Idle RPM, Automatic Transmission

[alchemist]

The idle of my 280SL with automatic transmission in park is 950 RPM and 650 RPM when it is in gear. The difference is 300 RPM which indicate something is wrong, perhaps a vacuum leak. Can anyone elaborates on this difference in RPM and suggest a possible fix. Thanks.

[ctaylor738]

280's don't have the constant speed solenoid activated by the pressure switches on the transmission like the 230 and 250.  Your idle behavior sounds typical to me.

Cheers,

[Larry & Norma]

My 280 has the CSS as does all 280 auto's I have seen. Idle should be around 800rpm I think and will drop a little
even with the CSS when in gear. I would suggest if the CSS is working then a good tune up following the tour on this
site should help.

[alchemist]

Thank you Chuck, Larry & Norma for your feedback. Chuck is right because he knows I am talking about my 1974 280SL with electronic injection (sorry Larry is not a Pagoda). Those systems do not have CSS and the idle usually drops about 200 RPM when the gear shifts from park to drive. Currently, modern Mercedes with naturally aspirated injection, the idle drops about 200 RPM. However, in my case, I have about 300 RPM drop and I wonder if vacuum leak may contribute to this drop.

[Larry & Norma]

Ok guess I assumed it was a Pagoda

[ctaylor738]

Actually, I was talking about the 113 280.  All that I have seen have the CSS connected to the AC compressor if they have AC, but not to the automatic transmission.

[stickandrudderman]

All 113 autos that I have ever seen have CSS.
107s do not.

[ctaylor738]

Agreed, they all have the CSS, but is it connected to the pressure switches?

[stickandrudderman]

Yes, they are all grounded by pressure switches on the transmission.

[ja17]

Later USA version 280SLs without AC have a CSS which is not hooked up. A vacuum dash pot on the intake linkage provides the boost in idle rpms when the transmission is put in gear. The CSS solenoids are then only connected if the car is equipped with AC. You can read more details on these systems in "The Linkage Tour".

[Rolf-Dieter ✝︎]

Hi Joe,

How about when the A/C was removed as on my car (you saw my engine last September in Williamsburg) should there have been a change made as far as the CBS is concerned?

Hope all is well with you and that business is booming