1973 280 M110 Ignition System question

[busman1965]

I have been working on my recently acquired 1973 280 sedan. The original (and only) owner before me did conversion on the distributor to a Crane/Allison XR-700 Electronic Ignition (basically a points eliminator).

I noticed today that the car has Resistor type Champion RN9YC spark plugs as well as copper/silicone ignition wires with 5K resistor metal boots on them. This seems to add up to over 10K resistance between the coil and the plug gap, which seems like a lot to me. I recall these engines used to use 1K resistor wires and non-resistor plugs originally.

The whole reason I went digging into the ignition system was I noticed the original owner listed fairly frequent replacement of spark plugs in his maintenance logs (he kept insanely detailed records of every single item that was changed on what date and the mileage, as well as what symptoms if any required the change). It appears he has used the Champion RN9YC for many years, and it appears he also had swapped out the original 1K ohm wires for the 5K wires many years after installing the XR-700 Electronics Ignition.

The last bit of info is that the coil is a stock internal ballast resistor Bosch coil. Currently the car starts easily cold or hot and runs really well with no hesitation or flat spots, and still has the original Solex 4A1 carb. Its actually one of the best running carb'ed M110's I have ever driven.

I guess my question is do I need to look at getting rid of the resistor plugs and/or going back to the 1k plug wires? My concern is that the set up is causing the plugs to fail prematurely based on the frequency of plug changes noted by the original owner, who sadly cannot remember much anymore. Any thoughts or ideas on this setup?

[Sugar Bear]

Since it runs well it is probably best to leave it alone unless you aren't happy with it. Changing spark plugs once a year was regularly done "back then". If you do decide to make changes try to do one at a time so if it doesn't respond well you'll know the most likely culprit. All the above being said, I don't care for the 10K wires.

Another issue with the older cars and spark plug issues is short trips and a light right foot. Get the car hot, keep the oil changed and PUSH it hard occasionally uphill and let it coast downhill.

If the plugs do foul and need to be changed I'd switch to NGK, non-resistor and one heat range hotter.

[Frank Reiner]

Originally Posted by busman1965

I have been working on my recently acquired 1973 280 sedan. The original (and only) owner before me did conversion on the distributor to a Crane/Allison XR-700 Electronic Ignition (basically a points eliminator).

I noticed today that the car has Resistor type Champion RN9YC spark plugs as well as copper/silicone ignition wires with 5K resistor metal boots on them. This seems to add up to over 10K resistance between the coil and the plug gap, which seems like a lot to me. I recall these engines used to use 1K resistor wires and non-resistor plugs originally.

The whole reason I went digging into the ignition system was I noticed the original owner listed fairly frequent replacement of spark plugs in his maintenance logs (he kept insanely detailed records of every single item that was changed on what date and the mileage, as well as what symptoms if any required the change). It appears he has used the Champion RN9YC for many years, and it appears he also had swapped out the original 1K ohm wires for the 5K wires many years after installing the XR-700 Electronics Ignition.

The last bit of info is that the coil is a stock internal ballast resistor Bosch coil. Currently the car starts easily cold or hot and runs really well with no hesitation or flat spots, and still has the original Solex 4A1 carb. Its actually one of the best running carb'ed M110's I have ever driven.

I guess my question is do I need to look at getting rid of the resistor plugs and/or going back to the 1k plug wires? My concern is that the set up is causing the plugs to fail prematurely based on the frequency of plug changes noted by the original owner, who sadly cannot remember much anymore. Any thoughts or ideas on this setup?

Re: Primary circuit resistance
The original (1973) points controlled circuit comprised a coil with an internal resistance of .38-.45 ohms and external resistance of 1 ohm (.4 +.6 separate resistors).
The XR-700 usually operates with a total primary resistance of ~ 3 ohms.

Re: Secondary circuit resistance
Internal coil resistance: 7K-14K ohms
External: Rotor - 1K, Spark plug connector - 1K, Wire ~ 0, Spark plug - 0

You are correct about the original secondary resistance; it was approx. 2K ohms. The use of resistor spark plugs is probably a function of availability; non-resistor plugs are harder to find.

[busman1965]

It sounds like I would probably be best served to leave the 5K wires in place and try some non-resistor plugs as a starting point. I was only concerned about this as going through the cars logs, it seemed like there were a fair number of fairly low mileage (under 5K miles on the plugs) plug failures in the last 20 years.

I know these old Mercedes were not the easiest on plugs, but I would expect more than 5K miles out of a set of modern plugs. I am guessing getting away from the resistor Champions and going non-resistor NGK's will hopefully help longevity. If not I will try bumping up to the next hotter plug.