From: Wally Plumley [wplumley@bellsouth.net] Sent: Thursday, September 14, 2000 1:46 PM To: 928 Subject: [928] Re: Knock sensor question. At 10:10 AM 9/14/00 -0700, Tom wrote: >Here is a question for the EE types. How do the knock sensors on the 32 >valve engines work. There are 3 wire connectors to each. What are they used >for. Obviously there is some feed into the computer,but what is fed? Is it >voltage, a change in resistance value? Why 2 sensors? Do they interconnect >before feeding the brain? > >I would like to be able to monitor the sensor(s) with an oscilloscope, but >not sure what I am looking for or how to hook up to it. (Not an expert with >the scope, just a newbie). Not being overly hampered by facts, I can speak freely on the subject - Knock sensors are normally zirconium (?) crystals, which generate a piezoelectric signal when they see a very specific range of mechanical vibration frequencies. These frequencies are typically generated only during abnormal combustion (detonation) events. The sensors are mechanically coupled to the block (Why not the heads, I wonder?) so as to detect these vibrations. The computer sees the output signal from the sensors, and retards the ignition timing slightly. My impression (Jim can possibly give more info) is that the ignition timing is mapped with a min and a max advance (a complex 3-D plane, not a simple table), and that if the sensors are functional, the computer advances timing until the onset of knock, retards slightly, advances, retards, etc. If the sensors are not functional, the fail-safe mode is retarded, which costs performance. Wally From: Gainey, Michael CONTR [GaineyM@rucker.army.mil] Sent: Wednesday, December 27, 2000 8:52 AM To: 928 Subject: [928] RE: knock sensors To the list, I didn't see a reply to this question so I thought I'd "chime-in". A knock sensor is a piezo-crystal device that generates current when vibrated. It is much the same as an accelerometer except it is designed to produce the maximum output within a given vibration range associated with engine "knock" or "ping". Being a piezo-crystal device means that it requires no "excitation" power. If its disconnected the engine management system would never detect knock or ping and wouldn't retard the ignition as designed. Also, your engine management system may set a fault code if the sensor is disconnected, as some systems check all sensors during initial "boot" diagnostics to verify that there are no "open" circuits. You *need* your knock sensors operating properly. Mike Gainey 86.5 Dunkleblau Otto "Open the pod bay doors HAL...." From: Pirtle, John [John.Pirtle@ceridian.com] Sent: Thursday, December 28, 2000 2:37 PM To: 928 Subject: [928] RE: Intake removed 88 S4 Well this won't help KrisBob since the intake is off, but for anyone else with an S4+ .... If you want to check your knock sensors using Chris Lockhart's patent-pending A$$tech 2000 - 1. Go stomp around town and do some floored runs up to redline. (One good test is get on the freeway and accelerate hard from 80.) 2. Go home and remove the right fuel rail cover (pass. side). 3. Unplug the knock sensor connector (large plastic possibly melted connector toward rear). 4. Secure loose connector ends (keep them disconnected). 5. Go stomp around town and freeway some more. If the 2nd stomp-around is not as fun as the first, then your knock sensors are probably okay. If the 2nd stomp-around is just like the first, well, call your favorite supplier and spend some money. :( (But hey - think about how much performance you'll get!) :) My PO had moved a knock connector and it disconnected when the duct tape melted (!). My automatic became MUCH more willing to downshift after reconnecting the knock sensor (surprised the heck out of me on a left turn in the middle of an intersection with rear wheels going sideways!!). John Pirtle Atlanta 87 Auto (knock sensors are my friends) http://members.rennlist.com/pirtle http://www.928oc.org