Thanks for inputs. The EZLs are inherently old so that may be an issue - however, even the "new" one I've just installed was made in 2003.

Battery is a new full size MB, much larger than the original MB battery.

Sunday 14th June update:

Checked all around diagnostic socket wiring, measured relevant earths etc. All appears good.

My only worry is the cheap-feeling brand new Beru plug wires. Similar to those that were installed (brand new NGKs) during EZL failures 2 and 3. Again, I had no trouble with older Beru wires installed 10+ years ago - WHY DID I CHANGE THEM AND BRING ON THIS MISERY???

So, the only thing I can do is get hold of a genuine MB set of wires and hope for the best.

And possibly get a new alternator to assure the smoothest current delivery.

Yes, I could find a true expert who could run diagnostics - oscilloscope etc - on the system to look for any voltage spikes that may be the cause of these repeated failures.

New 1200€ EZL now installed and car started from cold with its usual perfection. It’s like new!!!

I’m not going to use the car for a while as I reflect and plan.

Huh.

R

No EZL failures in 22 years and then 3 fail within 1 year?

That sounds very much like something outside the EZL and not the ezl itself.

Could be an intermittent short or something.

Finding the real cause of your problem is going to be a challenge I feel.

M104 with ke-jetronic was sold here only in the 300CE coupe/cab from 90 to 92 and 300SL same years I think.

cha-ching