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HOMESERVICE MANUALSCHEVROLET1997FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSIS (SINGLE PAGE)TRANSMISSIONAUTOMATIC TRANSAUTOMATIC TRANSMISSION DIAGNOSIS - 4L80-E (2-DIGIT CODES)DIAGNOSTIC TESTSDTC 24: OUTPUT SHAFT SPEED (OSS) SENSOR CIRCUIT LOWDIAGNOSTIC PROCEDURES
1997 Chevrolet Forward Control 5.7 R, Automatic

Diagnostic Procedures

1997 Chevrolet Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
  1. Connect scan tool to DLC. Turn ignition switch to ON position. DO NOT start engine. Using scan tool, record freeze frame and failure records for reference. Data will be lost when DTCs are cleared later in this test.
  2. Raise and support vehicle. Start engine and shift transmission into "3" (3rd gear) position. Gradually increase wheel speed. If transmission OSS increases with drive wheel speed, problem is intermittent. See DIAGNOSTIC AIDS. If transmission OSS does not increase with drive wheel speed, go to next step.
  3. Turn ignition off. Disconnect OSS sensor connector. Connect a DVOM set to AC voltage scale between OSS sensor terminals. Start engine and shift transmission into "3" (3rd gear) position. Slowly increase wheel speed. If voltage is more than .5 volt above 10 MPH, go to next step. If voltage is less than .5 volt above 10 MPH, replace OSS sensor, then go to step 11).
  4. Reconnect OSS sensor connector. Disconnect VSS buffer module connector. Measure voltage between OSS sensor input low and input high circuits at VSS buffer module connector. See WIRING DIAGRAMS . Start engine and shift transmission into "3" (3rd gear) position. With wheels rotating, slowly accelerate to 2000 RPM. If voltage is less than .5 volt, go to next step. If voltage is more than .5 volt, go to step 6).
  5. Check for open in OSS sensor circuits between OSS sensor and VSS buffer module. Repair as necessary, then go to step 11). If circuits are okay, check for short to ground or circuits shorted together. Repair as necessary, then go to step 11).
  6. Turn ignition off. Measure voltage between ground and ignition feed circuit at VSS buffer module connector. Turn ignition on. If voltage is more than 10.5 volts, go to next step. If voltage is less than 10.5 volts, check for open in ignition feed circuit. Repair as necessary, then go to step 11).
  7. With ignition on, measure voltage between VSS buffer module ignition feed circuit and ground circuit at VSS buffer module connector. See WIRING DIAGRAMS . If voltage is more than 10.5 volts, go to next step. If voltage is less than 10.5 volts, check for open in ground circuit. Repair as necessary, then go to step 11).
  8. With VSS buffer module connector disconnected and ignition on, measure voltage between ground and OSS sensor signal circuit at VSS buffer module connector. If voltage is 4.8-5.2 volts, go to step 10). If voltage is not 4.8-5.2 volts, go to next step.
  9. If voltage in previous step is more than 5.2 volts, check for short to voltage in OSS sensor signal circuit. Repair as necessary, then go to step 11). If voltage in previous step is less than 4.8 volts, check for open or short to ground in OSS sensor signal circuit. Repair as necessary, then go to step 11).
  10. Reconnect VSS buffer module connector. With DVOM set to DC volts and connected to a known good ground, backprobe OSS sensor signal circuit at VSS buffer module. Start engine and shift transmission into "3" (3rd gear) position. With wheels rotating, slowly accelerate engine to 2000 RPM. If voltage is 1.5-3.5 volts, check PCM/TCM for faulty or intermittent connections. Repair as necessary, then go to next step. If connections are okay, replace PCM/TCM, then go to next step. If voltage is not 1.5-3.5 volts, replace VSS buffer module, then go to next step.
  11. After repair is complete, select DTC on scan tool. Select "Clear Info" function. Select "Specific DTC" and enter DTC 24. Drive vehicle under steady acceleration. PCM/TCM must see an output speed of more than 200 RPM for 3 seconds. If DTC 24 is not present, repair is complete. If DTC 24 is still present, repeat test.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.