Danger: RESISTOR SPARK PLUGS - Page 6 - PeachParts Mercedes-Benz Forum

Carson357 · #76 12-13-2007, 12:50 PM

we just had an issue with a customer a few months ago, for what we had we were dealing with bosch plug FR8DPP33, one of the techs here was diagnosing a rough idle for a 112 V-6 engine, the rought idle was obvious, but not enough to set a misfire code, when he checked the plug number installed in the car it was F8DPP33, knowing that through bosch all that was available to the aftermarket was FR8DPP33 he assumed that someone had previously installed incorrect plugs, so he recommended to replace them with the FR8DPP33, once that was done, the rough idle was obviously still there, the customer told us that mercedes had installed the set of plugs, so we did some checking and found that untill the supply of the F8DPP33 plugs were exhausted mercedes would be the only one who will sell them, so we began ordering all our plugs from mercedes directly instead of worldpac, we wanted to install same as what dealer was using, i guess their supply ran out because about 4 weeks later we started getting the FR8DPP33 plugs from mercedes, due to the higher cost from mercedes we started getting them from worldpac again. so the R plugs will be replacing the NR plugs for some applications. by the way, the rough idle was caused by defective plug wires, replacing all 12 wires soved the rough idle issue.

below are some examples directly from a bosch chart
Old#- Replaced by
F7KTCR - FR7KTC
F8KTCR-FR8KTC
H8DC AND H8DCO-HR8DC+ (YTTRIUM ADDED*)
H9DC AND H9DCO-HR9DC
W7DC AND W7DCO-WR7DC+ (YTTRIUM ADDED*)
W8DC AND W8DCO-WR8CD+ (YTTRIUM ADDED*)
W9DC-WR9DC
W9DCO-WR9CD+ (YTTRIUM ADDED*)
F8DPP33-FR8DPP33

*http://en.wikipedia.org/wiki/Yttrium

whunter · #77 12-13-2007, 10:37 PM

Quote:

Originally Posted by Marsden

Just a note to this thread.

I just purchased a set of H9DCO plugs for my 190E 2.6. As of 5:55 eastern standard time, there were, if I remember right, 10,447 of these plugs in the MB system in North America. Apparently data that any MB parts department can quickly check. They said that they did not expect to run out any time soon.

I went over to check this vehicle in person.

Compression test:
#1. 150
#2. 150
#3. 150
#4. 170
#5. 180
#6. 185

Weak yellow intermittent spark, roughly every 15 - 30 crank shaft revolutions.

Note:
Look at this link for the broken resistor.

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1992 Mercedes Benz 190E 2.6
Part: Aux. Fan Resistor
Note: Auxiliary Fan
.2 OHM : Black bracket
Chassis: F000000-F637529
G5040-51923 Aux. Fan Resistor Beru IN STOCK

Carson357 · #78 12-16-2007, 01:36 AM

i agree with your findings, but not all have a mercedes dealer close by or prefer to buy parts from them due to cost, i posted my findings so people would not panic if their replacement plugs were resistor type.

psychiatry · #79 01-01-2008, 06:14 PM

After restoration of my 1990 420 SEL last year it soon began to miss at idle, then even under load -my mechanic fixed it, three plugs were fouled. This problem kept occuring until I replaced the UAP resistor plugs with non-resistor plugs. I do not know why the resistor plugs did not work but I have put 20,000km on the new plugs and the thing still runs great....

stevebfl · #80 01-13-2008, 12:21 PM

Just to muddy up the works I can tell you that the ammount of energy used in every coil firing is the same. The secondary disapation of that energy is done by voltage over time. The conditions the coil sees on the secondary are varied. They include the wires, the state of the plugs, the entire current path, but most importantly the environment in the cylinder. The disapation is easily observed with a secondary ignition scope and I can tell you from thousands of observations that neither the activation voltage nor the burn time is changed by resistor plugs. The condition of the spark could change, but the premise of this discussion is the load on the module. It is my position that the module could care less.

I don't believe any of this affects modules as they only control the current through the primary which is determined by the primary resistance and the modules internal circuit design. MB uses current limitation by anticipated firing point. The only way to change the current is to increase the rate of current ramp.

OzC36 · #81 01-15-2008, 08:26 AM

Steve,

I am not quite clear on what to deduce from your post.

Are you saying that it doesn't matter whether the plugs are non-resistor or resistor type as far as ignition/engine performance goes?

And are you further saying that there is no chance of damage to the module when using a resistor plug where a non-resistor plug was previously recommended?

Cheers,
Oz

stevebfl · #82 01-15-2008, 10:06 AM

I am making no statement about engine or plug performance. What I am stating is that the energy in the coil is the same no matter what happens on the secondary side. The energy is disapated differently depending on all the conditions that can occur including rich/lean mixtures, fouled plugs, open shorted wires. The disapation of that energy is a factor of voltage/current over time.

The load to the module is unaffected unless an open circuit causes a high voltage flash back. The load the module sees on the coil is affected by the resistance of the coil and the current limiting nature of the circuit.

OzC36 · #83 01-15-2008, 10:19 AM

OK, I think I now understand the point you are making...

The great unwashed masses out here in MB land, respect and rely on your knowledge, experience and recommendations!

Many thanks!

Xsbank · #84 01-15-2008, 01:22 PM

Actually, its usually the mechanics that are unwashed and the masses are clean!

barry123400 · #85 01-25-2008, 12:40 AM

Is not adding resistance going towards the way of an open circuit in the secondary to some extent? The coil field no longer totally collapses as fully as the designers expected across the plug gap. Instead the remaining undischarged portion of the field discharges from the low turn primary of the autotransformer.
The designed discharge path across the plug is stopped too early by the larger amount of resistance in the secondary. The resultant inductive spike from the primary of the coil could easily take out a transistor. Or just overstress it till failure. You have simply limited the original intended discharge path to some extent with the increase of resistance in the secondary.

A little is within design paremeters. More resistance than the design paremeters allow for might be created by using the resistance plugs. The remaining field generated inductive type of pulse is going to hit the control module. The potential in that field has to go somewhere as it was not able to totally discharge across the plug as designed since the resistance limited that. If this is a given the manufacturer has allowed some capacity in his module to deal with reverse pulses unless the amount of the field remaining is producing pulses larger than design paremeters.

Might be interesting to have a look with a scope in the primary and see if that pulse is present if at all or larger with a resistive plug than a non resistive plug. It is getting late at night and I am tired but the above and I think it might be true is enough of a reason not to set yourself up. Critique my thoughts if you wish but to me with my limited intellect it makes some sense.
Basically my position is why play russian roulette with expensive control modules? Hunter brought up a valid concern. The thing that attracted my attention was the cost of the module if a good used one cannot be found. Just make sure you do not leave the possible cause of the failure still in the circuit before using a new one if you ever have a failure.
This is also probably why newer replacement ignition wire sets have in some cases lower reistance than the originals had when new.
If a balast type of resistor is in the primary coil circuit. If this resistor increases in value with age the car might still run okay but the ability to partially filter or moderate any pulse coming out of the coil primary is getting reduced as well. The car battery is used as a capactor as well in this circuit and increasing balast resistor values are further reducing any pulses to some extent from its filtering effect.

Tims450SL · #86 02-06-2008, 04:39 PM

Older MBZ should never use a platinum plug. They make the car run bad!

skhagen · #87 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.

skhagen · #88 02-11-2008, 08:52 PM

http://www.eng-tips.com/viewthread.cfm?qid=168506&page=1

pmckechnie · #89 02-12-2008, 07:50 AM

What Barry123400 said. If you want to know what resistance in the secondary does to the primary then find someone with an automotive scope and see the difference in the pulse induced into the primary when the plug fires with low resistance and high resistance. ALWAYS replace plugs with the same plug the designers used. If they are no longer available find something close. I use NGK BP5ES plugs in my 500 SEL and they work fine. When I tried resistor plugs I had random missfires. There is more to this than the electrical circuit design. Combustion design must also be considered. Since most of us (well none of us) has the facilities to see what is happening during combustion we MUST rely on the design people.

GDC · #90 02-17-2008, 09:38 AM

Earlier in this thread there was a Link to Arthur Dalton's post regarding why resistor plugs are bad in wasted spark ignition systems. I have what is probably a foolish question regarding the answer. in a nutshell the answer was that the resistance is doubled since each coil fires 2 cylinders. Wouldn't the secondary side of the coil in a wasted spark ignition system be a parallel circuit from the coil to the plugs? If this is true, wouldn't the resistance be halved?

Just trying to learn.

Greg