Jorg,

You are correct. The 90 300E should have a MAS which incorporates the fuel pump relay, the A/C relay, control of heater current to the oxygen sensor, and talks back and forth with the CIS unit. The 91 model included everything in the CIS unit. As Brewtoo pointed there are slop over year model changes. I believe the 90 model of the 124 chassis had the first convenience system, which I think is really neat. Hold the key in the locking position, and it closes all windows and the sunroof if left open. They never included this system in the 126 chassis. Right now I have 2 300SELs in my stable. One is a 88 and the other is a 90. The 88 has fuel pump relay, and the 90 has a MAS unit. The 90 is hard to work on, because you need a 35 pin breakout box to pull codes stored in the MAS unit. The codes do not go away with the battery disconnected though the CIS codes will. You can retrieve codes on an 88 or 89 at the round plug on the left fender.

Hope the info helps. I'm happy to answer any questions you have.

Peter

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