Hirnbeiss-I forgot to write that I did lay the coil & boot and the #3 wire & boot across the engine and nothing, no arcing, no sparking - completely dead.


Deanyel- I d/c the socket at the module and was trying to measure/verify continuity between coil #2 and the rest of the harness, backwards to the module. I may have a break in the wire somewhere that leads to cylinders 3 & 4. I thought (since I couldn't see a number), that I could attach one probe of the voltmeter to the coil feed and then systematically touch every single socket on the harness receptacle. I was expecting no continuity after touching every receptacle, instead I found continuity on numerous receptacles - which makes no sense at all.

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Remember, Bad Decisions Make For Good Stories

There must be a ground wire socket at the harness plug but you should get no more than two circuits between the coil 2 lead and the harness plug. More than that would indicate a short or mulitiple shorts which would be the classic sign of crumbling insulation harness failure.

Someone must have a 124 service CD to look up the wiring diagram for the harness sockets.

Here are the wiring diagrams, I hope they are the correct ones.

Attached Files

	24072001.pdf (46.8 KB, 101 views)
	24072002.pdf (42.3 KB, 121 views)

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2004 E500 4Matic 138,000 miles

Thanks for the diagrams... I'm almost wondering if the diagnostic module is defective. Maybe it thinks it sees a problem with cylinders 3 & 4 so it shuts them down. I heard this happened to a W140 whereby the module thought one of the cylinders was misfiring, so it shut that cylinder down for no real reason.

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Remember, Bad Decisions Make For Good Stories

That's a possibility, but I would get proves of that before replacing it. If the inputs of the module are correct, and the outputs are not, it's defective.

The short finder is not as sensitive as an ohm meter. For this reason, you could find a "short" even if there is a coil in the middle. Getting "shorts" on several pins I think is normal. The coils have one primary side common, the other sides go to 3 individual pins and that is where they are triggered.

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2004 E500 4Matic 138,000 miles

How are you testing continuity? Are you using an ohm meter? What ohms are you reading? The injectors probably all have source resistors going to a common rail (+12) so it would not be unusual to read that resistance, but one pin should read short and the rest some resistance. The schematic shows they all go directly to +12. Compare the readings to the same tests on a good coil circuit.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Hmmm, that's showing the same 9, 10 and 21 sockets at the engine control module, but not showing the harness plug. I'm wondering if the numbering is the same. Have you pulled the engine control module to check circuits from that point? Thanks Andras.

Have you tried disconnecting the three terminals of each coil and taking three resistance measurements with an Ohm meter? The three coils should measure the same. If not, there is a problem with the coil that is different, which in turn can cause other components to fail.

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2004 E500 4Matic 138,000 miles

Can you verify the pin by the wire color going into it? Per the diagram it should be blk/wht, but obviously it should be whatever color is at coil 2.
At this point I think once you verify your wiring is good, you're looking at the engine module, which probably means shop scan or swapout to confirm.
Crumbling harness on a replaced harness would be a big surprise.

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Prost!

I got sidetracked here for a couple of weeks but I'm back. I just measured the resistance of all three coils. Two of them are a year or two old and the third is brand new. All three coils measure 9.4Kohms (using a Fluke 88).

Can anyone answer how I'm getting 12 volts at the low voltage side of coil #3 but the plug isn't getting any power? I don't understand that one. The only thing in the way of the 12 volt supply and the plug, is the coil - and that's brand new. What is my next logical step?

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Remember, Bad Decisions Make For Good Stories

There are 3 measurements possible between 3 terminals of each coil, and you are only listing one.

The spark is triggered on the secondary (plug) side of the coil, when the ground is interrupted on the primary (low voltage) side.

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2004 E500 4Matic 138,000 miles

I hate these threads with NO resolution!!

The resolution was that he probably got rid of the car.

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1990 190E 3.0L

Probly right, but this car is fabulous when running correctly, just can't figure if I need to do plugs boots coils wires....expensive to do all at one time

Mite just start with plugs