Constant Speed Solenoid (CSS) Not working

[campin8tor]

Good evening! I have a 1970 280SL Automatic Transmission with Air Conditioning that stalls randomly while in gear with brake applied. Based on things we have tried, fuel/air adjustments, verified valve within vacuum line from Brake Master Cylinder to intake manifold functionality, fuses, etc, we believe we have narrowed it down to possibly the CSS.  We also tried to ground the CSS by placing a ground directly on one of the wires that goes into the the CSS.

I have researched the forum and there is a lot of information on CSS issues but I wasnt able to come to a conclusion on where to start. I am seeking anyone's advice on how to troubleshoot this issue end to end.

I appreciate anyone's input. Thank you vehttps://www.sl113.org/forums/index.php?action=post;board=16.0#ry much.

[Kevkeller]

If it’s not in gear and you apply the brake does it stall?

[campin8tor]

Hi KevKeller - thanks for the reply. No it does not stall when it is not in gear with the brake applied.

[Cees Klumper]

When you put the car in gear, can you see the CSS engaging, ie does the little pushrod extend? If so, it could just need to be re-adjusted (extended further). It may also be at the end of its life, where sometimes it manages, and sometimes it doesn't. Making the linkages move more smoothly could help, but perhaps you need a new CSS. Another possible culprit could be the pressure sensor on the transmission, or maybe a loose or broken wire.

[Pawel66]

Is your Constant Peed Solenoid connected to anything? I see you have 1970 280SL - is it the US car? I hear you hava the AC as well.

On the late US cars CSS was not used for idle increase when selecting gear. It was used more often for increasing idle when AC is turned on or it was not used at all, left not connected to anything. On your pictures I do not see it connected.

If this is the case, idle increase on gear selection is effected by Emission Control System. This is where you would need to look for culprits.

[Shvegel]

On the inboard side of the throttle you should have a vacuum device that operates the throttle from below.  This would be connected via vacuum line to the inboard vacuum fitting on the throttle body.

[Peter]

I suggest to check fuse 5 and the adjustment and working of the 6 way switch.

Peter

[campin8tor]

Thank you for your response Pawel66 - I will be the first one to tell you that I am not very knowledgeable about the vehicle, but what you are saying makes sense. I dont see that it has any wires coming from elsewhere as is shown in the picture to provide power or ground other than the two wires you see in the attached pictures in the original post.

As for the Emission Control System, is there anywhere you would look first or in what order? Again Im learning as I go. Your help would be greatly appreciated.

[campin8tor]

Thank you all for your responses!! Lots to learn for me on this vehicle but with all of your help we will get her running smoothly when in gear again!

[Pawel66]

On the positive side: you found out you might have a spare worth some $$$.

Now you need to follow advise from Members fluent in Emission Control System fixing.

Best: become a Full Member.

[Shvegel]

If that is a picture of your constant speed solenoid it isn’t connected.  This is common on later 280 automatics where idle was controlled using the spring loaded vacuum diaphragm. It should be below the inboard side of the throttle body.  If it is missing you will have 2 clues. The first is you will have a plugged vacuum port on your throttle and the second is the linkage rod that connects to the throttle body will have a sliding section on it.  If you open the throttle from the pedal you will see the linkage expand a bit before the throttle begins to open.  This allows the vacuum diaphragm to oen the throttle without moving the linkage. If you don’t have it you should source one.

 The other think that very well may be in play is a dirty or miss adjusted throttle switch.  The black switch on the outboard side of the throttle controls the solenoid (3 position) that adjusts the pressure in the transmission. If it is dirty or miss adjusted it can cause the transmission control pressure to not drop at idle which increases the drag on the engine. Your problem sounds intermittent so the throttle switch would be my first suspect.

You will be time and money ahead to become a full member as the few dollars spent lets you behind the curtain and most of what you seek can be found here:  https://www.sl113.org/forums/index.php?topic=5003.0

Joe Alexander was kind enough to share his years of experience with all the various linkages and adjustments and it is all in one place and some that pertain to your car (mine is a 1970 as well) are not available anywhere else.


 

[FresnoBob]

I had similar issues and one wire was frayed and grounding to the transmission case. 

[afibbe]

If you discover you don’t need your CSS, I would be interested in buying it.  My car didn’t come with one