RX300 YR2000 knock sensor fault

Owelly · November 16, 2017

Greetings.

I've done the usual searching but not found anything specific to my problem so here goes:

I bought a 2000 RX300 that had sat for two years due to needing welding and rear brakes. I did all that but the EML was on. It showed dozens of faults which were probably caused by a flat Battery. Once reset, the car had a very high tickover and the EML came back on showing the P0325 and P0330. Cleaning the AFM sorted the tickover problem but the P0325/P0330 still popped up. These numbers are obviously the two knock sensors and it's very unlikely that both have failed at the same time, but as I don't know the history of the car, they could have been on for a long time before it was parked up. Not having the time (or wanting the expense) to replace both sensors, I ran the car with a scope attached to the knock inputs at the ECU and they both seemed to be steady at 4k but as the fault was intermittent, it wasn't easy to replicate the fault without driving with the scope attached!! Anyhoo....I decided to bypass the two knock sensors and join the two wires under the bonnet nd fit another knock sensor on a bracket on top of the gearbox where it was easy to reach. It's made no difference to the EML! It still comes on showing the same fault codes. It does the same with no sensors attached and the loom unplugged at the ECU?!

Any suggestions as the car needs new tyres and exhaust which it won't get if I can't sort out the knok problem! I may just cut my losses and weigh it in!!

Cheers guys.

iwonder · November 16, 2017

I agree its unlikely to be both although it can happen. Based on that I would be checking wiring between ECU and the connector that links both sensors. Try scoping at the ecu aswell rather than the sensor and check for the same results as before

Owelly · November 17, 2017

Thanks for the reply. I did scope at the ECU end of the loom and it checked out fine but as it wasn't 'real' (ie not being driven to ping fault) it didn't prove much. I think the next step is to do a continuity check between the ECU and engine bay and go for a drive.

sorcerer · November 17, 2017

14 hours ago, Owelly said:

It does the same with no sensors attached and the loom unplugged at the ECU?!

Surely that points to an ECU fault then doesn't it? If sensors and loom are disconnected from the ECU but the ECU is still putting the light on, it can only be an ECU problem. Maybe corroded/wet/shorted connections or some electronics problem within the ECU itself maybe?

NemesisUK · November 17, 2017

Showing a fault condition when an expected connection isn't there would be normal, surely?

Joining the two inputs and fitting another sensor may well bring up a fault condition because the input is not within the expected range? Did you check to see if the test load exhibited the same values as the designed two sensors input?

iwonder · November 17, 2017

21 minutes ago, NemesisUK said:

Showing a fault condition when an expected connection isn't there would be normal, surely?

Joining the two inputs and fitting another sensor may well bring up a fault condition because the input is not within the expected range? Did you check to see if the test load exhibited the same values as the designed two sensors input?

Spot on. The latest code can relate to wiring fault, which is exactly what disconnecting the connector will reproduce. Sensor on a gearbox wouldn't replicate the expected range and therefore code will remain. Testing at ECU end with scope would cover the complete circuit and can be test at idle and revving.

Owelly · November 17, 2017

The confusing bit is that without the loom plugged into the ECU, the fault doesn't ping immediately. It would be good to know the 'pending' and 'fault' parameters because at the moment, it all seems a bit random!! The originals and my replacement sensors all show 1.7vA/C ay 4kHz with the engine running.

The other thing to mention is the ignition timing. It's doing as I'd expect and adjusting with the load. If there was a geniune knock, I'd expect to see the timing advancing to counteract the knock.