'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?