Engine Shuts Off While Driving...
 

'87 420SEL just died as I was riding down the street. No hiccup or anything. I was going down hill and did not notice I had lost engine power. I first noticed the power steering was not working but then I realized my power brakes went out. All the diagnostic lights on the dash lit up just as if I had put the key in and had turned the key to position one. I coasted to a parking spot; strange thing was the radio never shut off until I turned the ignition off, but then the car started up as normal and I drove home. I don’t know where to begin troubleshooting this problem as it right now is transient. The only thing I suspect is the ignition cylinder has been getting stuck more and more as I’m trying to turn on the engine and I’ve had to jiggle the key more to get it to turn. Can a contact be wearing out in the key cylinder that keeps the electrical flowing? Any other suggestion on where to begin troubleshooting would be appreciated.

126 shuts off
 

My 2 pennies: had same problem w/ my 420SE; the fuel pump relay located behind the firewall would come loose going over bumps; cause: worn contact plastic plug in; fix: a small sleeve on one of the male contacts to tighten it up; had same deal with A/C relay next door; same fix; no problem ever since. another penny: get a new ignition switch tumbler before the key gets stuck and you can't turn the key over; cheap easy fix; a lot of piece of mind.

Does jiggling the key make the engine turn off? At any rate, if the tumbler is going, replace it soon. If the tumbler jams and you can't turn the ignition, it makes replacing it a lot more difficult - they'll have to drill the tumbler out. That, plus you'll be stranded wherever you are. :)

Fuel pump relay could be a cause, bad/worn solder joints will fail open intermittently, cutting off fuel to the engine. Happened on my 300E a few years ago, and a new one was the fix.

Just solved the same problem with my 91 420sel. The fuel pump relay was the cause. Re-soldered it and it's fixed.

Fuel filter ?

For what it's worth. I had a very similar problem with my 87 560SL. I thought it was the fuel pump relay as well, based upon what I had read in various posts in the forums. It turned out to be something that has received some attention in these forums, but not as much as fuel pump issues: it was a crank position sensor. You might search for crank sensor and see what's been written. BTW, it was a local independent shop that did a thorough check to find the faulty crank sensor.

Disclaimer: I do NOT know if there are any similarities between your motor and mine! Good luck!

thanks for the replies. The bracket that holds the fuel pump relay and another relay broke off from the fire wall so the two were not secured very well. I am working to find a new bracket from a junk yard as I just returned from MB Dealer and find out the part has been discountinued.