Constant Speed Solenoid - Wiring

[Lew]

I realize this topic has been discussed many times.  Having read as much as I can find, I’m still confused.  Surprise, surprise!  I have a 1971 280sl with a/c and auto trans.  The CSS is installed but not wired.  If I understand correctly, the CSS adjusts the idle for both automatic trans, and a/c. Would the CSS work for both tasks at the same time? Or does it serve one or the other….  Where does the positive lead wire into the wire harness?  At the 6-way switch, or to some connection that is “live” only when a/c is turned on? Thanks for any replies.

[lpeterssen]

Dear friend

Attached is the wiring diagram in German for the CSS.  It works in conjunction with the automatic transmission.  There are two conditions that have to be met at the same time in my opinion to activate the CSS

1. IDDLE SWITCH ACTIVE. That means when the IDDLE switch pases signal through it, that information is brought to the automatic transmission
2. PRESSURE SWITCH. If the car is cruising for example downhill coasting without pressing the accelerator pedal, THE IDDLE SWITCH will be active as intake manifold butterfly is clossed, but as car RPM are above IDDLE set point (800 RPM aprox) the pressure of the transmission fluid is above a certain point, therefore there is no need for CSS activation.  Therefore, only when IDDLE SWITCH is active and the pressure inside the transmission is low,  the transmission sends a signal to activate the CSS.

I understand that positive lead to CSS comes directly from fuse box, but the negative lead needed to activate the magnet of it comes from the transmission when above mentioned conditions are met.

You can use the CSS selenoid for both purposes by mean of using some relays and diodes .  You can have CSS activation with ACC Compressor catching in and when the transmission fluid reads low pressure.

Write me if you want a harness for the ACC control.

Best regards
Eng.Leonardo Peterssen

[lpeterssen]

Looking carefully at the automatic transmission diagram it appears that the CSS is controlled ONLY by two pressure switches at the transmission.

The components labeled 57V y 57R.  ONe is “Vorwards” and the other “Rueckwards”.

So the information coming from the IDDLE switch may help the automatic transmission decide whether it has to make a shift to a larger gear or something like that.

Best regards
Lp