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ksengage31, ASE Certified Technician
Category: Car
Satisfied Customers: 61
Experience:  Master Certified Technician Specialist in driveability and engine diagnostics.
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knock sensors on my 1995 4.3 vortec

Customer Question

i have replased both knock sensors on my 1995 4.3 vortec motor and want to by pass the knock sensor, because weather they are hooked up or not the motor still runs the same. slugish and like the timing isnt advancing like it should
Submitted: 4 years ago.
Category: Car
Expert:  ksengage31 replied 4 years ago.

ksengage31 :

Hi, my name is XXXXX XXXXX i'm happy to offer you some help today.

Expert:  ksengage31 replied 4 years ago.
My first inclination as a technician would be of course to test the timing advance and confirm that it is indeed the problem. However, i have read that the knock sensors can be eliminated/fooled by shorting the signal wire to a 5v+ reference wire. Can't tell you exactly how the car's computer will react to not having this input, but some of the DIY fuel injection tuners are doing this in addition to their custom ecm programming.

That said, if you have access to a timing light, i could walk you through the process of testing the knock sensors to determine if they are indeed an issue.

Customer: replied 4 years ago.
Yes I have a timing light
Expert:  ksengage31 replied 4 years ago.
Great, this test is really easy. With your timing light hooked up, and the est wire connected (normal ecm control of the timing) monitor the timing mark with your light. Have a helper tap on the engine block, lightly, near the knock sensors with a small hammer. It doesn't take much to get a response from them, and you should see the timing change significantly at the pointer under your timing light. The same can be accomplished monitoring with a scan tool capable of reading timing advance on the ecm, but this simple test works just fine.

If for some reason you are already seeing a timing number less than zero degrees on the pointer at idle and higher rpms normally, i would suspect an issue. Does this engine have an unusual amount of wear/miles/noise?

Customer: replied 4 years ago.
Only 30000 miles. I try that already. Does nothing. The timing is set right. It bounces around from 4 rev to about 10tdc. And when I check normal timing by unpluging the brown wire under the glove box like it says to do in the timing procedure and set at 0tdc it is set right. This is why I want to bypass the knock sensors. That there is 2 of. I have custom intake and exhaust and headers on this rig and have heard that the knock sensors are sensitive. And that the headers can set the knock sensors off.
Expert:  ksengage31 replied 4 years ago.
Gotcha. Youre right on with the headers, they will likely cause some interference. Try shorting the signal wires and retest in the same way.
Customer: replied 4 years ago.
What do you mean shorting the signal wire. I have reset the computer by disconecting the battery and it runs fine unfilled I take of and then its goes slugish again. It doesn't run right till high done and even then it only does that some times.
Expert:  ksengage31 replied 4 years ago.
In order to "bypass" them you need to provide the pcm with a constant value to read. Thats the idea with running a 5v wire spliced to the knock sensor signal wires (sensors unhooked).

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