Engine dies when clutch let out

[BHap]

I have a 70,000 mile 1970 280SL, 4 speed manual. I took it out of storage today and drove about 3 miles with no issues until it stalled at an intersection.  Car started right back up, checked that i was in 1st and not 3rd and same thing.  Over revving allowed me to get car off the road.  Started car again and put shifter in neutral. As soon as clutch is let out, engine dies. Any thoughts / ideas appreciated.

[ejboyd5]

If it stops suddenly when releasing the clutch pedal I would suspect the clutch linkage is disturbing an electrical connection crating a short or disturbing power to the engine.  This however does not square with your "over revving" solution to get the car off the road unless the failure mode was not yet complete as it is now.  If the engine starts and runs with the clutch depressed it seems as if you have substantially limited your area of inquiry.  Might your car have a switch that prevents the engine from being started unless the clutch is depressed?

[450sl]

Can you push the car back and forward without issues?

[BHap]

Yes, car rolls freely.  Engine fires right up with clutch engaged and revs smoothly. If placed in neutral and the clutch is let out, the car dies immediately.  I am thinking that I have a stuck relay or a bad switching box for the fuel shutoff solenoid circuit.  I'll let you know what testing these switches reveals.

Thanks

[waltklatt]

Had that with another non-MB(toyota pickup) and it was the engine setup with the vacuum.
Engine might run perfectly while unloaded, but once a load was applied it stalled.
Turns out the idle increase from injection pump was not working.
Rev the engine and let the clutch out to see if it changes.
Walter

[ejboyd5]

Engine fires right up with clutch engaged and revs smoothly.

Not clear what you are now describing.  Does this mean that motor will start and run normally if transmission is placed in neutral, your foot is removed from the clutch pedal. i.e., "clutch engaged," and the starter is then activated?

[BHap]

The starter is acting normally. Clutch pedal pushed in, car starts and revs normally. As soon as clutch pedal is let out, the car dies. If I press gas pedal while letting clutch out, engine will continue to run but when gas pedal released car dies. Car acts the same whether in gear or in neutral.

[ja17]

I would temporarily did connect the fuel shut off relay on the injection pump.

[BHap]

Yes - disconnecting the fuel shutoff solenoid does fix the problem...  Now to figure out which component is at fault!

[450sl]

You could just leave it that way  ; my 280-70-stick does not even have the solenoid..........

mark

[Tyler S]

On the 4 speed, there are 2 switches on top of the transmission that are actuated by the shift rod from the stick shift. They are depressed in 3rd and 4th gears respectively. If either of these switches is stuck in the depressed position, or the switch is shorted internally, power will transfer through to the fuel cut solenoid. The microswitch on the firewall also needs to be adjusted correctly to be depressed when the acc linkage is at idle position.
An easy check is to slightly depress the throttle while releasing the clutch. If the engine stays running, check the 3rd and 4th gear switches. These switches are externally mounted on a bracket above the trans

[BHap]

Thanks Tyler - any idea how to access the switches?  I have the fuel shutoff solenoid disconnected with a voltmeter connected to the wire that was on the solenoid.  Starting car cold, no voltage to solenoid no matter where the clutch or accelerator pedal are.  As car warms, 12v is present at the solenoid wire.  Pressing either the clutch or the gas pedal cuts off the 12v to solenoid, which tells me the car "thinks" it is in 3rd or 4th and decelerating. One of the switches is most likely stuck, or as you say, the wires are shorted. Can the switches be accessed through the opening in the floor or can you reach them from underneath ? Hopefully without dropping the transmission!

Edit: so after looking at the 3rd & 4th gear switches from the engine bay and from under the car, the 4th? (the one towards the front of the car) is stuck "in". I'm not sure this can be changed and adjusted without lowering the transmission? I'll drive it for a while without the fuel shutoff solenoid attached until I have another reason to lower the transmission.  The access plate in the floor also might be an option.

Thanks for your help.

[Tyler S]

Sorry for the late reply. Yes you can kind of access them through the tunnel covers. It a reach around kinda job.
What you can also do for now is simply unplug the offending switch with a long pair of needle nose. If its the 4th gear switch you may want to leave it unplugged anyway. It can cause a surge when cruising or slight downhill at light throttle if everthing is not adjusted perfectly.