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Randall C
Randall C, ASE Master Tech
Category: Chevy
Satisfied Customers: 12825
Experience:  ASE Master, L1, 32 years, Diagnostic / Electrical Specialist
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1988 chevrolet: 350 engine..a knock sensor code, but i..voltage

Resolved Question:

i have a 1988 chevrolet 1500 truck with a 350 engine. when i drive it for a while i get t/c i know this is a knock sensor code, but i put a new knock sensor in a while back. what voltage does the knock sensor work on and any other ideas would be appreciated. thanks
Submitted: 7 years ago.
Category: Chevy
Expert:  Randall C replied 7 years ago.

Hello, I wish I could tell you 100% what the problem is but a scanner needs to be hooked up to fully test the knock snsor and signals sent. I included the diag flowchart, wiring daigram as well as explanation in an effort to try and help. It is very possible there is an internal noise developing after hot and when oil is hot and thin that the sensor is detecting even if you can not hear the internal noise. they are fairly sensitive



Wiring Diagram For Code 43 - Electronic Spark Control (ESC) Circuit


Circuit Description :

Electronic spark control is accomplished with a module that sends a voltage signal to the ECM. As the knock sensor detects engine knock, the voltage from the ESC module to the ECM drops, and this signals the ECM to retard timing. The ECM will retard the timing when knock is detected and RPM is above about 900 RPM.

Code 43 means the ECM has been low voltage at CKT 485 terminal "B7" for longer than 5 seconds with the engine running or the system has failed the functional check.

This system performs a functional check once per start-up to check the ESC system. To perform this test, the ECM will advance the spark when coolant is above 95~C and at a high load condition (near W.O.T.). The ECM then checks the signal at "B7" to see if a knock is detected. The functional check is performed once per start-up and if knock is detected when coolant is below 95~C (194~F), the test has passed and the functional check will not be run. If the functional check fails, the "Service Engine Soon" light will remain "ON" until ignition is turned "OFF" or until a knock signal is detected.

Test Description : Numbers below refer to circled numbers on the diagnostic chart.

  1. If the conditions for a Code 43 are present, the "Scan" will always display "YES." There should not be a knock at idle unless an internal engine problem, or a system problem exists.
  2. This test will determine if the system is functioning at this time. Usually a knock signal can be generated by tapping on the right exhaust manifold. If no knock signal is generated, try tapping on block close to the area of the sensor.
  3. Because Code 43 sets when the signal voltage on CKT 485 remains low, this test should cause the signal on CKT 485 to go high, The 12 volts signal should be seen by the ECM as "no knock" if the ECM and wiring are OK.
  4. This test will determine if the knock signal is being detected on CKT 496 or if the ESC module is at fault.
  5. If CKT 496 is routed to close to secondary ignition wires, the ESC module may see the interference as a knock signal.
  6. This checks the ground circuit to the module. An open ground will cause the voltage on CKT 485 to be about 12 volts which would cause the Code 43 functional test to fail.
  7. Connecting CKT 496 with a test light to 12 volts should generate a knock signal. This will determine if the ESC module is operating correctly.

Diagnostic Aids :

Code 43 can be caused by a faulty connection at the knock sensor at the ESC module or at the ECM, Also check CKT 485 for possible open or short to ground.

See Diagnosis By Symptoms - No Trouble Codes Stored.



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