280SL Engine Stalling when selecting Drive

[Nicolas Aristodemou]

My 1970 280SL auto is stalling when the auto box is automatically downshifting from 3rd to 2nd gear when it has reached normal operating temperature. It also does the same when shifter selects from P or N into positions 4 or R.

Egine idles smoothly are about 800rpm in N and also car is accelerating and running very well. I should also mention that when the auto box is downshifting into 2nd gear just before car comes to a stop the downshift is accompanied by a judder followed by an engine stall ( switching off)

Any ideas please as to what could be causing this?

When the car is cold everything seems to work ok though........

[jeffc280sl]

interesting.  on another thread we are speaking about failures in the emissions system causing the fuel cutoff solenoid to engage.  attached is a pic of the system.  the circuit has temperature, transmission and fuel components.  try disconnecting the fuel shutoff solenoid.  its easy to try and return if its not the issue

[ctaylor738]

Check if your throttle switch is working.  The symptoms you describe can occur when the switch isn't closed when the throttle is in the idle position. 

A quick check.  Turn the ignition on with the throttle at rest.  You should hear a click as the transmission solenoid moves to "idle" (reduced modulator pressure) position.  Then depress the throttle slowly.  You should hear a click as the solenoid moves to "driving" position.  Then return the throttle to at rest, and there should be a click as the solenoid moves back to "idle."

Good luck,

CT

[teahead]

What's your idle speed when in 4 or R?

[Nicolas Aristodemou]

Thanks to everybody I will go through the checks suggested tomorrow and post the results.

I personally suspect that the vacuum operated device under the throttle body could be the cause. I have read several times how to check it and adjust it but I am not successful in doing that......

[Nicolas Aristodemou]

Idle speed is around 750-800.... when engaging 4 or R it drops to 500 and immediately afterwards it dies......

[Nicolas Aristodemou]

Just run all the suggested checks.....  it is not the fuel cutoff solenoid, I disconnected the lead to it..... no difference...... the throttle switch seems ok..... I can hear a solenoid clicking on and off when I open and close throttle with ignition turned on and engine off.....

[Pawel66]

I understand ypu have the emissions control system and the good old CSS is not responsible for keeping the idle while you select gear, correct?

[Cees Klumper]

I was also thinking Constant Speed Solenoid, if the car has one it may no longer be working properly.

[Pawel66]

naristodemou, do you have CSS? If so, is it hooked to switches on the gear box or to the air conditioner (if you have air conditioner)? It also may not be hooked to anything.

[Nicolas Aristodemou]

Yes I do have a CSS and is hooked to the aircondirioning system. It engages upon activation of the compressor clutch. In fact at the moment the only way to drive the car without constantly switching off is to have the AC running...... bur of course I do not want to keep AC on all the time.....I thought the vacuum operated controller located under the throttle body should prevent my idle speed from dropping at idle with gear engaged but it doesn't. Maybe not adjusted properly,, although I have tried several times to adjust the spring tension.

[Cees Klumper]

I thought the css was also activated when putting the car in gear; I have no A/C but an automatic transmission, and a CSS that definitely engages when the transmission is put in gear to maintain the RPMs. You could, as a temporary solution, raise your engine's neutral idle RPMs while this gets figured out.

[Nicolas Aristodemou]

From what I have read in the forum and in the technical manual's linkage setup the CSS on late USA autos like mine  was hooked to the Airconditioning or even left unhooked on automatic cars without AC. Maybe I will get my CSS to activate both when gear is selected and when the AC is on by using a relay for each task.......

[ja17]

Even if your CSS is not working at all, the engine should not stall. It would drop in RPMs (150-200 rpms) but keep running.  I suspect you may have a basic tuning issue (spark plugs etc.) also or possibly low compression on one cylinder?

[Pawel66]

naristodemou, then, following advise of the Members below, perhaps it is not only the questoin of the vacuum device adjustment but a question of followng the entire linkage tour predecessed by valves, timing and emission control system check... Also: making sure you have the correct plugs and ignition leads.

[Cees Klumper]

The fact that the car runs fine when the engine is cold/warming up seems to confirm the issue is with the tuning, rather than with solenoids/emissions systems? Sounds like the idle mixture may be too lean.

[Nicolas Aristodemou]

That was it  .... idle mixture too lean......I turned the idle mixture adjustment screw clockwise 8 clicks and now all is good...... thanks Cees Klumper for the tip..... I thought my mixture was set spot in but apparently it was not........ Now car downshifts without switching off and idles at about 600 rpm with gear selector at 4 or R without switching off.... thanks to everybody for valuable tips.....