Yes and NGK's work better in older models that can accept the equivalents. And Champion resistor plugs are indeed very unreliable and have high failure rates even out of the box, based on my experience.
But we aren't talking experience here its what Benz has to say in some obscure bulletin that is hearsay at the present time.

Lets have some technical reasons for resistor vs. non resistor plugs. Its going to be sticky because the types of ignition systems differ radically over the past say 5 decades. I know some ign systems fault when they see a short, and putting plugs with platinum coated tips cause faults or fault codes to come up because the pt flakes off over time (bad idea IMHO, just a way to sell more expensive plugs to fools who are eager to part with their money).

Are you seeing what is involved here, its more than just whether or not a plug may have a resistor in it or not.

I have also been wondering about what models/years fall under the no resistor rules. All I know is my 94 e420 gets a smooth 27-29mpg with the Bosch P4s, and a rough running, hard starting 23-24mpg with the "recommended" copper bosches.

If the P4s are both a gimmick and a resistor type, how is what I am experiencing possible? It should be the other way around. My car also came with P4s when I bought it at 82K. It now has 112K.

Shouldn't my ignition module have broke by now from the extra resistance?

That's right, and the only experience that's worth two cents is that of Mercedes/Bosch engineers and I'm still waiting to to what they've published.

Yeah, use non resistor plugs. Itll run sooooo much better.


My car had a slight miss at idle, new non resistor plug fixed it.

I believe the M119 and the late M104 engines are less susceptible to the resistor issue. Both engines are a double overhead cam design with 4 valves per cylinder and variable valve timing on the intake side with fully electronic and integrated ignitions. One coil for two cylinders. There is a lot of adjustability in these engines, not as much as the newer engines MB has now but I would say enough computer control to compensate and benefit from platinum resistor plugs.

The M103 and M102 engines on the other hand are traditional single coil designs with no variable timing. Resistor plugs do cause issues especially if the coil is an old (original) one. But this is just based on my experience, no technical document or lab test was performed lol :D

This makes for some interesting reading & it's in layman's terms:
http://www.ultralightnews.com/enginetroublshooting/resistorcapsandplugs.htm

The problem with resistor plugs is not the current but the voltage. The firing voltage may get too high and damage ignition system components.

Huh?!? Wanna run that one by us again? Please explain.

When resistor plug used, you add the resistance of the plug to the total resistance already there - in the plug connector and in the rotor or cover (not sure were the resistor on the distributor side is). The resistance of the path from the coil to the plug will be higher than normal. This high resistance puts additional demand on the secondary voltage to initiate and support the spark. It is like having a bad high voltage wire.
Mike

True.

True.

False. I’m sorry Mike, but the addition of resistance “in series” to the secondary side of any transformer (which is what a simple automotive coil is) does not place additional demand on the secondary voltage. This simply causes less current to flow from the secondary side of the coil. In essence, less spark energy would be created. If anything, it would be less stressful on the coil because the increased resistance from secondary through the high voltage wire through the resistor spark plug allows less current to flow. Less secondary current flow equates to less energy being coupled through the coil from the primary side. The coil runs cooler and less stressful.

To illustrate through an extreme case, imagine removing all the high voltage wires from your engine and attempting to start it. Of course it will never fire, but if the battery could stay charged forever and you kept cranking the starter, you would never see current flow from the secondary – it’s impossible… no path for the current. The coil would last forever because there is no current being generated in the secondary. In fact, the primary side is also protected, but without getting into electrical/magnetic theory, you’ll have to accept my word on it.

All automotive coils generate energy by allowing 12v current to be pulsed through their primary windings. The pulsing action (timed to the when the spark needs to fire) transfers energy to the secondary windings of the coil. The ‘voltage’ created on the secondary side of the coil is in direct relation to the ratio of wire windings of the primary to the secondary. The amount of energy transferred is completely dependent upon the "resistance to ground" of the secondary circuit.

In a perfect zero resistance scenario, the energy builds in the coil and the output voltage of the secondary rises rapidly. When sufficient energy is achieved to overcome the spark gap, the gap arcs and suddenly there is current flow until the energy in the coil drains enough to not sustain an arc. The spark gap – only when arcing – is considered a resistor and has a certain limiting effect on the high voltage current. It is important to remember that other than when arcing, the spark gap offers infinite resistance. The current flow is indirectly proportional to the resistance – higher resistance to the spark gap, less current. Thus, resistor plugs should create a weaker spark, but be less stressful on the coil. It is this 'weaker spark' that greatly reduces the infamous radio interference noise.

However, your example of a bad high voltage wire is valid for coil stress, but for a very different reason. When a high voltage cable begins to fail, it allows current through/across its insulator jacket. This equates to current flow directly to ground, so a portion of the secondary energy bypasses the spark plug gap. This means that the coil will produce more current (i.e.: work harder) to generate sufficient energy for the spark. In extreme cases, enough secondary energy may be drained off through the insulating jacket to the extent that the sparking arc may not be sufficient for initiating a complete fuel/air mixture burn – i.e.: misfiring. This scenario can cause stress on the coil, secondary side, in particular. Whether that stress causes ignition module issues is debatable and is very dependent upon the electrical engineering design of the ignition module.

It is a common misconception that increased spark plug resistance causes coil stress. High voltage cable breakdown does. A partially shorted spark plug does. But spark plug internal resistance should lower spark current, not increase it.

Simple.
The resistance is the airgap between the rotor and the distributor cap. ;)

No, there is a resistor in the rotor, about 1.2k. Just checked it on my old rotor from M120 engine.
Mike

This is missing the point - less current flow at some point will not sustain the plasma at the plug tip, where the resistance will jump from near 0 Ohms to no connection - open circuit, infinite Ohms. The coil voltage output at this point of the discharge cycle may be high enough to damage its insulation dielectric, but insufficient to re-create a plasma. With less series resistance initially, plasma would be sustained due to higher current flow, until coil output voltage falls to a safe level.

This speculation is simply to show that increasing secondary resistance at some point will likely cause a problem. Don't screw with the engineered parameters!

Steve

This is not a good example. You won't get any sparks in this case. Now if you remove the wire from the plug a move it away a little to create a second spark gap, you'll see secondary voltage demand increase to bridge both gaps. In your example the secondary voltage on the coil will be very high and could damage the coil insulation.
I'm not talking about bad insulation. The bad wire in terms of increased resistance. This is very unlikely with MB copper wires, but could happened to wires with distributed resistance.
I agree the the resistance has no effect on initiating the spark, but it will put higher demand on the secondary voltage to sustain the spark.

With all due respect, I'm not sure I agree with your line of reasoning. The secondary winding will have stored the maximum amount of energy for that duty cycle, immediately at the moment of spark ignition. Then, the energy is drained off the secondary at a rate that is defined by the total resistance to ground. Assuming for this discussion that the high voltage wires do not have insulation leakage, the energy in the secondary winding drops for the duration of the spark until there is not enough energy to sustain a spark. In essence, the secondary winding 'sees' maximum voltage and energy potential at the instant before spark ignition - regardless of type of spark plug used. In other words, once a spark starts, there is always less energy remaining in the secondary.

In the real world, there is leakage through high voltage cable insulation. In this case, energy begins to drain from the secondary winding immediately, through a continual resistive path in parallel to the spark gap, even before the spark is initiated. Thus, less energy is presented to the spark gap. This is why old high voltage cables cause misfiring (i.e.: weak spark).

In an additional real world scenario, aging rotors/caps increase the resistance from the secondary winding to the spark gap – just like adding resistor plugs. As we know, the carbon build-up and erosion of the rotor/cap interface can add substantial resistance to the spark circuit. Now we know that aging rotors/caps are not good for performance, but I have never heard of old rotor/cap taking out a coil. The vehicle may run like poo because of a really bad rotor/cap, but the coil is basically undamaged. I am not a professional mechanic and invite real examples, if they exist.

However, you are correct in that adding more resistance would create a weaker spark. (If I mis-paraphrased your thoughts, please correct me.) But the secondary windings have long since seen the maximum amount of energy by the time the spark extinguishes. Since the elusive TSB cannot be presented for whatever reason, I believe this is the most probable line of reasoning for not using resistor plugs – a weaker spark. How that can cause damaged ignition modules/components I do not know. But I’m keeping an open mind and continue to think about it and question it.

Cheers,
Jeff