N55 fresh rebuild with N54 crank: Crank-no-start, voltage drops, strong starter draw (0 fault codes)

This BMW E92 N55 has a fresh rebuild, an N54 crank with a modified N55 reluctor wheel, about 150 rpm while cranking, reported fuel, spark, and compression, an occasional catch, a voltage drop, strong starter draw, and no DME or CAS faults. My advice is to stop treating the visible rpm and zero faults as proof that synchronization is correct. Capture battery voltage and starter current together, check relative compression, and compare the crank and cam signals before chasing parts or software.

N55 fresh rebuild with N54 crank: what the symptoms prove

The reported rpm matters, but only in a limited way. It says the DME is seeing enough crankshaft sensor activity to calculate engine speed. It does not show that the reluctor pattern is indexed to the engine's true mechanical position. A clean-looking rpm value can exist while the signal arrives at the wrong crank angle.

The same caution applies to the absence of fault codes. A DME can receive signals that look electrically plausible without those signals describing the correct mechanical relationship. Fault memory is useful evidence, not a certificate that mechanical timing and synchronization are right.

Fuel, spark, and compression also need sharper definitions. Fuel at the rail is not the same as commanded and delivered injection. A spark seen outside the cylinder does not prove it occurs at the right time under cranking load. A compression result does not establish that every cylinder contributes evenly or that valve events agree with crank position.

An occasional catch is useful. It suggests the engine sometimes gets close enough to a combustible combination, but it does not identify why. I would use it as a marker in a recording. What changed in voltage, starter current, crank signal, cam signal, spark, or injection at that exact moment?

Start with cranking power and mechanical load

Voltage drop and high starter draw belong in the same test. Looking at one without the other can point the diagnosis in the wrong direction. Low voltage may reduce the operating margin available to the DME, ignition, injectors, and sensors. High current may also pull system voltage down because the starter is working against excess resistance or mechanical load.

I would record battery voltage and starter current through the complete crank event. The shape of the current trace matters as much as the peak. Repeating rises and falls can provide a relative view of cylinder loading. An irregular pattern can direct attention toward uneven compression, valve timing, bearing drag, starter trouble, or another mechanical issue. It is a screening result, not a final verdict.

Then I would perform a relative compression check and, if the result calls for it, confirm compression mechanically. On a fresh build, this step should come before assuming an electronic mystery. Assembly condition, cam timing, valve sealing, and rotational drag can all affect the way the starter sounds and the current it demands.

Voltage-drop testing also needs to cover the complete high-current path while the starter is loaded. A resting battery reading cannot reveal resistance that appears only during cranking. I would inspect the power and ground sides as measured circuits, rather than judging cables and connections by appearance.

Prove crank and cam correlation

The conversion itself should not be declared impossible, and the modified wheel should not be named as the cause without evidence. It does make crank reference integrity a central test because the DME depends on the relationship between the crankshaft signal and camshaft signals to place ignition and injection events.

I would capture the crankshaft sensor and relevant camshaft sensor waveforms at the same time during cranking. The questions are concrete: Is the crank pattern stable? Are teeth or edges malformed? Does the pattern shift when voltage falls? Do the cam transitions repeat at a consistent location within the crank pattern? Does the relationship stay stable when the engine briefly catches?

A stable waveform can still be stably wrong. Correlation has to be tied to known mechanical position and a trustworthy reference for the exact engine arrangement being tested. That may require confirming true mechanical position and checking how the reluctor wheel is located relative to it. A generic waveform image from another engine is not enough if its trigger arrangement or setup differs.

The N55 sensor choice may be correct for the control system, but sensor type alone does not establish signal polarity, air gap, wheel geometry, or indexing. I would inspect the waveform at the DME side when practical because that is the signal the controller receives. Back-probing at only the sensor can miss a wiring or connection problem between the sensor and DME.

Measure combustion inputs after synchronization

Once cranking voltage, starter loading, relative compression, and crank-to-cam correlation make sense, I would move to combustion inputs. The goal is not another yes-or-no claim that fuel and spark exist. The goal is to establish whether the DME commands them, whether the hardware delivers them, and whether they occur at a useful time during the same crank event.

For fuel, I would verify supply and pressure behavior during cranking, then determine whether injection is being commanded and delivered. For ignition, I would confirm primary control and secondary output under load where the test method allows. I would correlate those events with the crank and cam capture instead of testing them on separate attempts and assuming conditions stayed the same.

If spark and injection are present but occur against an incorrectly indexed position signal, the engine may cough without running. If the electrical events disappear when voltage collapses, power supply or excessive cranking load moves higher on the list. If relative compression is uneven, mechanical work comes before more sensor substitution.

This sequence also limits unnecessary parts replacement. A new sensor cannot correct a mechanically mislocated reference. A stronger battery cannot correct excessive drag. Rechecking assembly cannot repair a voltage drop in the starter circuit. Each test should separate those possibilities instead of blending them into one symptom.

What I would do next

I would keep the next session disciplined and record each result:

1. Capture battery voltage and starter current together through cranking and any brief catch.
2. Use the current pattern as a relative compression screen, then confirm mechanical compression where indicated.
3. Capture crank and cam signals together and assess signal quality, repeatability, and correlation to verified mechanical position.
4. Verify power and ground integrity under starter load.
5. Measure fuel pressure, injection command, and ignition operation during the same controlled cranking conditions.
6. Compare the traces from an ordinary no-start attempt with the attempt that catches.

That order answers the central question without assuming the N54 crank conversion is either valid or at fault. The strongest clue may come from the relationship between tests: voltage falling as current rises, one uneven compression event repeating in the current trace, a cam edge displaced within a stable crank pattern, or spark and injection stopping at the moment supply voltage sags.

Zero DME or CAS faults does not remove those possibilities. About 150 rpm does not prove indexing. A fresh rebuild deserves mechanical verification, and a modified trigger arrangement deserves waveform correlation. I would make those measurements before replacing more parts.