What is this? [Fuel Shutoff Switch]

[awolff280sl]

Does anyone know what this is:

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It is attached to the firewall next to the throttle linkage. It has 2 badly beaten up wires attached, and I would doubt if it still functions.

Andy
Sarasota, FL

[JPMOSE]

This is switch is part of the Fuel Shutoff Emissions on all USA models 1968 and forward.  It is an idle switch that is engaged when the throttle is released (or car is at idle).  In order for the Fuel Shut Off Solenoid to engage on the back of the Fuel Injection Pump, this circuit must be closed (along with engagement of the Oil Pressure Switch on the transmission and Neutral Safety Switch in off position).  Additionally, there is an RPM sensor that will shut off the fuel when that idle switch is engaged and the engine RPM is above 1100 RPM.

Most W113 owners have this system disconnected, as they were troublesome in the first few years.  Most unreliable is the Speed Sensor device.  I became interested in restoring mine (1968 250SL) last year until I priced the Speed Sensor.....$750 wholesale.   [:0][:0]  I will keep it hooked up in appearance but enjoy the popping sound of the car as it decelerates!

IT SHOULD BE NOTED THAT THERE IS ALSO AN IDLE SWITCH ON THE VENTURI (AIR INTAKE) WHICH ENGAGES AT IDLE AND CHANGES THE VACUUM MODULATOR PRESSURE VIA THE AUTOMATIC TRANSMISSION SOLENOID.....THIS IS COMPLETELY INDEPENDENT OF THE IDLE SWITCH MENTIONED ABOVE.

Best Regards,

J. P. Mose
1968 250SL
1987 560SL

[awolff280sl]

Thanks. If I were to disconnect this "system", should I simply disconnect the wires from the idle switch, or can I remove all the hardware, including the fuel shut off solenoid on the pump, and all the other devices that you mentioned?

Andy
Sarasota, FL

[JPMOSE]

I believe this system is usually disabled by disconnecting the wire on the back of the Fuel Shutoff Solenoid (the lower solenoid on the back of the Injector Pump if you have two solenoids).  Just wrap some electricians tape on the end of the wire.

Then again, as the old saying goes...if it aint broke don't fix it!  You may want to just leave it alone if your car isn't stalling.

Best Regards,

J. P. Mose
1968 250SL
1987 560SL

[wwheeler]

I have one wire attached to my fuel shut off solenoid on my '68 M130 engine. However, I also have a wire that is wrapped with electrician's tape right next to the solenoid. Is there supposed to be two wires going to the shutoff solenoid? One from the microswitch and one from the speed sensor relay?

Also, I was told that the speed sensor relay kept the fuel solenoid slightly OPEN when the throttle is released until the rpm dropped below 1800 when it then allowed the solenoid to shut off the fuel. If the throttle is released, the microswitch would energize the fuel shut off at any engine speed. If the speed sensor also energized the fuel shut off above 1800 rpm, wouldn't that be duplication?

Thanks for the help!

[Ron]

Oops, I did the first try at reply incorrectly, sorry.

However, what does the idle switch on the venturi do on a manual trans car?  My car has the engine out, as purchased last month.  Before installing it, I'm trying to figure this all out.  The throttle switch is broken off, and there's one wire on the harness that is any unknown near where that switch will go.  The wire goes to a fuse box on the right fender, then on through the firewall on the passenger side.  Do I need to buy a new, or functioning throttle switch?  Thanks, Ron


IT SHOULD BE NOTED THAT THERE IS ALSO AN IDLE SWITCH ON THE VENTURI (AIR INTAKE) WHICH ENGAGES AT IDLE AND CHANGES THE VACUUM MODULATOR PRESSURE VIA THE AUTOMATIC TRANSMISSION SOLENOID.....THIS IS COMPLETELY INDEPENDENT OF THE IDLE SWITCH MENTIONED ABOVE.