I had to replace the MAS not long ago. Someone correct me if I'm wrong, but I do not believe these are owner-rebuildable, nor can they be opened.

Dealer price was just over $400.00

Remans are less but who would trust an electrical reman?

The knob on top simply turns - then you lift the MAS off by pulling up.

To test fuel pump relay, jumper pins 1 & 2. If you hear the pump turn on, you're looking at replacing the MAS unless your able to open it up and determine what's wrong inside.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"