Hello Guys,
I have a really nice S320 that WAS a pleasure to drive. It started running rough and sure enough the upper engine harness was falling apart (typical). I bought a new one and put it in but the coil on cylinders 3 and 4 is still not firing. I have a spark analyzer (By OK Spark) and when I probe the other cylinders they work fine, 3-4 is dead. switched coils with another that I just observed working, stays dead. Tells me I'm not getting a signal from the EZL or Ignition Module. So I start ohming out wires until the harness disappears into the body work and figure it *might* be the EZL, maybe my missing insulation sorted out the output to the coil. I looked on the WIS for the location of the EZL and I swear it is NOT there. Picture says drivers side fender well, all I got is ABS and fuse box. Can anybody help me out here? A picture, crayon art, cave drawing, ANYTHING? Would love to ring that wire out all the way back to the EZL and have a smoking gun before I swap parts. Not that I could, as I CAN'T find the stupid thing...

Additionally, I just pulled the coil input connector off of 3-4 and had no change in running, further proof that that coil wasn't doing anything. I put a an oscilloscope on one that was working and compared it with 3-4 and didn't see a signal. Really need to find the ignition module...

Are you sure the car is a 90?

There are 2 systems I know about.

If the car has a EZL it should have a single coil and distributor. My 91 300SL M104 3.0-24 Valve has this. The fuel control is Bosch K jet / CIS

If the car has multiple coils, they are triggered by the engine computer. My 97 SL320 M104 ME 2.1 engine control.

I had though the 93 - 96 HFM -SFI had multi coils and used engine computer to trigger the ignition.

what year? I have a 99
I found out to replace coils together all 3.I bought three at 52. dollar each from Ebay new Bosch. Try sreaching for the part,and you'll know what it looks like

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Sorry, typo on my part. It is a 1995 S320. Do you know which module in the box is the ignition? I'll post some pictures here in a minuite.

I did exactly that. WIS says my VIN has EZL and says its near the abs solenoid. I clearly do not have one. I think it must be controlled by one of the modules in the computer box.

Drivers side fender well:


Computer Box:


Close up:

I defiantly DO NOT have an EZL. I traced the coil signals back to this module and I can say with near 100% certainty that it is the ignition module.
Here is the unit that is controlling the coils:


One thing that really got me off on a wild goose chase was that the coil connectors don't use the center pin as signal and the outer sheath and ground. EXACTLY the opposite.

That metal band around the center pin is the coil signal:


These three pins are where the coil leads end up (Black Sharpie on tips):


What a good coil input signal looks like:


Now I just have to find a replacement module from someone that won't rape me. Anybody got one in their junk box from a boneyard trip?

Update: A. Dalton straightened me out on the circuit for the coils. The center pin of the coaxial connector is +12VDC and all three are wired to the same point in the harness (that's why my clumsy ass thought it was a ground while snooping around with my Ohm meter and no schematic). The outer connection is what goes back to the ECM which switches the ground path on and off to fire the coil.

The black box is the engine computer / ECU / HFM_SFI box. Before you get another box, be sure the wires from the coil to ECU plug are not shorted to ground / shorted to power. Wiring harnesses circa 92 to 95 had poor insulation that would crumble. You will need a wiring diagram to find out what pins on the ECU match to the coils.

I'm pretty sure the calibration chip is removable so swap it to your new unit if you make a change.

If the wiring is good, open the ECU and have a look. Follow the coil pins into the box and to a power transistor. It is likely this transistor is bad and changing it might be possible.

Have a look at car-part.com , it is a online salvage yard inventory system.


From left to right in your pics.

GM General Module ( this is a relay box for misc functions . )

016 545 62 32 HFM-SFI ECU

CARB Diagnostic module

HGS ( this might be transmission cold up shift delay, first gear start, kickdown )

ASR Acceleration Skid Reduction ( also controls ABS function )

E-GAS Operates the electronic throttle assist

Thanks 97 SL320! As I stated earlier, I have replaced the upper harness and did in fact ohm the wires out from the coil connectors to the base of the transistors in the ECU, that all checked out. Arthur Dalton has been helping me along behind the scenes but I want to capture as much here as possible so it helps the next guy out. Tonight I hope to verify the Crankshaft Position Sensor operation. Arthur informs me that it is possible to have a magnet fail and loose one of the pulses potentially causing the 3-4 (Coil #2) to not me triggered. I have a used "working" module en route with the capability of returning it so I should have a solution soon. More to come! I might dig out the good O'Scope and capture some signals to post here as I go.

This has proven invaluable in understanding DIS Wasted Spark Ignition systems:
Diagnosing Waste Spark DIS Ignition Systems Autolite Spark Plugs - YouTube

As promised, here is the CPS signal that Artur thought might have been errant. It looks pretty solid and stable. I was able to access it by drilling a very small hole in the back of one of the connectors and using a piece of small gauge wire (wire wrap wire) through the hole so it would make contact with the center electrode. This is a coax connector and it can be accessed right where the upper engine harness terminates in the fancy connector. If you've replaced this harness you'll know exactly where I'm talking about. I'm feeling a lot better about ordering a replacement ECM now, even though it is returnable, my risk is much lower now. Next time I go junk yard crawling I'm getting matching connectors for this and the cam position timing cables so I can make a proper breakout lead. Here are the pics of the tap and the signal:

Well, I am embarrassed to report that the replacement ECU arrived and did not fix the problem. Hooked a test lead to the coil primary and the ecu was signaling it all along. Just ordered new coils, wires, and plugs which all ohm out ok. Oh well, live and learn...more to come.

try the benzworld.org forum the w140 section for help.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I've been told that twice now and I have had the same post over there. After a while I stopped updating the benzworld post because there was no traffic or support for it there. I've learned an awful lot the last week or so, so if that's all I take away from this, I'm ok with it. I've made a nice test light for checking ECU function, I've captured some CPS signals for reference use, I've learned that despite the fact that my Fluke DMM says that my coils and wires are "OK" does not mean too much "at the engine". The one thing I can't really check with my test gear is leakage and performance under load which is likely where these parts are failing. New ones are on the way, the ECU is headed back for a refund and the weather has been pretty good so I've *had* to use the motorcycle to get back and forth to work. In the end, its made me a better, smarter problem solver as we learn far more from our mistakes than we do from our triumphs. Peace!

wruehl we used to have a guy called MBDoc,he was the greatest mechanic with over 40 years with MB.I sure wish he was back sometimes.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Did you "loose" him in the "no longer above grade" sense? Or the "I'm taking my ball and going home" sense? It's a shame either way, man like that would never thirst for beer with me around...