DTC P0502 - VEHICLE SPEED SENSOR (VSS) CIRCUIT/LOW INPUT Isuzu Trooper II

1. Perform On-Board Diagnostic (OBD) System Check. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
2. Check speedometer. If speedometer works, go to step 10). If speedometer does not work, go to next step.
3. Disconnect VSS harness connector and turn ignition on (with engine off). Connect a test light to positive battery terminal. Probe VSS connector ground terminal. If test light illuminates, go to step 5). If test light does not illuminate, go to next step.
4. Repair sensor ground. Retest system.
5. With sensor disconnected, turn ignition on. Using DVOM, probe VSS connectors ground and voltage supply. If battery voltage is not displayed on DVOM, go to next step. If battery voltage is displayed, go to step 7).
6. Repair open or short to ground. After repair ensure meter fuse is not blown. Retest system.
7. With sensor disconnected, turn ignition on. Using DVOM, measure voltage between VSS connector ground and speedometer wire. If DVOM reads 7.5-8.0 volts, go to step 9). If reading is not as specified, go to next step.
8. Repair open or short circuit between speedometer and VSS. Retest system.
9. Replace speedometer. Retest system.
10. Turn ignition off. Disconnect MAF sensor connector. Remove air cleaner assembly. Disconnect 16-pin connector, located behind left headlight assembly. Connect jumper wire between battery voltage and transmission side of 16-pin connector Blue/Yellow wire. Using DVOM, measure voltage between ground and 16-pin connector Light Green wire. Disconnect Blue connector located next to 16-pin connector. Connect jumper wire between VSS ground at Blue connector Black/Red wire and ground. Raise and support rear of vehicle. Place transmission in Neutral. Rotating rear wheels by hand, observe DVOM readings. If reading toggles between .6 volt and 10 volts, go to step 12). If readings are not as specified, go to next step.
11. Replace VSS. Retest system.
12. Repair open or short between PCM and speedometer. Retest system. If problem was not found, go to next step.
13. Replace PCM. Retest system.

Diagnostic Aids

Check for poor connection, rubbed through wire insulation or a broken wire inside insulation at PCM and IAC motor. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection, and damaged harness. Ensure air intake system is not restricted. Check throttle body for blockage and carbon build up. Clean as necessary. Inspect for a large vacuum leak at brake booster or PCV. If DTC cannot be duplicated, check scan tool FAILURE RECORDS to determine mileage since last DTC was set.

1. Perform On-Board Diagnostic (OBD) System Check. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
2. Using scan tool, read DTCs. If any other DTCs are set, diagnose them first before proceeding. If no other DTCs are set, go to next step.
3. Start engine. Turn all accessories off. Using scan tool, monitor ENGINE SPEED and command engine speed up to 1500 RPM, down to 500 RPM, then back up to 1500 RPM. If ENGINE SPEED remains within 50 RPM of desired RPM for each command, check for an intermittent problem, see «DIAGNOSTIC AIDS»(ref-16268-S28347384262001010800000) . If operation is not as specified, go to next step.
4. Disconnect IAC harness connector. Install IAC Noid-Light (J37027A) in IAC harness connector. With engine running, monitor noid-lights while commanding engine speed up to 1500 RPM, down to 500 RPM, then back up to 1500 RPM. Both noid-lights should cycle Green and Red, but not OFF as RPM changes from 500-1500 RPM. If noid-lights flash as specified, go to step 6). If noid-lights do not flash as specified, go to next step.
5. Check IAC "A" high and low and "B" high and low circuits for an open, short to voltage, short to ground or poor connection at PCM. If a problem is found, repair as necessary. Retest system. If no problem is found, go to step 9).
6. Ensure throttle stop screw has not been tampered with. Check for stuck IAC valve or throttle linkage. Check for restrictions, collapse or blockage in air intake system. Check throttle body bore and IAC valve for excessive deposits. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
7. Check for faulty connection at IAC valve harness connector. If faulty connection is found, repair as necessary. Retest system. If connection is okay, go to next step.
8. Replace IAC valve. Retest system.
9. Replace PCM. Retest system.

Check for poor connection, rubbed through wire insulation or a broken wire inside insulation at PCM and IAC motor. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection, and damaged harness. Ensure air intake system is not restricted. Check throttle body for blockage and carbon build up. Clean as necessary. Check for vacuum leak. If DTC cannot be duplicated, check scan tool FAILURE RECORDS to determine mileage since last DTC was set.

1. Perform On-Board Diagnostic (OBD) System Check. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
2. Using scan tool, read DTCs. If any other DTCs are set, diagnose them first before proceeding. If no other DTCs are set, go to next step.
3. Start engine. Turn all accessories off. Using scan tool, monitor ENGINE SPEED and command engine speed up to 1500 RPM, down to 500 RPM, then back up to 1500 RPM. If ENGINE SPEED remains within 50 RPM of desired RPM for each command, check for an intermittent problem, see «DIAGNOSTIC AIDS»(ref-16268-S08919879452001010800000) . If operation is not as specified, go to next step.
4. Disconnect IAC harness connector. Install IAC Noid-Light (J37027A ) in IAC harness connector. With engine running, monitor noid-lights while commanding engine speed up to 1500 RPM, down to 500 RPM, then back up to 1500 RPM. Both noid-lights should cycle Green and Red, but not OFF as RPM changes from 500-1500 RPM. If noid-lights flash as specified, go to step 6). If noid-lights do not flash as specified, go to next step.
5. Check IAC "A" high and low and "B" high and low circuits for an open, short to voltage or short to ground. If a problem is found, repair as necessary. Retest system. If no problem is found, go to step 9).
6. Ensure throttle stop screw has not been tampered with. Check for stuck IAC valve or throttle linkage. Check for correct PCV valve installation. Ensure no vacuum leaks are present. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
7. Check for faulty connection at IAC valve harness connector. If faulty connection is found, repair as necessary. Retest system. If connection is okay, go to next step.
8. Replace IAC valve. Retest system.
9. Replace PCM. Retest system.

If DTC sets when an accessory is operated, check component for poor connection or excessive draw.

1. Perform On-Board Diagnostic (OBD) System Check. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
2. Using DVOM, measure voltage at battery. If reading is greater than 11.5 volts, go to next step. If reading is 11.5 volts or less, charge battery before proceeding to next step.
3. Install scan tool and select IGNITION VOLTS. Place vehicle in Neutral or Park. Start engine, raise engine speed to 2000 RPM and load electrical system (turn on headlights, blower, radio, etc.). If scan tool reads 12.8-14.1 volts, go to next step. If reading is not as specified, see ALTERNATOR & REGULATOR article.
4. Turn ignition off. Disconnect PCM harness connector. Turn ignition on, engine off. Using a DVOM, measure ignition voltage at PCM terminal A4 (Red/White wire). If output is equal to battery voltage, check for excessive current draw with ignition off. If voltage is not equal to battery voltage, go to next step.
5. Check for poor connections at PCM harness terminals. Repair as necessary. Retest system. If no problem is found, go to next step.
6. Check for an open ignition feed circuit to PCM. Repair as necessary. Retest system. If no problem is found, go to next step.
7. Replace PCM. Retest system.

If DTC sets when an accessory is operated, check component for poor connection or excessive draw.

1. Perform On-Board Diagnostic (OBD) System Check. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
2. Using DVOM, measure voltage at battery. If reading is less than 11.5 volts, go to next step. If reading is 11.5 volts or more, go to step 4).
3. Charge battery and clean terminals and cable connections. If voltage is less than 11.5 volts, replace battery. If voltage is 11.5 volts or more, go next step.
4. Turn all accessories off. Install scan tool and select ignition voltage parameter. Place vehicle in Park or Neutral. Start engine and increase engine speed to 2000 RPM. If scan tool reading is more than 2.5 volts greater than voltage measured in step 2) or 3), see ALTERNATOR & REGULATOR article. If voltage is not as specified, go to next step.
5. Check for excessive current draw with ignition off. If excessive current draw present, see «DIAGNOSTIC AIDS»(ref-16268-S35884706702001010800000) . If no excessive current draw is present, go to next step.
6. Replace PCM. Retest system.