The Hall sensor is timed fairly accurately relative to the camshaft. I am not sure that the Hall is the reason for the delay in the engine firing, that is more likely the EZK sorting out where it is relative to the gap in the teeth of the timing ring (flywheel).

The Hall is used to help the EZK work out which cylinder has detonation. The EZK then retards the ignition on a per cylinder basis. Clever stuff...

If the flywheel speed sensor fails, then the EZK uses the Hall signal to give a "get you home" spark timing, although this is not as accurate as the flywheel sensor.
John '86 Euro S2

On my EZK test jig, if I am testing an EZK ECU and I switch off the Hall signal, then the ignition retards by the expected 6 degrees at high loads/rpm. If I then switch on the Hall signal, it still stays retarded.

If I switch off the ignition, and on again, with the Hall signal restored, then normal timing is resumed.

I will remember your experience.....we are all on a learning curve about the mysteries of the 928's electronics !

John '86 Euro S2

"If the flywheel speed sensor fails, then the EZK uses the Hall signal to give a "get you home" spark timing, although this is not as accurate as the flywheel sensor."

- John Speake -

Actually. once the flywheel sensor fails, the engine will die as there's no information from the Hall sensor to keep the engine running other than a TDC for a certain cylinder, or a relative engine timing position versus the cams.

Furthermore, a Hall sensor is really not needed to determine which cylinder needs to be retarded because of pinging, but Porsche decided to use it for this purpose on the 928. A Hall sensor (cam position) is really only needed for sequential  injection. Even for direct ignition, the ECM can determine the proper timing relative to a TDC crank sensor.

"I had heard of another instance where the Hall sensor fault was not cleared by a battery disconnect."

There's NO non-volatile memory device in either the LH or the EZK ECUs.
Thus, once the battery voltage is removed, both ECUs lose their faults.

The fault codes are stored in an SRAM (volatile memory) in the diagnostic ECUs. The codes are maintained by a permanent battery supply. In most cases, the codes can be cleared by a battery disconnect.

The only case contrary to that, which I am aware of is the Airbag controller of the 928 which requires a diagnostic tester to clear the stored fault codes.
John '86 Euro S2


If the sine is fixed at 60Hz despite of rpm of the car, you are looking at background noise from the 110v electric circuits. If it is related to the rpm of your car, that will make me reconsider :) It is possible that some Bosch sensors have a signal amplifier and shaper to produce that nice (digital) square wave form. I can't tell for sure.

Your 928S4 has +12v from the EZK going from pin 5 to the sensor on pin 1, and has ground from the EZK pin 4 to the sensor pin 3. EZK pin 22 is the signal output which is shielded by the ground connection. The sensor conducts when the marker passes, and switches the +12v to ground. 12v could also be 5v, not sure about that. That is how I think it works. I'd say it does not have an active circuit inside.

I check on my own web and came up with this:
Measure the disconnected sensor with a DVM ohm meter:
Sensor Pin 1 to 3= 685 ohm
Sensor Pin 1 to 2= open(!)
Sensor Pin 2 to 3= 640 ohm
The pin 2 is obviously the output.
You can do this from the EZK plug, and pinning is like this:
EZK Pin 5 to 4 = 685 ohm
EZK Pin 5 to 22= open(!)
EZK Pin 22 to 4 = 640 ohm

Be aware that since the sensor is in the car, it might become active when you use your voltmeter (current induced) and the result on pin 2 (ezk 22) may become different.

I'm not always correct, do not pretend to know everything, so please see my idea's as suggestions. But I do know my way with electronics.

Here's some reading material:

Yes, the bad hall sensor will cause a retarded ignition. You will feel that. But there should be no difference between cold and hot. You explicitly say that there is. So...

The transition is never smoothly ... It just switches maps. Both EZK as LH. But the EZK will hang on to a retarded -6 degrees whatever rpm and map position you're at. That is how I think the system responds. The EZK is a bit unclear, but the LH superimposes criteria over a base map. These super-imposed conditions are fuel mapping corrections.

Can't help with that hall sensor replacing. I've never done it, but it seems in a very tight spot.

1992 928gts Midnight Blue
1988 928s4 Cherry Red (Sold in 2006)
The Netherlands


The Hall sensor allows the knock sensors to work. The crank sensor tells the ECUs where the crank is - on every revolution. This means that the ECU doesn't know when the cylinders are firing and when they are on exhaust.

The Hall sensor tells the ECUs where one cam is. This enables the ECUs to keep track of individual firing events.

The ECU calculates cylinder location from waveforms received by both knock sensors. This allows the EZK ECU to retard ignition on individual cylinders. The system doesn't work in groups of cylinders.

My understanding is that if the Hall sensor or either knock sensor is faulty, the entire system dies, and the timing is automatically retarded.

Perhaps one of our ECU experts will correct this if it is wrong.
Wally Plumley


It is not so easy to remove the Hall sensor, but a few special tools do help....

Got it out !!
No... I did not have to remove the lifting eye, it was flexible enough to work around.....pulled it toward the fender to get the screw out and then pushed it against the engine slightly twisting it counter clock wise to remove the sensor. The hook is very soft metal its pretty easy to move it around. The metal is not spring steel so when you move it stays put. I would assume bending it many times would cause it weaken.... but that is not required for this task.
You should remove the one spark wire holder 8mm nut and a allen screw. This is the holder on the valve cover.
You should also remove that 10mm nut on hook holding the wire harness and grounding the ground wire.
I created a allen wrench by cutting the long part of a allen wrench off and stuffing it into a 4mm 1/4 inch socket. I attached that to a 12inch extension....this gave me about 16 inches of extension point straight back. Avoid using a universal will only cause you problems when turning.
Like I said the vertical hammering on the screw while applying twisting force is the only way you can get enough break force on the screw if your head are rough like mine.
Ahh one more detail.... i used a vacuum hose connect to a can of penetrating oil to clean the grime out of the screws before sticking the allen wrench in the hole....this will get you a tiny bit more bite on that head. Mine were filled half filled with grim.

FYI: Feel like I committed a little sin on that hook, figured the hook would get pulled back and forth if it was actually used for pulling the engine. So the sin is not as bad as it sounds. Others might not feels the same...