knock sensor resistance?

[emilio]

before reassembly, i decided to test my knock sensors per the service manual. this was mostly out of curiosity since they weren't throwing any codes, though i didn't get a chance to pull DTCs before removing the engine.

the manual says to measure pins 1 & 2 (next to the connector tab) for "infinite" resistance, and anything else is a fail. OK, pretty simple...

all of mine measure around 4.9 megohms with a quality multimeter (Ideal 61-486, 40 megohm max range). i even tried torquing one to the block because they're piezo devices, but that didn't change the resistance value. the testing procedure doesn't specify anything about temperature or other conditions.

these probably weren't bad since it wasn't throwing codes; monitoring knock sensors is a huge part of the ECU's job, so it would know. could 4.9 Mohm indicate incipient failure? or, since they all show they same value, is 4.9 Mohm actually fine and maybe Audi equipment measures a smaller range that would show megohms as infinite?

bottom line, i'm not sure whether to replace them. if i don't, at least it's just service position and removing the IM to get to them.

- emilio

[Rol-co]

Once i had a knock sensor fault code, forgot to torque the bolts, i tested the sensors by connect them to the car and tap on them with a spanner and look at VCDS measuring blocks. You wil see reaction on the voltage.

[emilio]

i did some research and all the Bosch knock sensor data sheets say the impedance is >1 Mohm. that seems to contradict the Audi manual spec of any continuity being a fail.

bosch knock sensor data.jpg

i also ran across a BMW forum post that states 4.95 megohm is the typical value for Bosch sensors. so, i'm gonna send it.