Engine Controls - System & Component Testing: Diagnosis Isuzu Axiom I

Circuit Description

A properly operational EGR system will directly affect air/fuel requirements of the engine. Since exhaust gas being introduced in air/fuel mixture is an inert gas, less fuel is required to maintain a correct air/fuel ratio. Introducing exhaust gas into combustion chamber lowers combustion temperatures and reduces formation of NOX in the exhaust gas. Lower combustion temperature also prevent detonation. If EGR pintle were to remain closed, eliminating EGR, air/fuel mixture would be leaner. PCM would compensate for the lean condition by adding fuel, resulting in higher long term fuel trim values.

Diagnostic Aids

An EGR valve pintle constantly in the closed position could cause detonation and high NOX emissions. It could also result in high long term fuel trim values in the open throttle cell, but not in the closed throttle cell. An EGR valve pintle constantly in the open position would cause a rough idle. A EGR valve mounted incorrectly (rotated 180°) could cause a rough idle. Check EGR passages for restricted or blocked EGR passages. Check MAP sensor. If MAP sensor calibration that has shifted will affect fuel delivery.

Canister purge is controlled by a solenoid valve that allows manifold vacuum to purge EVAP canister. PCM supplies ground to energize the solenoid valve (purge On). EVAP purge solenoid On time is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. Duty cycle is calculated by the PCM and the purge solenoid is enabled when appropriate conditions have been met

1. Engine run time after start is 60 seconds or more.
2. Engine coolant temperature is 86°F (30°C) or more.
3. Fuel control system is operating in closed-loop mode.

1. Make a visual check of vacuum hoses.
2. Check for damage to throttle body.
3. Check for faulty malfunction indicator.

Diagnostic Procedure

1. At throttle body disconnect vacuum hose that goes to EVAP purge valve solenoid. Turn ignition on. Using a hand-held vacuum pump, apply 10 in. Hg (34 kPa) to EVAP purge valve solenoid. If valve holds vacuum, go to step 3 . If valve does not hold vacuum, leave ignition on and go to next step.
2. Disconnect EVAP purge valve solenoid harness connector. Using a hand-held vacuum pump, apply 10 in. Hg (34 kPa) to EVAP purge valve solenoid. If valve holds vacuum, go to step 4 . If valve does not hold vacuum, go to step 7 .
3. At throttle body plug disconnected vacuum port. Connect Tech 2 or equivalent scan tool to Data Link Connector (DLC). DLC is located under small cover on lower left side of instrument panel. On scan tool select MISCELLANEOUS TESTS and then EVAP test. Using a hand-held vacuum pump, apply 10 in. Hg (34 kPa) to EVAP purge valve solenoid. Start engine and run at 2500 RPM. Using scan tool, command EVAP purge valve solenoid On. If vacuum dropped when valve commanded on, go to step 8 . If vacuum did not drop when valve was commanded on, go to step 9 .
4. Check for open or short to ground in Red/Blue wire (purge valve control signal) between purge valve and PCM. See «WIRING DIAGRAMS»(ref-133147) article. If problem is found, go to next step. If no problem is found, go to step 6 .
5. Repair open or short to ground in Red/Blue wire (purge valve control signal) between purge valve and PCM. After repair, retest system.
6. Replace and reprogram PCM. See POWERTRAIN CONTROL MODULE REPLACEMENT under POWERTRAIN CONTROL MODULE in appropriate SELF-DIAGNOSTICS article. After repair, repeat OBD-II system check. See ON-BOARD DIAGNOSTIC II SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.
7. Replace EVAP purge valve solenoid. After repair, retest system.
8. Turn ignition off. At throttle body, install vacuum gauge at vacuum port. Start engine and run at 2500 RPM. Momentarily snap throttle open, allowing engine to return to idle. If vacuum maintains at least 10 in Hg. (34 kPa), testing is complete. If vacuum is not maintained, check for intermittent problem. See «DIAGNOSTIC AIDS»(ref-133140-S15623463482002092400000) .
9. Turn ignition off. Disconnect EVAP purge valve solenoid 2-pin harness connector. Connect test light between the harness connector terminals. Turn ignition on. If test light illuminates, go to step 7 . If test light does not illuminate, go to next step.
10. Using test light connected to ground, probe EVAP purge valve solenoid harness connector terminals one at a time. If test light illuminates at both terminals, go to next step. If test light does not illuminate at both terminals, go to step 12 .
11. Repair short to voltage in Red/Blue wire (purge valve control signal) between purge valve and PCM. After repair, retest system.
12. If test light illuminated on EVAP purge valve solenoid harness connector terminal No. 2 (Black/Yellow wire), go to next step. If test light did not illuminate on EVAP purge valve solenoid harness connector terminal No. 2, go to step 14 .
13. Check for open in Red/Blue wire (purge valve control signal) between purge valve and PCM. If problem is found, go to step 15 . If no problem is found, go to step 6 .
14. Repair open in Black/Yellow wire (purge valve ignition signal) between fuse box and EVAP purge valve solenoid. After repair, retest system.
15. Repair open in Red/Blue wire (purge valve control signal) between purge valve and PCM. After repair, retest system.

For fuel evaporative system testing, see related DIAGNOSTIC TROUBLE CODE DEFINITIONS in appropriate SELF-DIAGNOSTICS article.

FUEL PRESSURE TEST

Note. For fuel system pressure testing, see FUEL PUMP PRESSURE TEST procedure in BASIC DIAGNOSTIC PROCEDURES article.

FUEL INJECTOR BALANCE TEST

Note. Fuel injector Tester (J 39021) and Injector Switch Box (J 39021-90) are required for testing on all models.

Note. Fuel Injector Balance Test checks the mechanical (fuel delivery) portion of the fuel injector. An engine cool-down period of 10 minutes is necessary to avoid irregular fuel pressure readings due to "Hot Soak" fuel boiling.

1. Perform On-Board Diagnostics II (OBD-II) System Check. See ON-BOARD DIAGNOSTICS II (OBD-II) SYSTEM CHECK under SELF-DIAGNOSTICS in appropriate SELF-DIAGNOSTICS article. After performing OBD-II system check, go to next step.
2. Turn ignition off. Release fuel system pressure. See «FUEL SYSTEM PRESSURE RELEASE»(ref-133140-S00335660722002091900000). Connect Fuel Pressure Gauge (J 34730-1) or equivalent to test port on injector rail. Turn ignition on. Using scan tool, command fuel pump ON. Place bleed hose of fuel pressure gauge in an appropriate container. Bleed air from hose. With fuel pump running observe fuel pressure gauge until highest pressure is observed. Fuel pressure should be 43-55 psi (296-376 kPa). If fuel pressure is within specifications, go to next step. If fuel pressure is not within specifications, see «FUEL SYSTEM DIAGNOSIS»(ref-133136-S33194148372002021300000) procedure in BASIC DIAGNOSTIC PROCEDURES article.
3. Turn fuel pump off. Observe fuel pressure for several minutes. If fuel pressure drops a little and then remains constant, go to next step. If fuel pressure does not perform as specified, see «FUEL SYSTEM DIAGNOSIS»(ref-133136-S33194148372002021300000) procedure in BASIC DIAGNOSTIC PROCEDURES article.
4. Turn engine off. Disconnect fuel injector tester 5 or 7-pin in-line harness connector. In-line harness connector is located near front of valve cover and is identified by Blue locking plate. Connect Fuel Injector Tester (J 39021-5V) and Injector Switch Box (J 39021-90) to fuel injector harness. (Scheme 4) Set amperage supply selector switch of Fuel Injector Tester to 0.5-2.5 amp position, and go to next step.
5. Using scan tool, command fuel pump ON until fuel pressure is fully pressurized. Turn fuel pump Off. Record fuel pressure. Energize first fuel injector by depressing PUSH TO START TEST button on fuel injector tester. Record fuel pressure after fuel pressure gauge needle stops moving. (Scheme 4)
6. Repeat previous step for remaining fuel injectors. Subtract second pressure reading from first reading for each injector. The difference in readings is pressure drop value. Record pressure drop value for each fuel injector. Add all pressure drop values to obtain total pressure drop. Divide total pressure drop by number of fuel injectors. If any individual injectors pressure drop is more or less than 1.5 psi (10 kPa) when compared against average pressure drop, go to next step. If all individual injector pressure drops are within specification, go to «FUEL INJECTOR COIL TEST»(ref-133140-S13427645382002021300000).
7. Retest any fuel injector that does not perform as specified. If faulty fuel injector pressure drop value is still not as specified, replace fuel injector. See «FUEL RAIL & FUEL INJECTORS»(ref-133146-S27698187692002021300000) in REMOVAL, OVERHAUL & INSTALLATION article.

Scheme 4

FUEL INJECTOR COIL TEST

Note. Fuel injector Tester (J39021) and Injector Switch Box (J39021-90) are required for testing on all models.

1. Perform On-Board Diagnostics II (OBD-II) System Check. See ON-BOARD DIAGNOSTICS II (OBD-II) SYSTEM CHECK under SELF-DIAGNOSTICS in appropriate SELF-DIAGNOSTICS article. After performing OBD-II system check, go to next step.
2. Turn ignition off. To prevent flooding of engine, relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELEASE»(ref-133140-S00335660722002091900000) . Turn engine off. Disconnect fuel injector tester 5 or 7-pin in-line harness connector. In-line harness connector is located near front of valve cover and is identified by Blue locking plate. Connect Fuel Injector Tester (J 39021-5V) and Injector Switch Box (J 39021-90) to fuel injector harness. Connect DVOM leads to tester. On tester, set amperage supply selector to COIL TEST 0.5 amp position. If engine coolant temperature is between 50-90°F (10-35°C), go to next step. If engine coolant temperature is more than specified, go to step 5 .
3. Set injector switch box to injector No. 1. Press PUSH TO START TEST button. Observe voltage reading on DVOM. After one second, record lowest voltage observed on DVOM. Voltage reading should be 5.7-6.6 volts. Move injector switch box to next injector and repeat test until all injectors have been tested. If voltage is erratic and does not stabilize or is outside of specification at any injector, go to next step. If all injectors voltages test within specification, go to «FUEL INJECTOR BALANCE TEST»(ref-133140-S06163627632002021300000) .
4. Replace suspect fuel injector. See «FUEL RAIL & FUEL INJECTORS»(ref-133146-S27698187692002021300000) in REMOVAL, OVERHAUL & INSTALLATION article.
5. Set injector switch box to injector No. 1. Press PUSH TO START TEST button. Observe voltage reading on DVOM. After one second, record lowest voltage observed on DVOM. Voltage reading should be 9.5 volts. Move injector switch box to next injector and repeat test until all injectors have been tested. If voltage is erratic and does not stabilize or is more than specification at any injector, go to step 4 . If all injectors voltages test within specification, go to next step.
6. Observe recorded voltages. Identify highest voltage reading (other than those above 9.5 volts). Subtract voltage reading of other injectors from highest reading. If subtracted value is 0.6 volts or more, go to step 4 . If subtracted values are as specified, go to «FUEL INJECTOR BALANCE TEST»(ref-133140-S06163627632002021300000) .

PCM controls engine speed with the Idle Air Control (IAC) valve. To increase idle speed, PCM retracts IAC pintle away from its seat, allowing more air to bypass throttle bore. To decrease idle speed, it extends IAC valve pintle towards its seat, reducing bypass air flow. Scan tool reads PCM commands to IAC valve counts. Higher counts indicate more bypass (higher air). Lower counts indicate less air is allowed to bypass (lower idle).

A slow, unstable or fast idle may be caused by a non-IAC system problem that cannot be overcome by IAC valve. Out of control range IAC counts will be above 60 as observed on scan tool if idle is too low, and zero counts if idle is too high. Following checks should be made to repair a non-IAC system problem

1. Vacuum leak (high idle) - If idle is too high, stop engine. Using IAC Motor Analyzer (J39027-A), fully extend IAC. Start engine. If idle speed is 800 RPM or more, locate and correct vacuum leak, including PCV system. Check for binding of throttle blade or linkage.
2. Lean heated oxygen sensor signal (high air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down, and disconnecting IAC valve does not help. If DTC P0131, P0151, P0171, or P0174 is stored in memory, perform appropriate test. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-133138-S22707626342002081900000) in SELF-DIAGNOSTICS - RODEO & RODEO SPORT 2.2L article. On scan tool HO2S No. 2 voltage will be less than 100 mV. Check for low regulated fuel pressure, water in fuel, or a restricted injector.
3. Rich heated oxygen sensor signal (low air/fuel ratio) - Idle speed will be too low. On scan tool IAC counts will usually be 80 or more. System is obviously rich and may exhibit black smoke in exhaust. H02S voltage will be fixed at about 750 mV. Check for high fuel pressure, or a leaking or sticking injector. A silicon-contaminated heated oxygen sensor will show an HO2S voltage slow to respond on scan tool.
4. Throttle body - Remove IAC valve and inspect throttle bore for foreign material.
5. IAC valve electrical connections - IAC valve terminal connections should be carefully checked for proper contact.

If intermittent poor driveability or idle symptoms are resolved by disconnecting IAC harness connector, carefully recheck terminal connections and IAC valve terminal resistance, or replace IAC.

1. Turn ignition off. Connect Tech 2 or equivalent scan tool to Data Link Connector (DLC). DLC is located under small cover on lower left side of instrument panel. Connect IAC motor analyzer (J39027-A) to IAC valve. Set parking brake. Block wheels. Turn A/C off. Start engine, and allow to idle. On scan tool, observe RPM. Using IAC motor analyzer extend and retract IAC valve. RPM should increase and decrease as IAC is cycled. If RPM changes as commanded, go to next step. If RPM does not change, go to step 3 .
2. RPM should change smoothly with each flash of IAC motor analyzer light. RPM should change from 700-1500 RPM. If RPM changes as specified, go to step 6 . If RPM does not change as specified, go to step 5 .
3. Remove IAC valve. Check IAC passages for blockages. If problem is found, clean any obstructions as necessary. If no problem is found, go to next step.
4. Replace IAC valve. See «FUEL SYSTEM»(ref-133146-S21790669332002021300000) in REMOVAL, OVERHAUL & INSTALLATION article. After repair, retest system.
5. Turn ignition off. Disconnect PCM Red 32-pin harness connector C1. PCM is located under center console in front of shifter. Following IAC motor analyzer instructions, install appropriate noid light to PCM harness connector. Turn ignition. Observe noid light while cycling IAC motor analyzer. Noid lights should cycle from red to green, but never off. If noid lights cycle as specified, go to next step. If noid lights do not cycle as specified, go to step 7 .
6. At IAC motor analyzer pigtail, use DVOM to measure resistance between terminals "A" and "B". Measure resistance between terminals "C" and "D". Resistance should be 40-80 ohms at both measurements. If resistance is as specified, go to step 8 . If resistance is not as specified, go to step 9 .
7. Check for poor, loose or damaged terminal connections in IAC valve. Check for open, short to ground or short to voltage. Check for poor, loose or damage terminal connections in PCM. If problem is found, go to step 12 .
8. At IAC motor analyzer pigtail, measure resistance between terminals "B" and "C". Measure resistance between terminals "A" and "D". Resistance should be infinite. If resistance is as specified, go to step 10 . If resistance is not as specified, go to step 11 .
9. Replace IAC valve. See «FUEL SYSTEM»(ref-133146-S21790669332002021300000) in REMOVAL, OVERHAUL & INSTALLATION article. After repair, retest system.
10. Check IAC valve and related circuits. If problem is found, check for intermittent problem. See «DIAGNOSTIC AIDS»(ref-133140-S41175045912002092400000) . If no problem is found, go to next step.
11. Replace IAC valve. See «FUEL SYSTEM»(ref-133146-S21790669332002021300000) in REMOVAL, OVERHAUL & INSTALLATION article. After repair, go to step 8 .
12. Repair or replace faulty components. After repair, go to step 5 .

When air conditioning and blower fan are selected, and system has a sufficient refrigerant charge, a 12-volt signal is supplied to the A/C request input of the PCM. A/C request signal may be temporarily canceled during system operation by the electronic thermostat in the evaporator case. When A/C request signal is received by PCM, PCM supplies a ground from A/C compressor clutch relay if engine operating conditions are within acceptable ranges. With A/C compressor relay energized, voltage is supplied to the compressor clutch coil. PCM will enable the compressor clutch to engage whenever A/C has been selected with engine running, unless any of the following conditions are present

1. Throttle is open 90% or more.
2. Ignition voltage is less than 10.5 volts.
3. Engine speed is more than 4500 RPM for 5 seconds.
4. Engine coolant temperature 257°F (125 °C) or more.
5. Intake air temperature is less than 41°F (5°C).
6. Power steering pressure switch signals a high pressure condition.

To diagnose an intermittent fault, check for following conditions

1. Poor connection at PCM. Inspect connections for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connection.
2. Inspect wiring harness for damage. If wiring harness appears to be okay, observe A/C clutch while moving connectors and wiring harnesses related to the A/C. A sudden clutch malfunction will indicate source of intermittent fault.

1. Perform On-Board Diagnostics II (OBD-II) System Check. See ON-BOARD DIAGNOSTICS II (OBD-II) SYSTEM CHECK under SELF-DIAGNOSTICS in appropriate SELF-DIAGNOSTICS article. After performing OBD-II system check, go to next step.
2. If any DTCs were stored in memory, perform appropriate test before continuing. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in appropriate SELF-DIAGNOSTICS article. If no DTCs were stored, go to next step.
3. Disconnect A/C pressure switch harness connector. Pressure switch is located on receiver/drier behind left side of grill. Using DVOM check continuity of A/C pressure switch. If continuity does not exist, repair cause of open. For testing procedure, see appropriate AIR CONDITIONING & HEATINGarticle. If continuity exists, go to next step.
4. Ensure A/C is switched off. Start engine and allow to idle for at least one minute. Observe A/C compressor clutch. If A/C compressor clutch is engaged even though switch is off, go to step 37. If A/C compressor clutch is not engaged, leave engine running with A/C on and go to next step.
5. Turn A/C and blower on. Observe A/C compressor clutch. If A/C compressor clutch engages, check for intermittent problem. See «DIAGNOSTIC AIDS»(ref-133140-S03064105242002092400000). If A/C compressor clutch does not engage, leave engine running and go to next step.
6. On scan tool, select MISCELLANEOUS TESTS, and then RELAYS test. If A/C REQUEST display indicates YES, go to step 26. If scan tool does not display as specified, go to next step.
7. Temporarily switch A/C compressor and A/C thermostat relays. Relays are located in underhood fuse/relay box. Repeat test starting in step 5. If A/C REQUEST display now indicates YES, go to next step. If scan tool still does not display as specified, go to step 9.
8. Replace original A/C thermostat relay. After repair, retest system.
9. Turn A/C blower on. If blower operates, go to next step. If blower does not operate, go to step 11.
10. Repair or replace blower as necessary. See appropriate A/C-HEATER SYSTEMSarticle. After repair, retest system.
11. Check for blown A/C fuses. On Axiom, Passport, Rodeo and Rodeo Sport, fuses are located in underhood fuse/relay box. On Trooper, fuses are located in dash fuse/relay box. If fuses are okay, go to step 13. If fuses are blown, go to next step.
12. Check for and repair open in circuit between fuses and A/C pressure switch. See WIRING DIAGRAMS in appropriate A/C-HEATER SYSTEMSarticle. Replace any blown fuses. After repair, retest system.
13. Remove glove box to gain access to A/C electronic thermostat harness connector. (Scheme 5) Disconnect A/C thermostat 2-pin harness connector. Using a fused jumper, jumper between ground and A/C electronic thermostat signal circuit (Yellow/Blue wire on Axiom; Light Green/White wire on Trooper). Turn A/C and blower on. On scan tool, observe A/C REQUEST display. If display indicates YES, go to next step. If display does not indicate YES, go to step 17.
14. Turn ignition on. Using DVOM, measure voltage between ground and A/C electronic thermostat signal circuit (Yellow/Blue wire on Axiom; Light Green/White wire on Trooper). Turn A/C and blower on. If battery voltage exists, go to step 17. If battery voltage does not exist, go to next step.
15. Check for open in A/C electronic thermostat signal circuit between thermostat and A/C switch. See WIRING DIAGRAMS in appropriate A/C-HEATER SYSTEMSarticle. If problem is found, go to next step. If no problem is found, go to step 17.
16. Repair open in A/C electronic thermostat signal circuit. After repair, retest system.
17. Check for open in signal circuit between A/C thermostat relay and PCM A/C request terminal. See WIRING DIAGRAMS in appropriate A/C-HEATER SYSTEMSarticle. If problem is found, go to next step. If no problem is found, go to step 19.
18. Repair open in signal circuit between A/C thermostat relay and PCM A/C request terminal. After repair, retest system.
19. Turn ignition on. Measure voltage between ground and A/C pressure switch harness connector terminal No. 2 (Brown wire). See WIRING DIAGRAMS in appropriate A/C-HEATER SYSTEMSarticle. If battery voltage exists, go to step 21. If battery voltage does not exist, go to next step.
20. Repair open in Brown wire (pressure switch voltage supply) between A/C fuse and pressure switch. After repair, retest system.
21. Check for open in Brown wire (pressure switch voltage supply) between A/C fuse and pressure switch. If problem is found, go to next step. If no problem is found, go to step 23.
22. Repair open in Brown wire (pressure switch voltage supply) between A/C fuse and pressure switch. After repair, retest system.
23. Check for poor, loose or damage terminal connections in PCM harness connector. If problem is found, go to next step. If no problem is found, go to step 25.
24. Repair or replace suspect PCM harness connector terminal(s). After repair, retest system.
25. Replace and reprogram PCM. See POWERTRAIN CONTROL MODULE REPLACEMENT under POWERTRAIN CONTROL MODULE in appropriate SELF-DIAGNOSTICS article. After repair, repeat OBD-II system check. See ON-BOARD DIAGNOSTIC II SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.
26. Remove A/C compressor relay from underhood relay. Turn ignition on. Using DVOM measure voltage between ground and A/C compressor relay socket cavities No. 2 and 5 (Brown wires at both cavities). If battery voltage exists, leave relay disconnected and go to step 28. If battery voltage does not exist, go to next step.
27. Repair open or short in Brown wire (A/C compressor relay voltage supply) between A/C fuse and A/C compressor relay. After repair, retest system.
28. Start engine. Turn A/C and blower on. Measure voltage between ground and A/C compressor relay socket cavity No. 4 (Gray/Red wire). If battery voltage exists, leave relay disconnected and go to step 32. If battery voltage does not exist, go to next step.
29. Check for open in Gray/Red wire (relay control circuit) between PCM and A/C compressor relay. If problem is found, go to next step. If no problem is found, go to step 31.
30. Repair open in Gray/Red wire (relay control circuit) between PCM and A/C compressor relay. After repair, retest system.
31. Check for poor, loose or damage terminal connections in PCM harness connector. If problem is found, go to step 24. If no problem is found, go to step 25.
32. Connect a fused jumper between A/C compressor relay socket cavities No. 1 and 5. Start engine. Turn A/C and blower on. If A/C compressor clutch engages, go to next step. If A/C compressor clutch does not engage, go to step 34.
33. Repair A/C compressor clutch as necessary. After repair, retest system.
34. Check for open in Brown/Yellow wire between A/C compressor relay and A/C compressor clutch. See WIRING DIAGRAMS in appropriate A/C-HEATER SYSTEMSarticle. If problem is found, go to next step. If no problem is found, go to step 36.
35. Repair open in Brown/Yellow wire between A/C compressor relay and A/C compressor clutch. After repair, retest system.
36. Service compressor clutch or replace compressor due to a faulty internal overheat switch. After repair, retest system.
37. Remove A/C compressor relay from underhood fuse/relay box. Ensure A/C is turned off. Start engine. If A/C compressor clutch is engaged even though A/C is turned off and relay has been removed, go to next step. If A/C compressor clutch does not engage, go to step 39.
38. Repair short to voltage in Brown/Yellow wire between A/C compressor relay and A/C compressor clutch. After repair, retest system.
39. Turn ignition off. Install A/C compressor relay. Disconnect A/C thermostat relay from underhood fuse/relay box. Start engine. If A/C compressor clutch is engaged even though A/C is turned off and relay has been removed, go to next step. If A/C compressor clutch does not engage, go to step 42.
40. Check for short to ground in Gray/Red wire between PCM and A/C compressor relay. See WIRING DIAGRAMS in appropriate A/C-HEATER SYSTEMSarticle. If problem is found, go to next step. If no problem is found, go to step 25.
41. Repair short to ground in Gray/Red wire between PCM and A/C compressor relay. After repair, retest system.
42. Repair short to ground in circuits between A/C thermostat relay and electronic thermostat. After repair, retest system.

Scheme 5

Compressor Clutch Control Circuit Diagnosis

For A/C clutch circuit testing, see appropriate article in AIR CONDITIONING & HEATER SYSTEMS.

FUEL TRIM CELL DIAGNOSTIC WEIGHTS

No fuel trim DTC will set regardless of fuel trim counts in cell "0" unless the fuel trim counts in weighted cells are also outside specifications. This means that vehicle could have a fuel trim problem causing a problem under certain conditions (i.e., engine idle high due to a small vacuum leak or rough due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or Heated Oxygen Sensor DTC may set). Use scan tool to observe fuel trim counts while problem is occurring.