Knock sensors on P2 turbos.

[precopster]

Had a 2001 XC70 come in for a service today with about 182,000kms with a low signal on front knock sensor. It would occasionally call up a engine system service light though that was probably the lack of thermostat internals which had been torn out by a previous repair shop causing a thermostat code.

I tried replugging the knock sensor terminal multiple times but the code persisted.

Removed the manifold which took me almost two hours as I wrestled with a ETM intercooler band clamp that was still in the factory position (about 8 o clock) and ETM wiring still behind the starter.

Tore a PCV cooling hose which failed to budge and finally got to the knock sensors. I had ordered a new Bosch pair of knock sensors from FCP.

The front knock sensor bolt was almost finger tight and rear knock sensor was tightened by a child. The front one was probably providing a low signal due to this however as I wasn't certain if a loosely fitted knock sensor would cause a code I went ahead and fitted the new pair. Really odd fault as it's almost unheard of for these to fail. It appeared to me that the factory hadn't torqued them properly. Original PCV still there though no pressure yet.

Any thoughts on this anyone??

[abscate]

My indie ( Volvo specialist ) told me knock sensors rarely fail. Even a sensor code is probably something else. I caught a set on special so I replaced mine anyway and the knock code didn't show up again....at least not yet.

All data does say dress the front sensor to 6 o'clock and rear to 3 o'clock....I found that doesn't work and front 3 o'clock , Rear 6 o'clock fits better.

[jimmy57]

KS makes a voltage when the piezo crystal inside is rattled. Loose bolt can be a cause for the code. The ECM diagnostics is to verify that the KS signal exceeds a threshold value whenever an engine speed and load is exceeded where the ECM can know the KS should have made at least some signal.
Connectors would wear from the vibration on the direct connect versions so the cable leads were added to isolate the vibration that could harm the terminals.
Newer engines can have direct connect but these use a more stable connector.

[precopster]

Thanks very much for both replies.

Knock sensor technology must be amazing. With so much vibration at varying frequencies already occurring on an engine how the crystals detect detonation frequencies must be quite a feat of technology.

We have another code on the car according to a message from the owner this morning. I hope it's not a KS code again. Will read it at 4:30pm and report.

[jimmy57]

The piezo sensors are "tuned" for the frequency range combustion "bad" noises make.

The first knock sensors were spring wires inside a copper or brass ring. The knocks would make the wires vibrate and touch the rings that were connected to the sensor body screwed to block. These sensors were crude and you could check them with a timing light. Hammer tap the exhaust manifold or block while watching a timing light pointed at the timing marks and you could see the retarding action. I'm talking about 25 plus years in the past.

[precopster]

Thanks jimmy your breadth of comprehensive mechanical knowledge always astounds me.

The car came back in with ECM-903F (throttle code) and a short term fuel trim code that I can't recall. No KS codes.

Could be the thorough clean I gave the throttle and some throttle cleaner entered the fine & delicate Italian componentry

[abscate]

The other thing that would be relatively easy to do is to "time gate" the KS signal to the combustion events determined by when the plug fires. That is, you know when to listen for knocking, so you ignore all the noise that occurs away from the cylinders firing. That would be simple software in the ECU

[draser]

The piezoelectric crystal is cut such that it favors the frequency of interest. Similar to the quartz crystal used in radio oscillators. The crystal does resonate on multiple frequencies but the dominant is several orders of magnitude higher.