Unknown electrical unit

[bluelemon]

Good morning, I'm new to the forum and very much looking forward to the support provided. USA 280sl auto. Theres a small electrical unit on the end of the bulkhead throttle bar, seems to disconnect when throttle opens. Can someone tell me what it does, thank you

[WRe]

A photo would be helpful.

[lpeterssen]

Dear friend

That should be the IDDLE SWITCH, which lets know the transmission that you are not pressing the acelerator pedal, and by combining that signal together with the transmission fluid sensor inside of it it then sends a signal to the Constant speed selenoid to increase a bit RPM if car is about to stahl.

Best regards
Leonardo Peterssen
Www.wiredoktor.com

[Cees Klumper]

I think Mr Peterssen nailed it, I have the same microswitch on my 1969 US version 280 SL and have seen it on others.

[Peter]

I'm not sure, please provide a photo.

[twistedtree]

On later model 280s I think it's also part of the emissions system.  Some injection pumps have a solenoid that cuts fuel when you decelerate.

[twistedtree]

And on an automatic, it's one of the inputs to the three position solenoid that controls transmission pressure.  With your foot off the gas peddle, the switch is closed and causes the three position solenoid to shift to the lowest pressure setting.  This cause the transmission to downshift softly as you decelerate.  When you hit the gas, the switch opens and the solenoid shifts to the mid pressure setting which gives crisp up-shifts as you drive.   The other input is the kickdown switch under the gas peddle that closes when you floor it.   That shifts the solenoid to the highest pressure setting which delays shifts until higher rpms, and provides the most firm shifts.

The constant speed solenoid I'm pretty sure is controlled by either of two pressure switches on the transmission.  One switch activates in reverse and the other in forward.  Either one activated will energize the constant speed solenoid.  The reverse sensor also controls the backup lights.

[Lori]

The reverse sensor also controls the backup lights.

The neutral safety switch controls the backup lights

[bluelemon]

Many thanks for your replies. I've attached a pic. I recently had the gearbox rebuilt and it doesn't seem that any electrical connections were re-attached!

[WRe]

And now you should tell us which car it is, probably not a Pagoda.

[BobH]

twistedtree is correct, depending on whether your car has emission control and fuel shutoff on the injection pump or not, there were two uses for this switch, have a look on here:

https://www.sl113.org/wiki/Restricted/ColorWiringDiagrams (you'll need to register as a full member to access)  this shows the switch (either fitted on the venturi or occasionally located on the bulkhead) controlling the three position solenoid.  You may want to search further to find out what part it plays in emission control

Some more discussions on here, with another picture:

https://www.sl113.org/forums/index.php?topic=624.msg32115#msg32115

[Peter]

This is part of the environmental system. When you release the throttle and the rpm drop below 1100, the injection pump shuts off. A very rudimentary system to comply with US regulations at the time. You won't find this on European versions.

[Peter]

Below the testplan:

[lpeterssen]

Excellent, Peter you are right.   My initial guess of something at intake manifold like the IDDLE SWITCH,was mistaken.   This is definitely related to the emissions control system.

Best regards

Leonardo Peterssen
Www.wiredoktor.com

[twistedtree]

That's not a switch that I recognize at all.

He mentioned that it's a US spec 280 SL with automatic, but we don't know the year.

I now see there is a separate switch for the fuel cutoff, which makes sense now that I think about it.  So the idle switch on the throttle body only controls the automatic transmission pressure.

[clunker]

Check the Technical manual under https://www.sl113.org/wiki/Electrical/EmissionControlSystem for the 4th relay - mention is made of the microswitch on the firewall - this is what we are looking at here. (For a manual 1969 car there is a different 4th relay which also uses that switch in its rudimentary emssions control system, that is shown under https://www.sl113.org/wiki/Electrical/IgnitionSystem) In each case, scroll down to the '4th relay' section.