Fuel Shutoff Solenoid

[mdsalemi]

Gents,

Trying to solve a long standing drivability issue.  Grasping at straws.

There is a solenoid valve on the fuel injection pump (1968 and above I believe) called the "fuel shutoff solenoid".  I am certain it is the LOWER of the two solenoids on the right hand side of the pump.  There is a single wire going to it.

Has anybody had any intermittant or failed issues with this solenoid or the relay(s) that control it?  What were your symptoms?  Does it cut off all fuel (which would cause you to stall) or just part of the fuel flow?

One new theory of my problem is that in the middle of driving, the solenoid is engaging, thus creating a fuel starvation problem.  Turning off the car and restarting seems to clear the issue, at least for a few miles.

If I disconnect this wire in order to "test" the theory, what kind of additional symptoms might I create if this "system" is working properly?

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[George Davis]

Michael,

with the solenoid disconnected you may get some popping/burbling in the exhaust when coasting down.  IMO that would be a bonus .

George Davis
'69 280 SL Euro manual

[JPMOSE]

I don't believe Michael's question was answered on this.  I would also like to know more about the Fuel Shutoff Solenoid.  What exactly is it's purpose (other than the obvious...it shuts off fuel)?  Can it act up while the car is running, causing it to shut off unexpectedly?  What is the reliability rate of this part?

Best Regards,

J. P. Mose
1968 250SL

[jeffc280sl]

I have only studied the function of this solenoid on the 1970 and 1971 models.  In those model years it is part of the emissions system which is described in the Haynes Manual on page 82-85. I just noticed a section on page 78 relative to the 68 and 69 models but I have not looked at it in detail.

In the later models the body solenoid is gound.  +12V is switched to the solenoid by a relay box under certain conditions to improve emmissions.  I'm most familiar with the manual transmission and the solenoid operates as follows on this model.  On deceleration (throttle valve closed) clutch released and transmission in third and forth gear the fuel shut-off valve is energized.  Push in the clutch, step on the accelerator or operate in 1st, 2nd or rev gear and the solenoid is off.  It works a little differently in the auto trans but essentially during in gear deacceleration the solenoid is turned on to reduce unspent fuel emmissions.  That's it.  Most of the time atleast on the later models it doesn't come into play and on most cars I suspect it doesn't work at all due to a failure in the relay, wiring or micro switch.

Jeff C.
1970 280SL 4-speed

[BHap]

Michael - to test the solenoid, connect a test light to the stopping solenoid and to ground. Drive the car in third or 4th gear. Take your foot off the gas and the light should come on. The light should go off below 1100 rpm. Next test is to drive the car in "3" at an engine speed of at least 1500rpm. take foot off gas and pull shifter into "2" position, light should go off. if this light comes on while you have your foot on the accelerator - you have a problem....  i would be more suspicious of fuel pump inlet screen clogging...

Bob Happe
Pittsburgh, Pa
1970 280SL, white/black 4 speed

[JPMOSE]

Thanks for the write-ups on this topic.  I will test mine this weekend.  

I think the difference between a good running and great running car is often achieved by attention to small details.  It is also a learning experience!

Best Regards,

J. P. Mose
1968 250SL

[JPMOSE]

There is a small switch (similar in size and shape to the idle switch located on the throttle body) located near the firewall and neutral safety switch (at least on cars with automatic transmission).  It appears to be engaged when the throttle is released.  Two questions:

1) Does this pertain to the Fuel Shutoff function?

2) If the answer to question 1) is yes, could the malfunction of this idle switch cause the engine to stall at idle?

Best Regards,

J. P. Mose
1968 250SL

[jeffc280sl]

J.P.

Fig 3.77 in the Haynes Manual describes the emission control circuit for the 68-69 auto trans SLs.  Don't know if this includes the 250SL.  The write up describes a micro switch on the regulating shaft.  This switch could be the one you are describing on the fire wall near the neutral switch.  According to the diagram this switch is connected in series to the starter inhibitor switch and other switches which control the operation of the fuel cut off valve.  

The answer to your questions appears top be yes.  I would disconnect the wire from the fuel shut off solenoid and ohm it out with the micro switch to confirm.  A failure in the circuit may cause the shut off solenoid to operate eratically.  Try disconnecting the wire to the fuel shut off solenoid and duplicating the conditions where the shut down occurred.  If the engine continues to run where it shut down previously I think one can conclude a problem in this circuit.

Jeff C.
1970 280SL 4-speed

[don p]

I had a stalling issue when down shifting from either 3rd or 4th gear. Took awhile but found this thread, I unhooked the single wire off the bottom right "fuel shutoff solenoid" on the injector pump and that has solved the problem. I plan on leavng it off unless I find a reason that this shutoff is needed. I did not get any extra "popping/burbling in the exhaust when coasting down"

Thanks to this group this is the 4th problem I have solved in the short time since buying this car in May, thanks to this group!

Thanks to all that share thier insight!

Don Pemberton
1968 250SL
113.043-10-004113
190g Graphite gray                     Connecticut

[TheEngineer]

I thought that there was a problem with my fuel shutoff solenoid as well: So I conected a small 12 volt bulb between the terminal of the solenoid and ground and taped the bulb to one windshield wiper. The bulb is supossed to come on when I coast and the engine turns over 1200 rpm. Sometimes it does and sometimes it doesn't. I have traced the intermittent failure to the Bosch sensing unit. And, yes, if the solenoid is actuated it cuts off all fuel!