3200GT Knock Sensor

[Mattserati]

Brain-picking time. My Mas is in for a service, and the check engine light came on as i was driving there. I am told that it is the knock sensor, and I need a new one (which involves removal of inlet manifold, new gaskets, may as well do cooling pipes, etc etc, please bend over sir). Just wondering if this is another of the sensors which constantly give false readings, or if there is an easy fix. Any advice appreciated,

Mattserati

[Pimpernel]

The knock sensors are fairly common on the 3200's.
There are two situated in the centre of the v, Front and Rear, if it’s the front one it can be replaced without removing the water well but still requires the plenum to be removed. If it’s the rear this one, conveniently placed under the starter motor which is in turn under the water well. If it is the rear one I would be inclined to have the front one replaced too but not vice versa. The water well gaskets or hoses aren’t a weak point so I wouldn’t have them changed only checked for tightness.
The SD2 print out should tell you if its Front or Rear, and no they don’t give false readings unless you have a serious mechanical fault. The symptoms of a knock sensor are: Engine light on
: Lack of boost (feels like turbos don’t work)
: Very difficult to get over about 70mph

Hope this helps Good luck!

[Mattserati]

Thanks Pimpernel. Just had one thought, which may prove to be a stupid one, but here goes anyway:
Is it possible that the knock sensor could have faulted due to different/dodgy fuel? Not really knowing how these things work, it seems logical in my head.

[Pimpernel]

Yes bad fuel can cause the fireing time to be inccorect which is what the knock sensors pick up on. You can delete the error in the ECU, fill the car with fresh fuel (not the whole car, just the fuel tank) then run the car before replacing any components,
Always worth checking before replacing!

[Nigelo]

Pimpernel said:

Yes bad fuel can cause the fireing time to be inccorect which is what the knock sensors pick up on. You can delete the error in the ECU, fill the car with fresh fuel (not the whole car, just the fuel tank) then run the car before replacing any components,
Always worth checking before replacing!

Not quite. The sensors are in fact Piezo-electric devices configured to detect vibration harmonics signalling the start of detonation. They are therefore preset to the precise frequency which detonation will create. For this reason, a noisy or clattery engine can interfere with knock detection and therefore the electronics are programmed to flag an error even where the engine operates outside its normal noise parameters. The sensors are in effect an electronic version of a doctors stethoscope mounted external to the engine and simply "listen". For these reasons, bad fuel cannot damage the sensors in any way but at worst would simply retard the ignition timing at the onset of what the system perceives as detonation.

SD2 / SD3 diagnostics will identify whether front (cyls 1,2,7 and 8) or rear (cyls 3,4,5,6) sensor is tripping and more particularly whether at "key on" or "in run" condition. There is a different fault finding procedure for each condition including continuity check on the associated wiring. If these checks pass, substitution of the malfunctioning sensor follows. If that does not cure the probs then the engine ECU is substituted to see if that provides a cure. This last point is expensive so ensure you trust your workshop as the unscrupulous will mug you.

If you have access to the Workshop Manual Electrics Volume, the continuity checks are as follows:

Cyls 3,4,5,6

• Item 1 Check there is continuity between the G wire from pin 2 of connector 11C and the G wire pin 1 connector 68 , restore if necessary.

• Item 2 Check there is continuity between the N wire from pin 28 of connector 11C and the N wire pin 2 connector 68 , restore if necessary.

• Item 3 Check that there is continuity between the ECU ground and the shield on pin 3 connector 68, verify that the shield is not in short circuit with one of the signals, restore if necessary.

Cyls 1,2,7,8

• Item 1 Check that there is continuity between the B wire from pin 1 of connector 11C and the B wire pin 1 connector 68 , restore if necessary.

• Item 2 Check there is continuity between the R wire from pin 40 of connector 11C and the R wire pin 2 connector 68 , restore if necessary.

• Item 3 Check that there is continuity between the ECU ground and the shield on pin 3 connector 68 , check that the shield is not in short circuit with one of the signals, restore if necessary.

Hope this helps