Thanks for your input.

The first EZL failed with a new Beru coil.

The second with an older genuine MB on I had in stock.

The third failed with another, different, new Beru coil.

This afternoon I installed a new, genuine MB coil.

I cleaned and copper greased all the connections and checked earth/ground resistances - all close to 0.2 ohms as measured from the battery negative. This is a great improvement on initial measurements after the third failure.

Next week, the local garage will install a "new", remanufactured Bosch (genuine?) starter motor and alternator. He will be instructed to check, clean and reinstall all cables, including the engine earth strap.

Beyond that, I'm lost!!!!

Yes, it's an expensive business. The car is operational now with my new 1200€ EZL. I've ordered another used EZL for 220€ and I shall use that one until confidence returns. The new one will be with me at all times, together with the tools to replace.

And I may get yet another used one for safety.

This may make me sound very rich - not true. I spent some cash getting my old 300CE-24 restored as my "last car" - I'm 68. So, I cannot go back; I MUST resolve this.

Life's fun isn't it?

Ray

this

Yes, always install heatsink paste.

R

Overvoltage can damage electronics. Rig up a high quality voltmeter so you can read it from the driver's seat while driving. Voltage should not exceed 14.5, even momentarily.

Duke

Thanks for that. I've ordered an automotive oscilloscope that might come into play next week.

R

13 July 2020

Hello again All,

Well, I new, secondhand EZL has been installed. I am reluctant to install the "new" 1200 € one from MB until I am certain that the issues are solved. Time will tell.

Since the 3rd EZL failure, the following has been done.

1. New plugs, MB.
2. New leads, Beru.
3. New coil, MB.
4. Complete cleanup of all areas including distributor cap and rotor - only 9 months old.
5. Complete cleanup and reinstall of all earth straps, including the main engine strap underneath.
6. New (Bosch reconditioned) starter motor.
7. New (Bosch reconditioned) alternator.

Of course, I have no idea yet if all this has solved the problem. I shall not have confidence until a year (?) has passed. Even then 100% confidence is unlikely.

Just as before, the car now starts, drives and stops like a new car. Since its overhaul at PCS in 2017 it has never been better. Apart from this costly EZL issue.

Do I notice any differences after this latest set of "improvements"?

Yes, it now starts, even when hot, like a new car. Before, when hot, it would sometimes fire and then not keep running unless I kept the starter motor turning a little longer. That was hardly a problem as it was just a matter of remembering to hold the key in the start position for an extra half second.

However, it now seems to spring immediately into life and settle into an even idle under all circumstances. I shall be doing a more typical run today and shall be able to confirm this with greater confidence.

More as it happens.

And, as always, comments welcomed.

All the best.

RayH

Non-resistor plugs are no longer in production I've heard.

Take a Multi meter and measure the resistance between the top terminal and the center electrode.

If it has ressistance then your best bet is to find new old stock Bosch F8DC plugs on ebay.

My 300SE needed plugs and I found NOS plugs on ebay for cheap.

The plugs I installed are genuine MB, made in France. I think they are Beru.

NGK supplies a suitable non-resistor, I've got the reference somewhere for those interested. Oddly, these EZL failures all happened on the NGK plugs. I can't believe there's a correlation.

And then there's Champion C11YCC,, as stated by MB.

I've used MB, Beru, Bosch F8DC4 and NGK and never noticed any difference.

My last diagnosis will be measurements of the cable between the EZL and the coil. I've already verified this briefly but I'll do it now more aggressively by measuring the resistance whilst moving the cable to see if there's any intermittence.

And so it goes on.

R

R

yes the model of the plug can tell you/us if it's a resistor one or not.

https://www.ngk.com/learning-center/article/174/ngk-numbering-systems

Ray, I always run Bosch F8DC4 plugs in our 124 vehicles per the advice of an experienced MB-trained tech at our local MB dealer 20 years ago. It's critical to keep an eye on the condition of the distributor cap & rotor. Check for evidence of moisture inside the cap. Replace as-required. As mentioned the thermal paste under the EZL is important too.

__________________
Fred Hoelzle

Thanks and, yes, all that noted.

And to christuna, NGK today only lists 5K ohm resistor plugs. That's a change.

Ah, but I did find the reference for the NGKs I had installed until recently: BP5EFS. Apparently available here:
https://www.jdm-sparkplugs.com/shop/standard/bp5efs/

R

The best way to make sure you have non-resistor plugs is to just remove one of them and measure.

These are what are now installed: A0031596803.

R

I'd follow what Christuna has noted.

Our local dealership supplies MB plugs alleged suitable but they are no longer resistor types

Next. Beru Leads a'int what they used to be.

Check the connections at the fuel pump for corrosion. In fact check every electrical connection you can find. Even go to the trouble of changing all your fuses. Any aluminum ones change them out.

Good luck with that car. It's a shame a rare EZL can render it down.

Ive heard the 24's were ezl killers but I haven't had that proof myself. My Siemens one failed abruptly last year and I was able to track down a bosch unit from a sl for $300. Replaced it and its been good. Interestingly enough, my alternator was intermittently dying for a week before my EZL failed, I suspect low voltage precipitated the failure.



The idea that lower voltage is causing higher current across the ignition transistor doesn't hold water to me, current decreases as voltage goes down. Less "pressure" to push the electrons across the coil, less overall power being dissipated. I'm thinking it might have been electrical noise from my failing alternator beating on the electronics in the unit but I don't know. My replacement unit has not failed yet after a year of DD.





I have one more EZL that works and still have my old siemens one for dissection/repair. I was thinking of giving the EZL a more stable life if another one fails. Move the harnesses and mount the EZL under the dash with a fan on the back of it. This will keep it from experiencing high temperature deltas literally every time you drive, especially in winter. It will also isolate the EZL away from the strut tower (i read one guy with an sl500 had his die after a pothole). I would also add a 500uF cap across power and ground at the unit to absorb noise. To top it off I thought of running the EZL's output to an aftermarket MSD ignition to take the work load off the EZL. At that point it may not even generate heat if its only being used to generate a signal. Im also wondering if my failed unit still works, ie still getting a signal to the transistor but the transistor is blown. When I have a place and garage soon I plan to open up my siemens one and poke around with a scope. Hope this posts gives you some ideas..At min replace your coil, they rarely go open circuit but their always weak and if it bleeds to primary your gonna kill an ezl. Also scope the voltage supply.

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1981 300SD 512k OM603