Knock Sensor Bad? - How to test?
 

Hey guys...I've been searching for the last couple of days for some insight to my problem, but forgive me if it's already been covered.

So prior to getting my engine rebuilt, I had it tuned by Steve Kan, and everything was fine and dandy, with the knock sensor usually hovering between 0 and 30.

I then had my engine rebuilt (the front rotor housing was warped), and everything else was basically reused (the motor was built less than 2000 miles ago...probably warped due to overheating...possible air bubble in the coolant...but that's another story) So anyway, the front rotor was replaced with a brand new unit, the exhaust port ported to match the rear exactly (was streetported), and the knock sensor was moved from the warped rotor to the new rotor.

I finally got the car back, and during the trip, I noticed that the knock sensor was now reading up to around 150/160, but there's definitely not knocking as the engine is running perfectly with no problems, and the knock is totally independent of load, boost, whatever. I noticed that it was fairly highly correlated with RPMs (see attached chart), and it didn't really matter if I was under boost, cruising, or even off the throttle.

Now before I go spending $200 bucks for a new sensor, I decided to try some tests. First I tried the knock sensor test as stated in the manual (jumper b/w Ten and Ground, voltmeter to Men, hammer, check for voltage), and nothing happened. Then I tried to see if the Commander picked up any knock with the engine off but the ignition on, but no matter where I tapped or how hard, nothing registered under Knock on the Commander. Then, just to make sure, I disconnected the knock sensor and started the car up, no knock. I reconnected the sensor, started the engine, crazy knock.

Does this sound like a bad sensor to you guys? Can you guys register any knock on the Commander with the motor off and ignition on with the use of a small hammer?

Sorry for the long read...and thanks in advance!
Allen

Typically, you should see some knock always. I typically have 20-30 knock count just driving around easy. Every knock sensor reads a little differently, it seems, so some could be higher, some lower.

I'd just find a used knock sensor to swap out with - they should be a dime a dozen.

If it's overtightened onto the motor, I could see the readings getting thrown off.

Dale

Thanks Dale. So overtightening can also throw the readings off that much?

I'm attaching a gif of the graph (couldn't save the pic on my comp at home last night)

When I first installed my PFC in my present vehicle, I noticed my knock readings were in the 80s in the 2000 rpm range. I tried the hammer test while logging with Datalogit, no noticeable readings were recorded. I attempted to shield the knock sensor wire to no avail. I also ran the car momentarily without the alternator hooked up, in both situations the readings were still in the 80s in the 2,000rpm range.

Finally I purchased a new knock sensor. However the magnitude of the readings remained the same. When discussing this with Steve Kan, his response indicated that knock readings were somewhat car dependent.

Even though my situation seems similar to yours, your readings seem extremely high. Have you looked at the sensor; sometimes they lose their potting material.

Seems I saw something about overtightening. Just to be sure, I'd just go ahead and pick up a spare one. Double-check the shop manual - I think there is a torque spec.

Dale

Yup, my car's knock readings while cruising in the 2000 RPM range I get anywhere from 80-120 on the knock meter with zero boost. I don't trust the factory knock readings at all.

Knock sensor readings are valid only when the engine and driveline are loaded together during throttle/acceleration; not while in "unloaded" idle or cruise conditions.