 on: May 08, 2020, 22:15:26 »
I re gapped the spark plugs to .030, and there was still a bad miss.  I then changed the HT line from the coil to the distributor with a Bosch brand wire, and the coil arch problem was gone.  While I was inspecting the rest of the HT wires, I got shocked.  I decided to change out all of the spark plugs wires as well, and the idle cleaned right up.  The existing plug wires were a very, very good copy of the Bosch brand, but were a knock off (a brand that I did not recognize). When the old black top coil was in the car, the aftermarket wires worked just fine, but when the red top coil was installed it would seem that that was too much current for the aftermarket brand to handle.  Seems to run great, and tomorrow I will give her a street test drive.  Now it is 100% Bosch and no more issues.  Thank you, GCW

Benz Dr.

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Re: Ignition miss
« Reply #3 on: May 09, 2020, 03:24:48 »
I was told by a Bosch representative that the black coil is 13 KV  and the red coil is 26 KV which is why you need a 1.8 ohm ballast resistor for the red coil in place of the .9 for the black coil. A lot of Bosch ignition wire sets come with a carbon core coil wire which will really cut your spark down - maybe that was part of your problem.
1966 230SL 5 speed, LSD, header pipes, 300SE distributor, ported, polished and balanced, AKA  ''The Red Rocket ''
Dan Caron's SL Barn

1970  3.5 Coupe
1961  190SL
1985   300CD  Turbo Coupe
1981  300SD
2013  GMC  Sierra
1965  230SL
1967 250SL
1970 280SL
1988 560SEC