BMW 120d non start

BMW 120d 2004 (204D4) non start following replacement engine.

I was called out to the above vehicle for a non start condition, after a replacement engine had been fitted.

The original engine had been a failure to run, with little back-ground information. (Some kind of sales related vehicle).  Instead of fully investigating the cause, it was assumed the engine was damaged and so the easy solution was to just replace the engine.  The owner of the car sourced a second-hand engine.

With the replacement engine fitted and not starting, I had a quick look at basic data.
I noted that fuel pressure appeared good, but noted no RPM value.  Unfortunately with some BMW’s I have noticed in the past, do not give an RPM value during non start.  And so out comes the scope.

After accessing the crankshaft sensor circuit, along with the camshaft sensor and giving the engine a cranking start command, I noted no crankshaft signal. (Sorry no scope capture).

I didn’t bother about keeping data on this vehicle at this time, because it was quite a clear and straight forward issue.  I was concerned only about the crankshaft signal generation, not really about the sensor.  There was no DTC for a bad sensor or circuit and in fact a circuit test suggested the sensor was ok.

So the sensor was removed so that I could test the operation, (not the nicest sensor to remove, and so a return visit was needed).

On my return, I was able to establish the sensor worked perfectly.  So onto the reluctor ring.

The use of a borescope, revealed no reluctor ring present. ( the price of a second-hand engine!)

My work was done! so off I went, leaving the garage to do the necessary.

Out came the gearbox and removal of the flywheel.  Then over to the original engine, removal of the flywheel and reluctor ring.

The reluctor was fitted to the new engine, gearbox re-fitted and; oh dear.. The engine failed to start, even although it did have a try.

So back I came, to further investigate.

This time it got interesting.  I noted that sometimes the injectors fired, but sometimes did not fire.  Refer to the images below.

120d non start crankshaft 1
Cam, Crank, Fuel pressure and injector current non start
120d non start crankshaft 2
Cam, Crank, Fuel pressure and injector current attempted start

So this waveform raised the question; what’s happening with the injector timing sequence?

Now have a much closer look at the crankshaft signal.

120d non start crankshaft signal zoom
Crankshaft signal anomaly

That signal anomaly repeats every 360 degrees of rotation, so that’s not a bad sensor but a bad reluctor..

And so.. out with the gearbox again.  A nice new reluctor plate fitted and all back together again.

Hooray!  the engine starts and runs perfectly.

120d start crankshaft signal good zoom
Crankshaft sensor signal after replacement reluctor plate fitted

Of course this makes you wonder.. was this all that was wrong with the original engine?

We’ll never know.

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