Constant Speed Solenoid Pressure Switches

[FergFuerte_12]

Hi everyone,

I recently got my hands on a Euro 1966 250SE coupe with a column-shift automatic. When I put the car in gear, the engine bogs down almost to the point of stalling. After a linkage adjustment, valve adjustment, and some other checks, I discovered that the constant speed solenoid (CSS) isn’t being engaged. I bench-tested it and confirmed it works, so I went to inspect the two hexagonal pressure switches on either side of the transmission (one for forward, one for reverse).

It turns out the transmission has been replaced with one from a 280SE, and these pressure switches are nowhere to be found.

I’ve included three photos:
- The data tag on my transmission (“GA 280 SE”)
- The side of the transmission where one of the pressure switches should be
- A reference photo from BaT of another 250SE with the switches installed

My question is: does the 280SE transmission regulate the engine differently, or are these pressure switches simply missing on my car? Any help would be greatly appreciated.

Cheers!

[SEB]

In the original K4C25 transmission (250SE), there were two pressure switches: one activated the constant speed solenoid (CSS) when engaging a forward gear, and the other for Reverse. The CSS received a 12 V signal from these switches to raise idle speed, compensating for engine load when the gear was engaged. In short, the transmission sent a signal, the CSS increased idle speed, and the engine did not stall.

With the 280SE transmission, especially in later models from the late 1960s, a newer automatic (K4C040 or K4C025) was used, in which external pressure switches were eliminated. The CSS signal was handled internally within the transmission or through another circuit in the engine wiring. In some 280SE versions, the CSS was controlled by a microswitch at the throttle linkage rather than through the transmission.

That is why in your case, the two hexagonal switches are missing; Mercedes moved their function to another part of the control system.

There are several options, depending on how much you want to preserve the original CSS functionality. The simplest solution is to connect the CSS through a manual microswitch at the gear lever in Drive or Reverse. The CSS then receives 12 V only when the transmission is in Drive or Reverse. This works like the original system and does not require modifying the transmission.

[wwheeler]

A couple of thoughts. My '68 280SE has the early trans with the pressure switches. I know 280SE V-8 cars had the newer trans and guessing the change to the newer trans occurred with the low grille cars ('69+).

Also, I am pretty sure the pressure switches send a ground signal to the CSS, not the positive signal. The positive signal is always there at the CSS. When the pressure switches are activated (closed), they ground to the trans case. 

When I bought my 280SE, the CSS was not functional. The PO simply raised the idle speed to prevent a stall when in gear. I have since changed that, but maybe an easy fix until you find a permanent fix.