Diagnostic Procedure

Perform On-Board Diagnostics II (OBD-II) System Check. See appropriate ON-BOARD DIAGNOSTIC II SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article. After performing OBD-II system check, go to next step.
If any DTCs were stored in memory, perform appropriate test before continuing. See appropriate DIAGNOSTIC TROUBLE CODE DEFINITIONS in appropriate SELF-DIAGNOSTICS article. If no DTCs were stored, go to next step.
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 A/C-HEATER SYSTEMSarticle. If continuity exists, go to next step.

NOTE: Before continuing testing, ensure that intake air temperature is 60°F (15°C) or more, and engine coolant temperature is less than 246°F (119°C).

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.
Turn A/C and blower on. Observe A/C compressor clutch. If A/C compressor clutch engages, check for intermittent problem. See DIAGNOSTIC AIDS . If A/C compressor clutch does not engage, leave engine running and go to next step.
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.
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 .
Replace original A/C thermostat relay. After repair, retest system.
Turn A/C blower on. If blower operates, go to next step. If blower does not operate, go to step 11 .
Repair or replace blower as necessary. See appropriate A/C-HEATER SYSTEMSarticle. After repair, retest system.
Check for blown A/C fuses. Fuses are located in underhood fuse/relay box. If fuses are okay, go to step 13 . If fuses are blown, go to next step.
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.
Remove glove box to gain access to A/C electronic thermostat harness connector. See Fig 1 . Disconnect A/C thermostat 2-pin harness connector. Using a fused jumper, jumper between ground and A/C electronic thermostat signal circuit. 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 .
Turn ignition on. Using DVOM, measure voltage between ground and A/C electronic thermostat signal circuit. Turn A/C and blower on. If battery voltage exists, go to step 17 . If battery voltage does not exist, go to next step.
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 .
Repair open in A/C electronic thermostat signal circuit. After repair, retest system.
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 .
Repair open in signal circuit between A/C thermostat relay and PCM A/C request terminal. After repair, retest system.
Turn ignition on. Measure voltage between ground and A/C pressure switch harness connector. 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.
Repair the open circuit between the 10-amp A/C fuse and the pressure switch. After repair, retest system.
Use an ohmmeter to check continuity between the pressure switch and the A/C thermostat relay. If problem is found, go to next step. If no problem is found, go to step 23 .
Repair the open circuit between the pressure switch and the A/C thermostat relay. After repair, retest system.
Check for damaged pin or terminal at F45 of the PCM. If problem is found, go to next step. If no problem is found, go to step 25 .
Repair or replace suspect PCM harness connector terminal(s). After repair, retest system.
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.
Remove A/C compressor relay from underhood relay. Turn ignition on. Use a DVM to check voltage at both of the wires at the A/C compressor relay socket. If battery voltage exists, leave relay disconnected and go to step 28 . If battery voltage does not exist, go to next step.
Repair the faulty wire between the A/C fuse and the A/C compressor relay. After repair, retest system.
Start engine. Turn A/C and blower on. Measure voltage between ground and A/C compressor relay socket. If battery voltage exists, leave relay disconnected and go to step 32 . If battery voltage does not exist, go to next step.
Check for an open wire between PCM terminal F4 and the A/C compressor relay. If problem is found, go to next step. If no problem is found, go to step 31 .
Repair the open wire between the PCM and the A/C compressor relay. After repair, retest system.
Check for a damaged pin or terminal at F4 of the PCM. If problem is found, go to step 24 . If no problem is found, go to step 25 .
Connect a fused jumper at the A/C compressor relay socket with either wire. 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 .
Repair the A/C compressor relay. After repair, retest system.
Check for an open circuit between the A/C compressor relay and the A/C 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 .
Repair the open circuit between the compressor Clutch and the A/C compressor relay. After repair, retest system.
Service compressor clutch or replace compressor due to a faulty internal overheat switch. After repair, retest system.
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 .
Repair the short to voltage between the A/C clutch and A/C compressor relay. After repair, retest system.
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 .
Use a DVM to check for a short to ground between the A/C compressor relay and F4 of the PCM. 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 .
Repair the short to ground between the PCM and A/C compressor relay. After repair, retest system.
Repair the short to ground between the A/C thermostat relay and the electronic thermostat. After repair, retest system.

Fig 1: Locating A/C Electronic Thermostat Harness Connector

Courtesy of ISUZU MOTOR CO.