Chris,

"If you turn the potentiometer, the idle will change dramatically."

My understanding is that this pot does not affect idle speed nor baseline mixture. Are you saying that if you set the pot to a different position, the speed changes permanently, or just briefly?

"1-3 is fixed at 4kOhm
1-2 varies first on the position of the MAS plate and second on the wear"

The trimpot is actually in series with pin 1 and the board, i.e. pin 1 does not connect to the board as do 2 and 3. Therefore, 1-3 is actually not fixed - it varies as the trimpot is turned. Since the trimpot is at the end of the board closest to 'idle', it will affect the idle voltage more than a condition of high demand.

However, I measured the trimpot values directly, and in the case of my old board, it was adjusted to 108 Ohms, and in the new one to 57 Ohms. Since this affects the voltage bias at the 'bottom' end of the potentiometer, I adjusted the new the same as old. Note that if measuring 1-3 on the connector, this is a difference between 4057 and 4108 Ohms - not a lot.

I guess we still don't know the function of the trim pot, though it might be an alternative way to adjust idle voltage. In general, circuits provide two adjustments to track a range, and in this case that is provided via the rotation and the trimpot. It's only useful if there is a procedure for adjusting at two points, though, and it sounds as though Mercedes only provides one.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7