Danger: RESISTOR SPARK PLUGS - PeachParts Mercedes-Benz Forum

whunter · #1 08-17-2007, 12:50 PM

Resistor spark plugs can NOT be used in many MB engine applications..
Follow Mercedes Benz specification on what spark plug to use.

What spark plug is in your engine at this moment, please remove one to check the number personally, if it has an "R" = resistor.

Do not trust service records on this.
Bad wires, cap, rotor and/or wrong spark plugs can wreck your ignition module, most applications this = $2,000.00 - $3,000.00 for a new one.

If you know your MB part number, here are links to verified Bosch Non Resistor Spark Plugs
Bosch Non Resistor Spark Plugs

Call or e-mail your VIN# to check application.

7450SE · #2 08-19-2007, 08:21 AM

whats wrong with them?

manny · #3 08-19-2007, 08:48 AM

Quote:

Originally Posted by 7450SE

whats wrong with them?

As the name ( Resistor ) implies, the resistance & load that is placed on all ignition components, with these plugs, could seriously damage the ignition module.
In simple terms, if you overload an electrical circuit in your house, you'll end up blowing a fuse/breaker.
In this case the ignition module is your fuse/breaker, but a very expensive one.

7450SE · #4 08-19-2007, 09:41 PM

That is not cool... Does this also apply to older mercs? ie '77 and back?

Tomguy · #5 08-19-2007, 10:36 PM

It sure does. No older Mercedes should have a resistor plug - resistance is built into the spark plug boot.

dpetryk · #6 08-20-2007, 09:50 AM

I have to challenge the thinking in this thread.

The previous statement:

Quote:

As the name ( Resistor ) implies, the resistance & load that is placed on all ignition components, with these plugs, could seriously damage the ignition module.

Is incorrect as it applies to ignition circuits of Mercedes Benz vehicles. I can prove it. As the resistance in a circuit goes up the "load" or current goes down.

Would the originator of this thread care to share with us the scientific or factual basis for his statements;

Quote:

Bad wires, cap, rotor and/or wrong spark plugs can wreck your ignition module.

If anybody wants to get into technical discussions on why I don't agree with this thinking I would be happy to engage them.

sbourg · #7 08-20-2007, 01:40 PM

Well, I for one don't KNOW why resistor plugs would or would not ruin an ignition module, but I can certainly dream up a technical argument for the damaging scenario...

The coil output must drop across the spark plug gap with enough initial voltage to jump the plug gap. A plasma is then created which drops the resistance of the air / fuel vapor to near 0 Ohms, allowing a lower source voltage from the coil to push enough current across the gap to complete the ignition process. If the series resistance of the wired path is too high, the lowered energy flow rate to the gap could be insufficient to sustain the plasma, and the spark will die. For the remainder of the spark cycle, the full output of the coil will then appear across the coil terminals, risking internal breakdown over time. Now I don't know that this would actually occur if resistor plugs are used, but I would like to hear the technical reason refuting this possibility.

Steve

whunter · #8 08-20-2007, 01:40 PM

Quote:

Originally Posted by dpetryk

I have to challenge the thinking in this thread.
The previous statement:
Is incorrect as it applies to ignition circuits of Mercedes Benz vehicles. I can prove it.
Would the originator of this thread care to share with us the scientific basis for his statements;
If anybody wants to get into technical discussions on why I don't agree with this thinking I would be happy to engage them.

Personal/professional experience and MB TSB/public announcement data on this issue is fact.

I do not have time to dig up all of the data.

If you desire to discuss your opinion, please start another thread.

dpetryk · #9 08-20-2007, 03:05 PM

Well, if you or someone else can get some time to dig up the TSB, I would love to see it.

I called 4 MB dealers in Houston and none of them could remember anything describing the TSB mentioned here. This does not prove that it does or does not exist, just they had no memory of anything related to the use of resistor plugs being detrimental to the ICU.

Owning 5 vehicles, and having resistor plugs and wires, I am concerned about the possibility or risk to the ICU. I would like to know for sure. I think everyone else would also.

Certainly someone here has access to the TSB's and can confirm this one way or the other. Is the TSB specific to all cars or only to certain years / models? We need to know.

For me personally, I am not concerned. But I would like to know the official word on the subject.

dieseldiehard · #10 08-22-2007, 07:33 PM

Quote:

Originally Posted by whunter

Personal/professional experience and MB TSB/public announcement data on this issue is fact.

I do not have time to dig up all of the data.

If you desire to discuss your opinion, please start another thread.

Awww, I think this thread is still the place to discuss the subject. As an EE I would like to know the reason for an announcement, to see if its based on technical or other basis. It could be a simple warning to keep sales up on MB instead of aftermarket parts. I think its pretty rude to try suggesting starting another thread. Tech help matters are open to discussion too aren't they????

Show us the details, Mr Hunter

skhagen · #11 02-11-2008, 05:42 PM

I found that the PN's for my 1996 I 3.2L have changed for plug wires and coils aswell as the coil resistance spec.
MB must have a reason to change the 3 Ohm coils to 2 Ohms. I assume it is to accomidate the resistor plugs.

Graham · #12 10-20-2007, 12:06 AM

Quote:

Originally Posted by Tomguy

It sure does. No older Mercedes should have a resistor plug - resistance is built into the spark plug boot.

That may be true if the original wires are used.

But, Bosch on their own site specify their part 09027 wire set for our '72 350SL 4.5L . Almost all on-line parts suppliers sell the same wires for 4.5L M117 engines.

These wires are not copper wires, but are what Bosch call Opti-Layer Mag wires - They say it is a nickel alloy (stainless steel). These wires measure about 1.6 kohms end to end and do not appear to have any resistors built in, at least on the distributor end.

The original 1972 vintage MB wires that I just removed measure 8 kohms and have an integral 1.6 kohm resistor at the distributor end (molded into the ceramic plug) (I know - I cut one apart!)

So now we have a Bosch recommended wire set that has 6.4 kohm less resistance than the original MB wires.

So the question comes up as to which plugs to use.

I was sent plugs with following designation:

7500
0 242 235 663 -74U
WR7DC+

Old plugs, which were W7DC, also had the 7500 designation? Does this mean that Bosch consider these "R" plugs a replacement for the non-resistor plugs? WR7DC used to have 7501 number. Can anyone verify this change and explain the thinking?

If I used non resistor W7DC plugs, would my overall resistance not be lower and could this mean a very short lived hot spark? How much resistance would the 7500 WR7DC plugs add?

There are two of us with this dillema, being discussed in a separate thread. But the experts are here, so I would really like to hear any comments.

BTW, the car is running OK with new wires/plugs , but always interested in improvements!

97civicdx · #13 10-20-2007, 10:05 AM

So everyone is saying that by using a Resistor plug like the autolite r103 can short out a coil?

MB-Dude · #14 10-26-2007, 09:59 AM

Quote:

Originally Posted by 97civicdx

So everyone is saying that by using a Resistor plug like the autolite r103 can short out a coil?

There are two sides to this excellent discussion - one side believes the use of resistor plugs can damage coils and ignition systems, while the other side believes the use of resistor plugs is acceptable. Also, this is not a one-size-fits-all sort of discussion; meaning there may be certain special mitigating circumstances in a particular model/year.

However, without additional conclusive and objective data from MB Engineering, we are sort of at an impasse and have to approach the subject with theory, personal experience and gut-feel. Therefore, you must use your own judgment based upon what you learn as to whether resistor plugs will work in your vehicle. Personally, I’ve enjoyed this thread immensely.

Cheers,
Jeff

CamelotShadow · #15 10-31-2007, 01:16 AM

Quote:

Originally Posted by Graham

That may be true if the original wires are used.

But, Bosch on their own site specify their part 09027 wire set for our '72 350SL 4.5L . Almost all on-line parts suppliers sell the same wires for 4.5L M117 engines.

These wires are not copper wires, but are what Bosch call Opti-Layer Mag wires - They say it is a nickel alloy (stainless steel). These wires measure about 1.6 kohms end to end and do not appear to have any resistors built in, at least on the distributor end.

The original 1972 vintage MB wires that I just removed measure 8 kohms and have an integral 1.6 kohm resistor at the distributor end (molded into the ceramic plug) (I know - I cut one apart!)

So now we have a Bosch recommended wire set that has 6.4 kohm less resistance than the original MB wires.

So the question comes up as to which plugs to use.

I was sent plugs with following designation:

7500
0 242 235 663 -74U
WR7DC+

Old plugs, which were W7DC, also had the 7500 designation? Does this mean that Bosch consider these "R" plugs a replacement for the non-resistor plugs? WR7DC used to have 7501 number. Can anyone verify this change and explain the thinking?

If I used non resistor W7DC plugs, would my overall resistance not be lower and could this mean a very short lived hot spark? How much resistance would the 7500 WR7DC plugs add?

There are two of us with this dillema, being discussed in a separate thread. But the experts are here, so I would really like to hear any comments.

BTW, the car is running OK with new wires/plugs , but always interested in improvements!

I have a feeling that there is a difference between resistance of a wire
& a resistor built into a plug.

They might not be of the same units so there fore you can't add apples with oranges & compare it with the origiinal resitance of the oem wires.

Guess thats a new thread
Low resistance wires...

I got those new Bosch opti layer nickel wound SS & just wonder if its hurting anything when paired w the spec non res plug?

I should hope to try to understand the physics
as I once was a physics major
but alas time has a way of clouding the mind
& I'm trying my best to if not figure it out
at least know the system is not compromised...

Resistance is a measure of the force that reduces the flow of energy
less resistance would mean a higher end voltage.
What happens when it gets to the plug?
less resistance would mean a more efficient flow thru the plug
sometimes resistance creates additional heat as electrons have to work harder & bump each other more

Help, I'm loosing it again