I understand your point, here is a little more history.

I had replaced the sensors about 3 years, ago. Then when the light came on again, about a year later, my first guess was the sensors again, so I replace the sensors again. The light went off with the new sensors but then it came on. Then it stayed off for a few months, then came back for a few months, then on again and now has remained on for about 10 months. So I took the car to a mercedes mechanic who was an ex-shop foreman, 22 years experience, who now just fixes cars out of his house. He had a box he plugged into the cable between the computer, or maybe it was directly into the box, and then followed the book on the test procedures. He showed me the book, and the failure point, spinning the wheels etc. and the conclusion in the manual. I was sceptical too, but I am tending to believe him since he performed the tests twice, confirming his orginal conclusion. He said it is rare for the unit to fail, but they do fail. He had never changed one before but has heard of others failing.

We swapped the both relays within the box with spares he had, but that did not fix things.

So, any advice on changing the unit??

__________________
1989 420 SEL