After reviewing the electrical schematics for the W116 300SD, it appears that in order for the transmission to start in 1st gear when D or S is selected, a signal must be applied to the AT solenoid valve.

The AT solenoid can be activated either by the kickdown switch or the Overload Protection Device. There is no other source for activation signal.

Since the transmission does not downshift under any circumstances when the kickdown switch is activated, the AT solenoid must not be working.

I will appreciate it if someone with knowledge can verify this diagnosis.

Also, where is the AT solenoid valve? How do I get at it?

Thanks

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JMF