FW: Q Knock Sensors

[Brian Jones]

This was a message I picked up on another board, but thought the
Information may be useful here to someone.

The knock sensors are just tuned contact microphones which listen for
sounds, the ecu has a filter and a time slot [there could be 1 knock
sequences every 90 degrees at anytime from the spark plug ignition
until roughly TDC optimum burn is at 17 ATDC]. So the ecu ignores
sounds not in this time window of roughly 25 degrees off for 90
degrees then on again.
Each bank has its own sensors and accumulator [which has a decay
function] so a voltage is generated representing the number of knock
spikes per rpm per bank. A few every once in a while is normal in
transistion throttle changes. Too many in a short time leads to the
voltage going up. The ecu calulates the timing advance from load
[TPS, rpm, temp, etc] at steady cruise the most advanced timing
possible is used to improve mileage. Say it calculates 40 degrees
BTDC but there are to many knocks on bank #1, it decreases the
advavance by 3 to 37 on that bank, still to many knocks down to
34....still to many down to 30; yet the other bank which had no
knocks is still at 40 BTDC.
When the sensor fails [open] or the harness is corroded the self
check at startup [and running] doesn't receive a signal [plus the
550k ohm resistance is different] the ecu goes into fail safe mode.
Since a sustained knock can destroy the piston and weaken the wrist
pins max timing advance is reduced by 10 degrees or more depending on
coolant temperature [extra hot coolant -- it must be summer].
The knock sensors are not critical to engine operation thus no check
engine light until 96 [OBD2], a code is stored but only readable by
Consult.
The engine operates but not well, the gas mileage drops a little
[especially highway] and acceleration is sluggish at cruise and there
may be a little bucking or surge. Something is missing just vague.
Idle is not affected. Max power is down by 50-70HP and WOT
acceleration is weak!
Very common problem after 100-140k [or 6-8years] on all models. If
the engine is allowed to overheat the sensor plastic body will crack
and open the circuit [prima fecia evidence of the overheat].
If [when] the plenum is removed to replace hoses etc, the $80 sensor
harness should always be changed even if the sensors show good! Same
with changing one bad $150 sensor change them both.

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