I've been trying to figure out exactly how these cars are supposed to work before I start guessing.

After studying the electrical schematics for my '79 300SD W116 I'm a bit confused. There is a "Transistorized" box on the schematic that's called "Overload Protection Device Switching Unit"

I believe this is the little black box located in the front passenger kick well on the right. This device looks like it works in conjunction with several switches and senders:

Intake Manifold Pressure Switch(Over boost sender on top of Intake Manifold)
Transmission Pressure Switch (???)
Switch Over Valve (AKA Changeover valve on firewall connected to Over Boost sender and ALDA)
Kick Down Switch (Behind Accelerator pedal)
Stop Lamp Switch (Brake light switch)
Automatic Transmission Solenoid valve (12V solenoid located on passenger side of transmission)
TDC Sensor (Not sure where this is located)

Here is a link to some good information on how this works:

Operation engine-transmission-overload protection

I heard there is a pendulum switch inside the overload protection device switching unit (not sure if this is true). This pendulum is supposed to sense when the vehicle has come to a complete stop. When this happens it will energize the 12V solenoid on the transmission to kick it down to first gear. If this box is not working properly your car will never start off in 1st gear, only second gear.

My question is: Where is the Transmission Pressure switch located?

I have quite a few projects going on with my car(s) and house. I haven't been able to crawl under it for a while to locate the transmission pressure switch. I was wondering if anyone knows where it might be.


Phil