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HOMESERVICE MANUALSCADILLAC1996SEVILLE STSREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODESDIAGNOSTIC TROUBLE CODESDTC P1520 - GEAR INDICATOR SYSTEMDIAGNOSTIC PROCEDURES
1996 Cadillac Seville STS

Diagnostic Procedures

1996 Cadillac Seville STSSECTION Diagnostic Procedures
  1. Perform On-Board Diagnostic (OBD) System Check. After performing OBD system check, go to next step.
  2. Ensure Park/Neutral switch and shift cable/lever are properly adjusted. Block drive wheels and apply parking brakes. Start engine. Move shift lever through each range and observe TRANS. RANGE and SEL. LEVER SW. in ENGINE 3 mode on scan tool. If scan tool displays match each other for the range selected, fault is not present. See DIAGNOSTICS AIDS. If scan tool display does not match the range selected, go to next step.
  3. If only TRANS. RANGE display matches each range selected, go to next step. If range selected does not match TRANS. RANGE display, diagnose using transmission DTC P1810.
  4. Turn ignition on, with engine off. Using scan tool in ENGINE 3 mode, observe SEL. LEVER SW A/B/C/P display. If all modes displays ON, go to step 13). If not all of the modes displays ON, go to next step.
  5. If any of the modes (A/B/C/P) stays on or off all of the time, regardless of selector position, go to next step. If only some of the modes stays on or off all the time regardless of selector position, go to step 15).
  6. If the mode(s) that did not change always display ON, go to step 9). If it displays OFF, go to next step.
  7. Store fluid life index as indicated on scan tool. Turn ignition to LOCK position and wait 30 seconds, then disconnect PCM harness connector C1 and transaxle range switch. Using DVOM, check resistance between ground and the circuit that were always off. Resistance should be 5 ohms. If resistance is less than 5 ohms, go to next step. If resistance is greater than 5 ohms, go to step 11).
  8. Repair short to ground on the affected circuits with low or no resistance.
  9. Store fluid life index as indicated on scan tool. Turn ignition off and wait 30 seconds, then disconnect PCM harness connector C1 and transaxle range switch. Using DVOM, check resistance between the transaxle switch connector and PCM harness connector C1 terminals on the circuit(s) that were always on. Resistance should be 5 ohms. If resistance is greater than 5 ohms, go to next step. If resistance is less than 5 ohms, go to step 12).
  10. Repair open in the circuit(s) that have high or infinite resistance.
  11. Reconnect PCM harness connector C1. Turn ignition on. Note SEL. LEVER SW A/B/C/P display on scan tool. If all the modes indicate ON, go to step 16). If not all modes indicate ON, go to step 17).
  12. Reconnect PCM harness connector C1. Turn ignition on. Note SEL. LEVER SW A/B/C/P display on scan tool. Connect jumper wire between ground and the transaxle range switch harness connector terminal circuits that were always on. If scan tool display changes from ON to OFF when circuit is jumpered, go to step 15). If scan tool display does not indicate a change from ON to OFF when circuit is jumpered, go to step 17).
  13. Turn ignition off. Disconnect transaxle range switch ground connector. Using DVOM, check resistance of ground connector to chassis ground. If resistance is 5 ohms or less, go to step 15). If resistance is greater than 5 ohms, go to next step.
  14. Repair open or high resistance in ground circuit to transaxle range switch.
  15. Check terminal contact at transaxle range switch. Repair as necessary. If terminal contact is okay, go to next step.
  16. Replace transaxle range switch.
  17. Check terminal contact at PCM. Repair as necessary. If terminal is okay, go to next step.
  18. Replace PCM. Program replacement PCM using required equipment.
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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.