Knock problem
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,469
Likes: 6
From: Winchester Ontario
Knock problem
I think it was here that I heard that the coolant temp sensor in the head and the knock sensor share the same circuit or something like that. Well, the connector to the coolant temp sensor is getting weak and needs to be replaced (getting old) and I sometimes get a low coolant light. The rad is topped up, so I know that's not it, and my mechanic has "burped" the system to get rid of air.
My car also knocks and pulls up to 7 degrees of timing at WOT and I wonder if it is due to the coolant temp sensor problem.
How do I check to see if the knock sensor is working properly. I have heard about tapping the block with a hammer while running and seeing it retard there. Is there a way to check the volts to make sure it's running properly and properly grounded and such?
My car also knocks and pulls up to 7 degrees of timing at WOT and I wonder if it is due to the coolant temp sensor problem.
How do I check to see if the knock sensor is working properly. I have heard about tapping the block with a hammer while running and seeing it retard there. Is there a way to check the volts to make sure it's running properly and properly grounded and such?
Drifting
Joined: Jul 2002
Posts: 1,488
Likes: 4
From: Frostburg MD
I don't think they share the same circuit, it's just that both sensors use the same style of connector. To test the know sensor, just hit the block near it, sometimes hitting the exhaust will work, and a count should show up on a scan tool if you have one. I'm not sure how to test it electrically.
Drifting
Joined: Apr 2003
Posts: 1,379
Likes: 4
From: rome ny
i am really annoyed with the stupid knock sensor... if a carb engine doesnt need it why do we. i am inclined to rip the dam thing off. it is too hard to tell if you are getting false or real knock. the only good thing about the knock sensor is if you get a bad load of fuel.. other then that if your timing is set right and you are getting true knocks then instead of pulling the timing back get it over and let the motor blow so i can build a new one... 
Advanced
Joined: Jul 2004
Posts: 80
Likes: 0
FWIW, the knock sensor gets a 5v signal from the ECM and the ECM expects a return voltage from the KS (that's why you can't just take it off the engine and discard it). Water cooled automotive engines vibrate at between 5-8 kHz when they knock and there's a piezoelectric disk inside the KS that changes the return signal to the ECM under knocking conditions based on the vibrations it senses. The ECM then retards the spark timing to try and avoid the knocking condition. On some engines the knock sensor can be removed from the block, grounded, and placed in a rag to fool the ECM - not sure about Corvettes. There's a lot of information in the literature on knock sensors; you can probably find some on the SAE website under technical papers. P.S. According to my OTC, my 91 retards the timing >10 degrees sometimes.
Race Director
Joined: Dec 1999
Posts: 11,182
Likes: 2
From: Almost all Skyline Cruises Vettes at Waterside 1-5
Cruise-In I Veteran
Cruise-In II Veteran
Cruise-In III Veteran
Cruise-In IV Veteran
Cruise-In V Veteran
Cruise-In VI Veteran
Cruise-In VII Veteran
Originally Posted by Old182
FWIW, the knock sensor gets a 5v signal from the ECM and the ECM expects a return voltage from the KS (that's why you can't just take it off the engine and discard it). Water cooled automotive engines vibrate at between 5-8 kHz when they knock and there's a piezoelectric disk inside the KS that changes the return signal to the ECM under knocking conditions based on the vibrations it senses. The ECM then retards the spark timing to try and avoid the knocking condition. On some engines the knock sensor can be removed from the block, grounded, and placed in a rag to fool the ECM - not sure about Corvettes. There's a lot of information in the literature on knock sensors; you can probably find some on the SAE website under technical papers. P.S. According to my OTC, my 91 retards the timing >10 degrees sometimes.
The Vete modules are too sensitive and signal the ecm to retard when they shouldn't!
This is mot the knock sensor itself, but the module that is attached to the passenger side hood support/lock mechanism
seeya
Drifting
Joined: Dec 2002
Posts: 1,861
Likes: 4
From: Houston TX
Yes, the knock sensor circuit is too sensitive on Vettes. On an LT1 I've been working on that has an angry cam in it, the valve ringing as the valves close is close enough to the resonant frequency of the block to trigger knock. There is a way within the ECM programming (at least there is on the LT1s...L98s I'm not sure) to disable the knock sensors. If you can't do it through the ECM program, you can do it by wiring a resistor into the circuit, but I can't remember off the top of my head what resistance....3k ohm rings a bell for some reason.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski