Ignition wires- 5 ohm or 1 ohm?

[Mike K]

A few weeks ago while out on a drive, my ‘71 280SL suddenly started running rough and misfiring. I eventually tracked it down to the ignition/spark plug wires.  On replacing these, everything went back to normal and the car’s running smoothly again.

I’m running a 123 ignition & NGK BP5ES spark plugs gapped at .035.

I initially sourced these cables: http://www.mbclassics.de/index.php?a=5814&lang=eng which have 5 ohm resistance.
Fitted these and immediately the rough running problems disappeared.

Having read that the collective wisdom on this site appears to be that 1 ohm resistance is ideal, I then purchased another set of wires from Authentic Classics: http://www.authenticclassics.com/Mercedes-280SL-Ignition-Spark-Plug-Wire-se-p/auth-004612.htm

(BYTW this turned out to be a rather expensive exercise, on top of the $125-  I paid for the wires incl. postage, I had to pay another €57- import duty into France).

I've experimented with and tried out both sets of wires and have found that the car runs smoother and cooler on the 5 ohm resistance wires. 

I was just wondering why and if this possibly has something to do with the 123 ignition?

Best,
Mike

[scoot]

I think you are supposed to be using 1 k ohm, not 1 ohm wires.  But I think that's what you mean.  Don't think it has anything to do with electronic ignition. 

[ja17]

Hello Mike,

With the standard original non-transistorized ignition, these cars used the 1k ohm resistor ends on the wires. If it is a late car and had the factory transistorized ignition, I believe they used the 5k ohm resistor ends. Now if you have a hotter replacement coil or a transistorized ignition the 1k ohm or 5k ohm resistors should work fine. With the l k ohm resistor ends,  you want  to use the standard BP6ES spark plugs which are a little cooler than the BP5ES.

MB allows 13k ohms total for non transistorized ignitions and 19k total ohms for transistorized ignitions. Never use carbon spark plug wires which add 5k resistance and break down sooner with age. You should also avoid resistor spark plugs which add another 5 k ohms to each circuit.
The factory wire sets also have resistor ends at the distributor 1 K ohms ea. The rotor adds 5K ohms to each circuit. If you added a hotter coil your system can handle a little more resistance. Make sure you are using the correct ballast resistor with your coil.

[Mike K]

Hi Joe,
Thanks for the input, it explains a lot, much appreciated.

Mine's a late 280SL, according to the data card she's a Feb. '71. It had the transistorised ignition, which I removed when it got fried. I've replaced that with a 123 ignition and the hotter Bosch coil. I've noticed that the garage that installed the 123 ignition bypassed the ballast resistors.

Since I've been using the hotter NGK BP5ES plugs I've also found that I have no sooting or carbon on the plugs, they're burning a nice clean rusty grey colour.

The bottom line I guess is that the 5k ohm resistor ends are fine for this set up.

Thanks & best,

Mike

[Benz Dr.]

Something to consider: the standard black coil is only 13 KV ( 13,000 volts ) and the red coil is supposed to be  26 KV.  This is why you need a 1.8 ohm ballast resistor or double the resistance when using the hotter coil - remember, the black coil uses .9 ohms.

Given that MB allows 13 K of  total ohms, there really wouldn't be much of a spark at that point with 13 KV of power. I've been going towards no resistors anywhere except at the spark plug terminals. If I bother someone's AM car radio - too bad. They need to upgrade to at least FM. 

 Check your coil wire. I find at least 50 to 75% of wire sets with carbon core. Save the terminal ends and boots then get some proper metal core wire and use the old hardware. It's one of the best fixes you can do and you will see a real difference in starting, idle quality, and overall performance. And, it costs next to nothing to do!

  GO CHECK THAT COIL WIRE RIGHT NOW!!!!

[Mike K]

Thanks for the input Dan. Along with the spark plug wires I also replaced the coil wire.
They're all 7mm Silicone jacketed with copper core.

Pic of the coil I've installed below.

Are you suggesting that I re use the original 1k ohm resistance spark plug connectors, with my set up, instead of the 5k ohm resistors?

Best,
Mike

[Benz Dr.]

I have no problem with 1K plug resistors and it's what I use.