'89 W124 300E cranks but no start
 

So I drive to the store (15min trip) and come out 10mins later and now it cranks but will not start. This has happened before but very intermittently (maybe one time per 2month), and always started after re-trying 1-2 times or waiting a minute. I walked home and plan to get a tow to my house tomorrow. Before I pay for a tow, what are some easy/obvious things I should check, requiring minimal tools? (I'll have to lug the tools with me to where I left the car).

Searching the forum, a possible suspect seems to be overvolt relay. Anything else I should check?

Thanks for any help!

Most likely you will find it is the fuel pump relay with a poor solder joints.typical relay fault
remove the relay and jumper it at pins #30, #87 ? . pump should come on and the engine start.

OK thanks mak.
I'll have to research the location of the relay, and location of those pins to be jumpered. So...I could connect the jumper, start the engine, and drive home. Right? It's OK safe to drive 2 miles in jumpered condition?

Yes its fine as all it does is kick on the fuel pump....assuming that's the problem.

While you're at it, check the OVP (sitting right next to the fuel pump relay). The OVP powers the relay, and it too gets bad solder joints and blown fuse(s).

Well it immediately started this morning no problem, so didn't need to use my backpacked toolkit.

I removed the OVP. The socket and pins are clean and dry and free of corrosion. No rattling of anything inside. Both of the 10amp fuses look good and ohmmeter test good.

Removed the fuel pump relay. The relay pins are clean and dry and free of corrosion. No rattling of anything inside. But the socket looks like maybe trouble. A couple of the sockets are kinda loose in the plastic block, as if the surrounding plastic has melted away or disintegrated away or something. Specifically, the socket positions which mate to relay pins #30 and #87k are a little loose in the plastic block, instead of being solidly held like the rest of the positions are. But on the other hand, once the relay is plugged in tightly, those big sturdy pins should hold everything in place, loose sockets or not.

I've seen threads about people repairing fuel pump relays by resoldering bad solder joints. I don't see any solder joints. Do you have to disassemble the relay to discover that? Mine doesn't look like it snaps apart.

Any further advice and recommendations will be appreciated.

Did you actually check to see if the pump comes on?

At the time of the no-start condition, I did not check the fuel pump. How would I go about checking it, for when it happens next time?

Have someone listen for it while you turn the key to "ON". It should run a couple seconds.

In the meantime, you might want to measure the current the pump is pulling.

The fuel pump is pulling 8.6amps to 9amps.

I have my emergency jumper ready made. What is the recommended diagnostic sequence to follow for next time the no-start occurs, in order to definitively determine FP relay vs. OVP vs. something else?

Thanks.

If you jump the fuel pump and the car starts, check to see if the ABS light is on. If it is, it's more than likely the OVP relay.

According to alldata, the FP relay will shut off if it doesn't recieve an RPM signal for more than a second, so that could be the problem as well.

The easiest way to rule out the FP relay would be to test it off the car with a 12V power source and a meter; or take the casing off and watch it while it's installed while taking voltage readings...

Bench testing it won't reveal an intermittent problem, can it? On mine, the casing looks molded on; appears impossible to open without destroying it. Am I missing something in how it is constructed? I would like to inspect for the solder problems mentioned in other threads.

Also, check the crank position sensor.

Sure it can. You can apply & cut voltage to it several times a minute on the bench - not something you'd want to try in the car.

Frankly, I'm not sure how people get them apart, but I haven't had the need. I think I've got a "bad" one lying around somewhere (came with the car) so I'll take a look...

It happened again last night. This time it stalled out as I was pulling out of a parking lot into the street. Luckily, some bystanders helped me push it to a safe place. I inserted the fuel pump jumper. Definitely heard the fuel pump running. But it didn't start.

I fumbled around in the semi-dark, tried to remove the OVP but couldn't get it out (not enough light to see what I was doing). Wasted time (~20minutes) figuring out what to do next...Replaced the FP relay, turned the key, and it started. No ABS light, no CEL.

This means I can rule out the FP relay, right?

Is the OVP still suspect?
How do I check the crank position sensor? Can it cause such intermittent symptom?

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.

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.

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...

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.

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.

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?

The CPS is always in the loop. To prove it (to myself) I just went out and started it, then pulled the CPS plug from the EZL and it instantly shut off, just like turning off the key.

no, that's not the crankshaft position sensor you are talking about... it's something but not that.....

the CPS in not in the loop at all once the engine is fired and running.
The CPS' job is simply to tell the ECU where TDC is and then hand the rest off to the ECU once confirmed...that's it...from that point forward the ECU runs the show and CPS just sits around waiting for the next restart...that's it....I would be happy to elaborate

I unplugged the connection at the upper left coaxial-looking port of the EZL.
It is the crankshaft position sensor connection.

More details in this other thread:
http://www.peachparts.com/shopforum/tech-help/263045-89-300e-crank-sensor-help-post2313926.html#post2313926




http://i179.photobucket.com/albums/w...ed/300EICM.jpg

Yes, that is the crank sensor connection to the EZL, and you are unfortunately getting unreliable advice about it.