When I switched on the AC the RPM dropped to a shaky low of 500.
Not wanting to play with the idle and rack damper any more and wreck my perfect 600rpm idle I added a solenoid to the throttle linkage.
The solenoid is from a Carter carburetor for this purpose.
The solenoid coil is wired up to the AC clutch and when actuated gives the throttle a hit to add a little more RPM. After some adjustment the RPM never changes when the AC comes on.
The solenoid is mounted so it can be swung to one side for the emergency shut off.

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1984 300SD turbo 126
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God made me an atheist and who am I to question His wisdom