Porsche Enthusiasts Club Forum

Why does an s2 need both and why did older 944's have two crank sensors instead and no hall sender?
Cheers

Mainly due to the advance in engine ECU controls

The cam sensor (hall) is used to sequence the injectors, not read up as to why they changed from 2 crank sensors to one and a cam sensor

On the older 2.5 engines one sensor is used to detect engine rotation and one to detect TDC. This gives accurate crank position, although as far as I understand will detect TDC on both firing and gas exchange strokes, so I'm not sure how it distinguishes. (Crank rotates 360 degrees, but due to 4 stroke, an engine has 720 degrees of rotation. Cam rotates 360 degrees for every 720 of the crank)

Later engines with a crank and cam sensor track the rotation of each. The relative position of the crank and cam are used to get the absolute position of the engine (across it's 720 degrees of rotation). Presumably this gets over any ambiguity over which stroke the TDC is on.

That's my understanding at least!

EDIT: This sparked (pun intended) a good conversation with a colleague. Our best guess is that the 2.5 engine uses batch injection (all injectors fire at the same time, priming the valves with fuel for the next 720 degrees of rotation) , so it doesn't actually matter which stroke TDC is detected on. Adding the cam sensor would allow for sequential injection to be used. That provides a good reason for the upgrade too!

That seems to be backed up by this rennlist thread: http://rennlist.com/forums/944-turbo-an ... iring.html

Hopefully someone who actually knows will comment now :p

Thanks for that, interesting.

has anyone had to test the crank sensor as per the clarks garage ? Wondered if using a multimeter on the plug to test ohms while the sensor is still in position in the bell housing would be ok or would you have to take it out completely? Knowing my luck the flywheel magnet/gap (or what ever it uses) would be against the sensor when testing.

That's basicly what I was getting at, he just put it over far better than me

You can test them in situ, the air gap use a penny

If the sensor needs changing does the sheath need to be replaced or is it a case that it falls to bits and has to be replaced?
Also out of interest, what's the purpose of the sheath?