The crank position sensor (L5) can definitely produce the symptoms you're describing. Fortunately, both the diagnostic test and the repair are quick and easy and inexpensive.

Here's the main diagnostic test for that L5 sensor (see FSM section 15-1250 for complete description). Engine cold. Ignition off. Multimeter set to Ohms. Unplug the L5 sensor at the EZL. Positive probe on inside of pin. Negative probe on outer part of pin. Spec is 680 -1200 Ohms. If no, rplc L5 sensor.

__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

I tested crank position sensor (cold) and it measures 800 ohms.

While testing, I noticed the small vacuum tube at the EZL had become disconnected. That vacuum connection does the advance/retard of ignition timing. I don't think it can cause the intermittent no-start and stall trouble I'm having though.

So...time to gamble.

Should I buy:

Crank position sensor (even though resistance measure is within spec)
or
OVP (even though looks good, fuses good, no rattling, no CEL, no ABS light)

Or continue looking for something more definitive?

Personally, I never replace a part unless I know it's bad - the only exception being ECU's that can't be tested. Even then, I like to verify it's bad before I put the new one in.

Of course, if you don't have the equipment to test things properly, you're in a tough spot. I've seen plenty of crank sensors that ohmed fine, but produced sporadic readings on an O-scope.

As far as ruling out the fuel pump relay, I wouldn't be too hasty. If you were driving with it and it stalled, and then you put it back in and it worked, that's not exactly proof that it's working exactly as it should - after all, the FP relay controls more than just the FP itself.

__________________
A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

I ruled out the FP relay based on:

1)While driving, it stalled and wouldn't re-start
2)Immediately removed FP relay and installed bypass jumper (and fuel pump ran)
3)Still no re-start.

Am I overlooking something with this line of reasoning?

The fuel pump relay controls more than just the fuel pump.

It will shut off power to the fuel pumps if it doesn't recieve an RPM input for more than a second from the electronic ignition control unit. If there's no output from the ignition control unit, that probably means no spark, so a (jumpered) running pump doesn't mean anything, other than the pump probably works.

The FP also controls the cold start injector.

It's why there's so many pins on it...

Personally, I think "fuel delivery module" would be a better description of what it does; a "regular" fuel pump relay only has 4 pins; 2 for the coil and 2 for the contacts that needs to be made to power the pump.

If it were me, I'd be looking at the ignition system, especially since you've somewhat verified that the fuel pump works.

You can either take it to an indy who has all the tools (scopes, etc) to correcly diagnose it and replace the correct part, or you can risk it and break out the parts shotgun and start blastin'.

I will say, though, when it comes to DIY'ers, it has been my experience that an hour of quality diagnostic time with quality tools would have saved those people hundreds of dollars in "this might be it" parts...

__________________
A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

based on all the tests and chatter that have occurred on this thread i would change the crankshaft pickup as it's the only part that commonly works again when allowed to cool off.

__________________
David S Poole
European Performance
Dallas, TX
4696880422

"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
http://www.w108.org/gallery/albums/A...1159.thumb.jpg

I had the exact same issue with the SL and it turned out to be the fuel pump itself. Not only would it fail to start some days, but a few occasions it would just quit while driving and fail to restart. I could swear I heard the thing running too.

Next time it does this, give the fuel pump a couple of smacks or shake it around. If it starts, then you found the problem. I stumbled upon it that way accidentally while fiddling around back there.

__________________
1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

Many thanks to all who have replied to my queries.
I value your collective experience, training, and advice.

I want to be as certain as possible about the Crank Position Sensor; $150 isn't peanuts.

I checked AC voltage at Crank Position Sensor terminal (while cranking) and it measured 0.6 VAC. I started the engine and just let it run at idle; it eventually began to stumble and shut off after 15-20 minutes. Would not restart. I measured the CPS again. Resistance was 900 ohms (compared to 800 ohms cold). AC voltage was still 0.6 VAC.

I read in another thread (vehicle was a 1990 190E) of CPS voltage actual 2-3 VAC and spec 1-1.5 VAC. Is it valid to compare my numbers against those?

Is this the definitive proof that my crankshaft position sensor is faulty?

I must, I don't think it's the crankshaft position sensor at all....it's something else....best of luck

however, let me ask, you do know what the crankshaft position sensor actually is/does right? Cause it sounds like you dont....

My understanding is:
During crankshaft rotation, the CPS sends a signal to the EZL (ignition control module), which in turn (thru some additional electronics) makes the ignition system work.
So, faulty CPS = no ignition.
And flakey, temperature-sensitive CPS = flakey, temperature-sensitive ignition.

roughly you are correct

cliffmac,

What is your reasoning for thinking it is not the Crankshaft Position Sensor?

Another thing is, when it starts stumbling, if I press the accelerator and increase the RPM, it runs smoothly at the higher RPM. And I read somewhere that the AC voltage from the CPS increases, when RPM increases.

I want to be corrected if I'm on the wrong path here.

OK....sorry...anyway, I don't think it's the CPS. This, to me, seems more like a coolant temp sensor (CTS) issue...the CTS is a very big player in variable engine dynamic managment which of course Mercedes Benz employs. Let me think here.....

OK - here is the most basic reason... if the engine starts, it
is NOT the CPS...that's really it....the CPS will NOT affect the engine
operation once the engine is running...that's it..it's (CPS) out of the loop.
Once you shut the car down and let it cool off..and then can restart the motor.... that eliminates the CPS thing...dig? or not?