AC Idle speed solenoid

[MikeL43]

I have a very late 67 250 SL that I bought from the estate of the original owner about 2 years ago. The car ws/is in great condition, no rust, runs good but needed the usual wear items replaced which, thanks to this forum, I have successfully made right.  I'm now down to the fiddly stuff which in this case is engine RPMs at idle with the AC on.  The car was purchased in late 67 and is titled as a 68.  It has a standard transmission with an AC unit (Kuhlmeister) installed by the dealer at the time of purchase so the factory made no provisions for an AC idle speed advance.  The problem I'm having relates to keeping the RPMs up at idle when the AC kicks on.  My old 66 250SE coupe (automatic) had an electromagnetic solenoid that advanced the idle when the compressor kicked on but in its place my SL has a hydraulic solenoid that supposedly advances the idle when the power steering saps idle RPMs,  As far as I can tell this hydraulic advance hasn't been necessary and I'm wondering if it is necessary or how I can work around or remove the hydraulic advance and replace it with an electromagnetic solenoid powered off of the AC clutch switch.  Has anyone dealt with  this problem before?  Any suggestions? 

[awolff280sl]

I have never seen a hydraulic solenoid for the AC idle kick-up.
Please post a picture if you can.
Thanks.

[Tyler S]

Andy beat me to it. Pic please
If its just a dashpot you are referring to the yes it can be removed and an idle solenoid put in its place to bump up the idle when the compressor kicks on.

[MikeL43]

I hope I have succeeded in attaching a photo of the hydraulic solenoid that I was referring to in my original message.  Just in case I will describe it as similar to a wheel cylinder in size with an adjustable screw in the front and a hydraulic line in the rear which I think goes to a power steering hose or line.  It is in the same location as the electromagnetic one found on many cars with automatic transmissions.  As I said previously I think the purpose of the hydraulic solenoid was to increase the motor RPMs at idle when the Power Steering was engaged.  I got that idea from some of the posts on the forum so I'm not certain that is correct, especially as I have never seen or felt it in operation.  The dashpot thingy is just below the hydraulic solenoid on the same bracket.  All of these are set up so that if they work they should increase the Idle RPMs.  I have read some recent posts about the purpose of the dashpot so hadn't considered replacing it with an electromagnetic solenoid though that sounds like a good idea.     

[Tyler S]

You may be able to leave everything as is if you can find this different bracket for the cross over rod. Found this pic on a thread about ac that was added to a manual transmission. Looks like this different bracket has a provision for mounting a CSS in a different location.
http://www.sl113.org/forums/uploaded/ramcdonald/200782173524_CSSsolenoidsmall.jpg

If the hydraulic system isnt working maybe remove it and install CSS in its place? Where does the other end of the metal pipe go on the hydraulic setup? Could it be isolated or capped without major surgery.

[MikeL43]

It seems like the simplest thing to do is  to replace the dashpot with an electromagnetic solenoid.  I am considering this since I don't think the dashpot is working of if so I haven't noticed that it significantly improved the idle.  The next question is will the new solenoid fit where the dashpot is located and most importantly where can I find an electromagnetic solenoid.  I'm reluctant to remove the hydraulic solenoid as I want to keep the appearance as near original as possible.  In regard to the question of where does the pipe that goes to the hydraulic solenoid originate I have traced it to the steering box just outside the firewall on the drivers side of the car.  Thanks for the ideas guys.   

[Tyler S]

The dashpot is only there to keep the throttle from slamming shut when you let off the gas. To prevent a stall. Basically just a damper. Used mostly just on manual trans cars because the idle setpoint is lower.
A MB CSS wont fit in its place. A css would normally go where your hydraulic unit currently resides or off the cross shaft as previously posted above. You might be able to get creative with a generic setup. See link below.
http://m.ebay.com/itm/Holley-46-74-Throttle-Solenoid-Cadmium-Plated-Universal-Each-/331625682499?nav=SEARCH

[ctaylor738]

It's actually a slave cylinder that is supposed to keep the revs up when using the power steering at idle, i.e. maneuvering into a parking place.  I believe this was only used on late 250s. I think you can add the constant speed solenoid (CSS) as well.  I worked on a 250 a while ago that had both.

[Tyler S]

Chuck, Was the CSS mounted from the right cross shaft bracket like my pic link above? If you remember.

[ctaylor738]

Yes, on the right side.  This was (still is) an automatic with AC.  I will try to get the owner to take a picture of the top of the engine showing how the two fit together.

The other car is a 4-speed, no air.  Attaching a picture of the little slave cylinder.

[awolff280sl]

So, it seems that the "PS hydraulic idle solenoid" was used on both automatics and manuals.
I do notice that turning my wheel can lower the revs a bit at times, but it's really nothing that I can't easily compensate for with the throttle in a manual 280SL if I have to.
Was this engineering overkill, or does the 250SL need it for some reason? I guess that it was used instead of an electromagnetic solenoid in 250SLs with AC, whether auto or manual.
Leaving the hydraulic solenoid in place and adding an electromagnetic solenoid onto the cross shaft bracket would be tempting in a car with AC.

[Tyler S]

My 250sl 4 speed with power steering but no AC does not have this. Since AC was installed at the dealer I dont think they would have come with an additional CSS from the factory unless perhaps it was automatic. They may have been installed at the dealer along with the ac system on manual trans. If there is an old service bulletin floating around about AC installation procedure would help. Maybe its possible that turning the steering wheel with the AC on is what caused it to stall so the hydraulic system was added at the dealer?

[kampala]

My late 250 manual gearbox (serial #4442) has this hydraulic piece that does raise the idle with the power steering.  No hint that the car ever had AC.

[MikeL43]

I'm convinced that the hydraulic idle speed solenoid was an original factory installation on my car along with the dashpot under it.  My setup is identical to the photo provided by Chuck Taylor.  If I can't find a used original CSS that I can use with the AC I'll try the one suggested by Tyler S.  As far as I know AC was not available as a factory option on my late 250 SL as mine was installed by the dealer at the time it was originally purchased.  I suspect that the dealer either cut a few corners by not adding the CSS or that it was not part of the Kuhlmeister AC setup provided by MB.   

[MikeL43]

Attached (hopefully) is a photo of the modification I made for the installation of a constant speed solenoid (CSS) to increase the idle speed of my 250 SL when the AC is engaged.  Thanks to forum member Meyer Nahon for the CSS.  The modification is essentially a 90 degree bracket (the shinier stainless steel part on the bottom) to which I mounted the straight part of the bracket that came with the CSS.  It should be noted that the mounting bracket for my 250 SL is located in a different part of the linkage than on the later cars (not at the end of the linkage, more toward the center).  Some bending of the bracket  was required to get the CSS into the position to increase the idle.  The CSS was energized by a wire attached to the hot wire going to the AC clutch.  A wire from the CSS to the intake manifold completed the ground circuit.  As previously discussed by me on this subject my standard transmission 250 SL did not come with this part from the factory and the CSS was apparently not part of the dealer installed AC option. So far it works fine.

[TheEngineer]

My car is an early 1969 with automatic transmission. The solenoid in question has a 5 mm travel and is energized when any forward gear or reverse is selected. You can verify this by connecting a voltmeter or a test bulb. It has nothing to do with the A/C. I have the original A/C installed because the car was delivered in San Jose, CA. It uses the York compressor which does not absorb a lot of torque at idle speed unlike the variable speed compressors in newer cars. Therefore, no increase of idle speed device was installed.

[AGT]

I have a '66 manual with power steering USA delivery.

This summer I noticed that it has a tendency to stall when doing a three point turn in our courtyard. The courtyard has a gentle slope and for the reverse part of the manoeuvre I leave it in first gear, dip the clutch and let the car roll back (lazy or what?). That's when it stalls. I was thinking this was just the idle set too low, unaware that there is a device to raise the revs when the PAS is activated. Can anyone say whether my '66 car would have this CSS?

I had a similar issue with my 420SL automatic which stalled when manoeuvring, shortly before needing billions spent on new valves @ 65,000 miles!

Regards
Andrew

[MikeL43]

In response to the engineers comments that the CSS "has nothing to do with the AC" I suggest he/her read all of the previous discussion concerning this post.  My old 66 250 SE coupe had a CSS that advanced the idle when the automatic transmission was engaged and/or when the AC clutch was engaged.  In addition the York piston compressor that was on my current late 67 250 SL (manual transmission) used considerably more power than the replacement Sanden rotary compressor I recently installed.  I thought the consensus was that a rotary compressor was a lot more energy efficient than the old piston compressors.  I suspect that if you put your car with the automatic transmission in park and engage the AC you will see that the CSS does increase the idle, or at least should if it  is working properly.