I'm guessing this pertains to the 4 speed non electronic controlled trans correct? Was there a 5 speed non electronic? ( My 97 sl320 has a 5 speed electronic shift with a Winter 2nd gear start / Sport 1 st gear start switch )

Some things to consider:

Build a bit of hystereses into the controller otherwise it might toggle if you drive at switching speed.

The KD solenoid might not be rated for continuos duty, adding current limiting ( pulling the voltage back, peak and hold ) or running PWM would help the situation. Finding a donor solenoid and running it at 14.7 V ish at over 200*F would be a good test.

The factory 1st gear switch sounds interesting, a look at the wiring diagram would be a good idea.

Moving the shifter to 2nd / 1st to engage 1st start sounds interesting and , if the trans is hydraulic controlled, you might only need to pulse the KD sol.

Anyone have a hydraulic diagram / apply chart of the trans in question?

Also a pressure chart is needed, the non factory 1st switch units might not have enough line pressure at light throttle to prevent slippage.

MSD makes a speed ( frequency ) operated switch as does Murphy. I've used the Murphy unit on industrial diesel engines that needed overspeed shutdown. ( diesels that work in a refinery / gas / oil well area need a overspeed air intake shutdown down because if there is a flammable gas leak they will run away. )

Yes the original target was Hit Man X's 300SEL which is the 4 speed non-electronic controlled transmission. This should apply to any 722.3/4 series transmission. The 722.5 were electronically controlled I believe so this would not apply to them.

Good idea on the hysteresis, would not want the unit to be constantly trying to engage the kickdown solenoid.

I'll have to see if I can find out more information about the solenoid. Adding current limiting circuitry is a good idea. Arc suppression as well as inductive kickback prevention as well.

Can you elaborate more on the usage of PWM in this case?

The factory 1st gear switch sounds interesting, a look at the wiring diagram would be a good idea.

From my brief testing in the 420SEL the KD solenoid should be engaged the duration one wishes first gear to be engaged. When I disengaged my switch the transmission shifted immediately into the next gear.

Hit Man X recommended about 7 mph for the switching speed. I think this is pretty close to what the normal first gear start cars shift into second.

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