Never did this myself, but I understand one uses an ohm-meter to check the resistance of the spark plug wires. Here's earlier posts on this topic by three experts:
Joe Alexander: The original spark plug wires are solid copper and very rarely go bad. However the screw-on spark plug ends have built in resistance and these do go bad. Chances are if you replace the whole set, you will never get another set that looks as good or lasts as long as the originals. I advise that you check each wire and replace the bad ends. Finding original ends may be a challenge, but I am sure some listers have spares. Make sure you still have the original solid copper wire and that it has not been replaced by those troublesome carbon resistance wires. If you do have some damaged plug wires, original solid copper plug wire is still available from some motorcycle and farm tractor sources. JC Whitney used to sell it by the roll. I usually check all my plug wires and ends with an ohmeter during a standard tune-up. Mercedes used several versions of wire ends during the 113 years. Same vintage Mercedes sedans used the same in many instances and are good parts donors! The coil wire was also solid copper originally, and had a screw-on resistance end.
Pete Lesler: the early, pre-transistorized ignition cars already had bakelite resistor terminals installed rated at 1K ohms, the steel resistor ends starting with the transistorized ignition was rated at 5K ohms. Don’t use resistance plugs or silicon resistance impregnated wire. Use of one or both will materially reduce the intensity of the spark. You may be forced to make up your own plug wires with solid copper core spark plug cable. I have used Packard 440 cable in the past. The terminations screw into the copper core and the ends that plug into your distributor are available as a kit as is the cable from almost any good auto parts supplier.
Dan Caron: be careful when buying ignition wires. The 230 SL uses wires with bakelite ends. K&K's wires are correct and is what I use and sell. Some are made by Beru but they are still correct. Stay away from any wires that have carbon core in them. All wires should have the 1K ohm spark plug ends and the best ones don't have any resistors at the distributor cap. I prefer the 230 SL wires over almost anything else. I also use the 26KV coil with a 1.8 ohm balast resistor. Very good set up. What you want is really low impedance wires so all the spark can get to the plugs. The standard black coil used on some early cars is only 13,000 volts , the blue ones are around 20,000 volts and the hottest standard coil is rated at 26,000 volts. This is the one I convert to. You need to use the 1.8 ohm balast or twice what the black coil uses which is .9. Total package with Bosch wires runs about $160 for all the parts you need.
p.s. - SL Tech sells correct wire sets.
