Porsche Enthusiasts Club Forum

91 S2 cab

When i put my engine back in( if i ever get it back ) ,can i refit the old knock sensors.
I have a spare secondhand set too.

Thanks

You can as long as they are good, and knock sensors are especially important with getting the best out of an S2 or S, as is the hall sensor, as either of the knock sensors, or the hall sensor, or all 3 can go down and the engine will still run.. just at reduced power and without the individual knock sensing ability of the S2 Motronic.

We usually check the hall sensor, crank sensor and knock sensors with a oscilloscope to test if they are working as they should, however, all 4 sensors use coil windings internally so all should be ok, as long as the ohms are in the right range.

I will be back in the workshop in a couple of days and see if I can find the ohms for all 3 types of sensors, even if I have to test new ones with a multimeter.

I suspect there are a lot of S and S2 models out there with non working hall or knock sensors though.

Thankyou for the reply.
I have a few multimeters so will be able to test them.
I shall await the figures,cheers

Did you have any luck finding those figures? I'd be interested to know so I can check my sensors before I put the engine back in my S.

I might be wrong but pretty sure the S2 has only one knock sensor at back of the engine reading the flywheel and a Hall Effect camshaft sensor at the front mounted behind the distributor on the end of the o/s camshaft .

Nick

Hi EDDIE, the S and S2 have a speed ref sensor on the bellhousing reading the flywheel, a hall effect sensor on the camshaft and two knock sensors mounted on the block under the inlet manifold, as shown here:

Cruddy engine, I know...but it got better!

Thanks for that I never knew they were there will have a look at mine.

Cheers

Nick

Oops, I just noticed this thread was still active with a question about the knock sensor resistances.

The correct resistance of the knock sensors should be between 270-330kohm.
However, I just checked a new one of a different part number which Bosch and Porsche specify as the original's replacement and it was reading in the range of mega ohms.

Which to be honest... I am more than a bit concerned about.

The knock sensor works as a microphone, and at the end of the day, as long as the signal getting to the engine management system is giving an accurate amplitude of sound level and an accurate signal quality, then it should work fine... However I am unsure if these new sensors will provide the correct signal levels/amplitude given the altered resistance.

So I have emailed Bosch for clarification on the issue.

So far it seems the original flat top knock sensors give a signal in the range of 270-330 k ohms, the new ones with a domed top give a reading in mega ohms.

I will let you know when I have more information from Bosch, as it could be a mistake that they have cross referenced this part number over.

Thankyou for the reply.
Not sure if mine are domed or not,will have a look then test,cheers

The knock sensors are piezoelectric devices (like crystal microphones) not magnetic coil, so I would expect resistance range of order of Megaohms.

The heart of the device may be fine and resistance reading normal, but it may still need replacing. Usual failure mode is the outer cable sheath and connector crumble, so it's susceptible to electrical noise interfering with sensor signal. The ecu can interpret spurious electrical noise as triggered by knock, so retards timing and power reduced.

Always good idea to replace sensors if you're in there unless inspection shows insulation, screening and connector are fine. They are in a very hostile environment!

^^^ That makes sense regarding the construction of the knock sensor, it couldn't be a hall effect or mag coil as it just bolts on the outside of the block.

One thing to be careful with is taking them off, remember that the bolts will probably be corroded in place, so all usual precautions regarding removing them must be taken. Also, the screened cable and connector is part of the engine wiring harness, not part of the knock sensor itself. So, if that is shot you will have to do a harness side repair.