This one has a number of people confused including me.

Generally fault is one of a hard start with fault code 79 RPM/TDC Sensor when engine has cooled down for some time or overnight. Disconnecting crank sensor allows a start (I assume here must be synchronising on CAM sensor to provide start) after extended cranking.Engine runs badly until it is revved several times and after a few minutes runs smoothly and restarts easily with crank sensor re-connected.

Crank sensor replaced with GSF unit and because it gave same fault a genuine BMW sensor (Siemens unit) was fitted but still have the problem. .

Continuity tests from ECU loom end to both Cam and Crank sensors all good, no shorts, open.Also resistance measurements for cam and crank sensors as specified in Autodata are correct.

Not seen an engine able to start without crank sensor before so was really surprised. When it starts in this mode , engine runs roughly and misfires, pops on revving , but as it warms up it starts to run OK and starts OK with crank reconnected.Everything OK no fault codes when warmed up.

I Pico scoped both cam and crank and I am really confused by what I see. The best signal is achieved by measuring on the middle pin of the 3 pin connector and this signal is constant whether engine is running or just with ignition on . Signal does not alter when engine is revved. I am measuring a constant sine wave of 4 volts peak to peak +2V/-2V about 0V on either sensor. The Cam sensor frequency is 100KHz and that of the crank is 150KHz. I read somewhere these might be AC excited sensors ,but I have looked for some time at the traces on all pins of both sensors and cannot find any break points to indicate TDC. These are not Hall effect as there is no digital switching as I would normally expect to see.

My question is what the hell am I measuring and how do these cam/crank sensors work?

By the way these are the correct sensors being measured and although the connectors are identical , they are fitted to the correct loom ends as I can generate fault code 84 when I disconnect the cam and run the engine and also create fault code 79 for the crank when I start the engine with this disconnected.

Bottom line since the crank sensor comes up as a fault and this has been replaced and wiring to the ECU checks out , I am of the opinion that the fault lies with the ECU and it goes away once the ECU has warmed up after a few minutes of running.

Any ideas??