knock sensor question

[compression]

I was diagnosing a knock sensor circuit malfunctuon, and it turned out to be a ground issue, or at least I think/hope , I don't have a transmission in the car so there is no ground on the engine . So I added one for diagnostic purposes and it went away no more check engine light or code.

But just for shits and giggles I tapped the block with a hammer a few times to test the reading , key on ecm logging raw knock and knock Ret, and I didn't get anything at all, it didn't pick up any knock and I was hitting the block pretty decent..

Is that normal ? Does it have to be an internal noise for it to be picked up? If could I Have a wiring issue ? I was checking both ends of the wires constantly trying to figure out the code, soo really doubt its a wire issue but its not out of the question .

 

[gmtech121]

where did you add the ground to?

 

[steve]

The ECU knows when to listen for real knock. If the CAS isn't moving, it knows the engine isn't running and can't be knocking now can it?

 

[We're on Boost]

The 1g knock sensor is an interesting subject. When I first bought my DSM I started reading about the AEM EMS. AEM claimed that the 1g knock sensor was kinda useless and recommended using a 2g sensor instead (which you could do if you were using a EMS - but not with a stock 1g ecu). The first time I took my car to the local DSM shop, the shop put in a new knock sensor (~130,000 miles on the car at the time). They didn't even bother to test it they just said it was a good idea.

A couple years later when I got my ecu+ working, I was finally able to log actual knock voltage (right from the sensor) at 25 samples per second, and also log "knock counts" at the same time from the ecu. As far as I can tell, the 1g knock sensor (when reasonably new) works very well. Average voltage output is about 0.3 or 0.4 volts while cruising at a lazy 40 or 50 mph. When actual knock happens the voltage spikes to a little over 0.5 volt (as I remember), usually only for an instant - like 1 or 2 samples on my logger, which triggers the ecu to pull timing and send a knock count signal which ramps back down over a period of a second or so.

Anyway my bottom line I guess is:
1.) Don't be shy about just replacing the darned thing with a brand new OEM knock sensor. It's a pretty important piece.
2.) If you have a way of measuring or logging the actual knock voltage while your car is running (not just knock counts) you should.