Did lots of search and couldn't find much topic associated w/ knock sensor failure. I cleared the code ~ 2 days ago and it came back again today.

My understanding is that K.S hardly goes bad. If that's the case, what else can trigger the code ? Any help would be very much appreciated.

Are you sure you have the right error code? I assume you're getting this from the DM. What engine do you have?

A 1994 W124 should have an M104 3.2 litre engine.

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Steven Harrison

1993 300E2.8 - 196K
2001 E320 4MATIC - 86K

Help !!

That was a lucky guess - could have been a 119 or a 606. In any case code 25 on the 104 motor DM is "knock sensors or engine control module faulty". I'd say next step is to build Arthur's code reader and check the other PINs.

http://members.aol.com/ajdalton7/ledtool.jpg

talked to the local Indy Mech - he has no clue. Then talked to the dealership Mech - he thinks that it maight be related to foul S plugs. S plugs were replaced less than a 1,000 miles ago....makes me wonder if this is related to wiring harness issue.

<< plugs were replaced less than a 1,000 miles ago>>


Did you use Plats ??????????? He may be right.

Update - Thanks to Phil (Mercedesshop.com), I got hold of an image of knock sensor !

Came home from work, disconnected wiring harness that was attached to the K.S module and started the engine. I expected the check engine light to come on, but to my surprise, no light. I even used a long screw driver and tap on the engine block to emulate pinging . Nothing.....

Next step - will drive the car and see what happens.

KS will bring up a code 25 if the sensor calls for full 12 degrees retard.
You can check the sensors outut w/scope and tapping near it with a metal tool/hammer.
It is a piezo crystal that sends an ac signal to the ECU. If it is open or unplugged . it will fail the self-test and pop the code.

Got to love this forum....

Now I see what triggers the K.S. You said to test it w/ a scope. Any written procedures that I can access to ?..would like to ensure that it's the sensor that went bad before ordering a new part.

Again, thank you very much for your help.

[QUOTE=last993;1273779]Got to love this forum....

Now I see what triggers the K.S. You said to test it w/ a scope. Any written procedures that I can access to ?..would like to ensure that it's the sensor that went bad before ordering a new part.

A/C coupling
Amplitude-50mV/div
Time base -1 mS/div
Auto-trigger.

I have been told that a simple diyer tool is to use a small speaker or headphones to pick up the audio output , but I have never tried it.
I would imagine it works the same way you can test a speaker by scratching the leads across a 1.5 dc flashlight battery... makes sense. I will try it next time I have one laying around.
You actvate the piezo [ Knock sensor] by tapping something metalic near its vicinity... no need to have key On power , as the sensor is an a/c crystal generator when vibration activated ...............this is not to compare to scope testing, but it should at least tell one if the sensor is working.