Diagnostic Aids

Perform this test when diagnosing a symptom. To prevent replacing good components, check following non-EEC components and systems:

• Refrigerant charge.
• Low ambient temperature (less than 45°F). This test is only intended to diagnose:
• Wiring harness circuits (WAC, VPWR, GND, POWER-TO-CLUTCH and ACD).
• WAC relay.
• A/C fan controller.
• Faulty Powertrain Control Module (PCM).

Fig 1: WOT A/C Cut-Out Circuit (Aerostar)

Fig 2: WOT A/C Cut-Out Circuit (Explorer)

1) Low/No Voltage To A/C Clutch  Check all A/C-related fuses in fuse panel before proceeding with this test. Turn ignition off. Disconnect A/C clutch wiring harness connector. Measure resistance between ground circuit terminal at A/C clutch wiring harness connector and negative battery terminal. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open ground circuit. Reconnect all components and recheck system operation.

2) Check Power-To-Clutch Circuit  Turn ignition off. Disconnect WAC relay connector. Measure resistance between power side of A/C clutch harness connector and power-to-clutch terminal at WAC relay harness connector. If resistance is less than 5 ohms, reconnect A/C switch and go to next step. If resistance is 5 ohms or more, repair open circuit. Reconnect all components and check system operation.

3) Check For Power On A/C Demand Circuit  Turn ignition on. Leave WAC relay disconnected. Turn A/C switch to A/C position. Measure voltage between chassis ground and A/C demand switch input terminal at WAC relay wiring harness connector. If voltage is more than 10.5 volts, go to step 5). If voltage is 10.5 volts or less, verify operation of A/C clutch cycling pressure switch and A/C demand switch. If components are okay, repair open circuit and recheck system.

5) Check WAC Circuit For Short To Ground  Turn ignition off. Leave WAC relay disconnected. Disconnect 60-pin PCM connector. Inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Leave PCM disconnected. Measure resistance between WAC terminal at WAC relay wiring harness connector and chassis ground. If resistance is less than 10,000 ohms, repair short and check system operation. If resistance is 10,000 ohms or more, go to next step.

6) Check For Voltage At ACCS Input To PCM  Turn ignition off. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Set A/C switch to A/C position. Turn ignition on. Measure voltage between test pins No. 10 and 40 at breakout box. If voltage is less than 10.5 volts, repair open circuit and check system operation. If voltage is 10.5 volts or more, go to next step.

7) Check WAC Relay  Turn ignition off. Connect WAC relay, leaving A/C clutch disconnected. Leave A/C and blower switch positioned as listed in step 6). Turn ignition on. Measure voltage between power-to-clutch terminal of A/C clutch wiring harness connector and negative battery terminal. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, replace WAC relay. Reconnect all components and check system operation.

8) Check Engine Coolant Temperature (ECT) Sensor & Throttle Position (TP) Sensor Voltage  Turn ignition off. Reconnect PCM connector to breakout box. Reconnect A/C clutch connector. Turn A/C on. Start engine. Measure voltage between test pins No. 7 and 40 at breakout box. Ensure ECT sensor voltage is as specified. See ACT & ECT SENSOR SPECIFICATIONS TABLE in CIRCUIT TEST DA. Also, measure voltage between test pins No. 40 and 47 at breakout box. With throttle plate at specified position, ensure voltage is as specified. See Figure in CIRCUIT TEST DH. If voltage for ECT sensor and TP sensor is as specified, replace PCM. Remove breakout box and check system operation. If ECT sensor voltage or TP sensor voltage is not as specified, perform QUICK TEST. Service any codes that are present. See DIAGNOSTIC TROUBLE CODE DEFINITION CHARTS. If no codes are present, check for open or shorted ECT/TP sensor wiring. Repair as necessary. Remove breakout box, reconnect all components, and repeat QUICK TEST.

15) No A/C Cutout At WOT: Check For VPWR To WAC Relay  Disconnect WAC relay connector. Turn ignition on. Measure voltage between chassis ground and VPWR circuit terminal at WAC relay wiring harness connector. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, repair open circuit. Reconnect all components and recheck system operation.

16) Check For WAC Cycling  Enter output state Diagnostic Test Mode (DTM). See OUTPUT STATE DIAGNOSTIC TEST MODE (DTM)  under ADDITIONAL SYSTEM FUNCTIONS. Once output state DTM has been entered, disconnect WAC relay connector. Set DVOM to 20-volt scale. Connect DVOM positive lead to VPWR circuit terminal at WAC relay wiring harness connector. Connect DVOM negative lead to WAC circuit terminal at WAC wiring harness connector. Observe DVOM, and depress and release throttle several times (to cycle output on and off). If voltage cycles high and low (about a one-volt change), replace WAC relay. Reconnect components and recheck system operation. If voltage does not cycle high and low, exit output state DTM and go to next step.

17) Check WAC Circuit Continuity  Turn ignition off. Disconnect 60-pin PCM connector. Inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between test pin No. 54 and WAC terminal at WAC relay wiring harness connector. If resistance is 5 ohms or more, repair open circuit. Reconnect all components and check system operation. If resistance is less than 5 ohms, go to next step.

18) Check WAC Circuit For Short To Power  Turn ignition off. Leave PCM and WAC relay disconnected. Turn ignition on. Measure voltage between test pin No. 54 and chassis ground. If voltage is less than one volt, replace PCM and recheck system operation. If voltage is one volt or more, repair short circuit. After repair is made, if symptom is still present, replace PCM.

30) Check A/C Input Circuit  Turn A/C switch to A/C position. Perform KOEO SELF-TEST. If Code 539 is not present, go to next step. On all models except 4.0L MFI, if Code 539 is present, PCM is receiving and recognizing A/C input from pin No. 10. Check for other possible causes of low idle.

31) Check A/C Input Circuit  Turn ignition off. Disconnect PCM 60-pin connector. Inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Install breakout box, leaving PCM disconnected. Turn A/C on. Turn ignition on. Measure voltage between test pins No. 10 and 40. If voltage is more than 10.5 volts, replace PCM and connect all components. Repeat QUICK TEST. If voltage is 10.5 volts or less, repair open A/C circuit. Remove breakout box, reconnect all components, and repeat QUICK TEST.

32) Cycle A/C Demand Switch  Turn ignition off. Disconnect PCM 60-pin connector. Inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Install breakout box, leaving PCM disconnected. Connect DVOM positive lead to test pin No. 43 at breakout box. Connect negative lead to test pin No. 40. Turn ignition on. Turn A/C on and off. If voltage cycles high and low when switch is cycled, EEC-IV system is okay and testing is complete. If voltage does not cycle, repair open ACD circuit, and repeat QUICK TEST.

Before entering this test, ensure A/C selector is in OFF position and shift selector is in Park (A/T). If A/C was on, repeat QUICK TEST. If Code 539 is present, go to step 40).

40) Code 539: Check A/C Input  Code 539 indicates ACCS input to PCM was high during SELF-TEST. Turn ignition off. Disconnect 60-pin PCM connector. Inspect terminals and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Turn ignition on. Measure voltage between test pin No. 10 at breakout box and chassis ground. If voltage is one volt or more, repair short to power in A/C circuit and repeat QUICK TEST. If voltage is less than one volt, replace PCM and repeat QUICK TEST.