Diagnostic Aids

WARNING: This page is about a different car, the 1995 Ford Aerostar. However, it is still accessible from the selected car via links, so may be relevant.

When ignition switch is in the ON or START position, the PCM connects FUEL PUMP circuit (PCM terminal No. 22) to ground. The fuel pump relay is energized and electrical current flows to the fuel pump. If PCM does not receive ignition PIP signal within 2 seconds, FUEL PUMP circuit is opened and fuel pump operation is terminated. Perform this test when instructed by QUICK TEST or CIRCUIT TEST AA. This test is only intended to diagnose:

Fuel pump relay.
Inertia Fuel Shutoff (IFS) switch.
Wiring harness circuits (B+, VPWR, FUEL PUMP, GND and POWER-TO-PUMP).
Faulty Powertrain Control Module (PCM).

Fig 1: Fuel Pump Monitor Schematic Diagram (3.0L)

Fig 2: Fuel Pump Monitor Circuit Diagram (3.0L)

1) Code 556: Check VPWR To Fuel Pump Relay Code 556 indicates a fuel pump primary circuit failure.

Possible causes for this fault are:

IFS switch not reset or circuit open.
Open or shorted circuit.
Faulty fuel pump relay.
Faulty PCM.

Disconnect fuel pump relay. Turn ignition on. Measure voltage between chassis ground and VPWR circuit at fuel pump relay wiring harness connector. If voltage is 10.5 volts or more, go to next step. If voltage is less than 10.5 volts, verify IFS switch is in ON position. If switch is okay, repair open VPWR circuit between EEC power relay and fuel pump relay. Connect fuel pump relay and repeat QUICK TEST.

2) Check Fuel Pump Relay Turn ignition off. Disconnect fuel pump relay. On ISO relays, measure resistance between terminals No. 85 and 86. See Fig 3 . On all other relays, measure resistance between VPWR pin and FUEL PUMP circuit pin at fuel pump relay. On all relays, resistance should be 40-120 ohms. Set DVOM to 10-k/ohm scale. On ISO relays, measure resistance between terminal No. 85 and terminals No. 30 and 87. On all other relays, measure resistance between FUEL PUMP circuit pin and both POWER-TO-PUMP and B+ pins at fuel pump relay. All resistances should be greater than 10,000 ohms. If resistances are as specified, go to next step. If resistances are not as specified, replace fuel pump relay and repeat QUICK TEST.

Fig 3: Identifying Fuel Pump Relay Connector Terminals (3.0L)

3) Check For Short To Power Turn ignition off. Disconnect fuel pump relay. 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. 22 and negative battery terminal. If voltage is less than one volt, go to next step. If voltage is one volt or more, repair short circuit. Reconnect PCM, and attempt to start vehicle. If vehicle fails to start, replace PCM. Repeat QUICK TEST.

4) Check For Short To Ground Turn ignition off. Leave fuel pump relay and PCM disconnected. Measure resistance between test pin No. 22 and test pins No. 40 and 60. If resistance is less than 10,000 ohms, repair short circuit and repeat QUICK TEST. If resistance is 10,000 ohms or more, go to next step.

5) Check Fuel Pump Circuit Continuity Turn ignition off. Leave fuel pump relay and PCM disconnected. Measure resistance between FUEL PUMP circuit at fuel pump relay wiring harness connector and test pin No. 22 at breakout box. If resistance is less than 5 ohms, replace PCM and repeat QUICK TEST. If resistance is 5 ohms or more, repair open circuit and repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 5) to step 10). No test procedures have been omitted.

10) Code 542 A KOEO Code 542 indicates one of the following conditions:

Engine Starts:

Fuel pump secondary circuit shorted to power.
Fuel pump relay contacts always closed.
Open FPM circuit between PCM and junction to POWER-TO-PUMP circuit.
Left/front HO2S short to power (dual HO2S applications).
Faulty PCM.

No Start:

IFS switch circuit open or not reset.
Open circuit in or between PCM and fuel pump.
Faulty ground connection at fuel pump.
Faulty fuel pump.

On models with dual fuel tanks, ensure tank selector is in same position it was when KOEO Code 542 was set. Attempt to start engine. If engine starts, go to next step. If engine does not start, go to step 15).

11) Check Fuel Pump Operation Turn ignition on. Wait 5 seconds. Listen for fuel pump operation. If fuel pump is off, go to step 13). If fuel pump is on, go to next step.

12) Check For Fuel Pump Relay Always Closed Turn ignition off. Disconnect fuel pump relay. Turn ignition on. If fuel pump is off with relay disconnected, replace fuel pump relay and repeat QUICK TEST. If fuel pump is on with relay disconnected, repair short to power in POWER-TO-PUMP or FPM circuit. (For models with relay block, also check fuel pump prime plug circuit). Repeat QUICK TEST.

13) Check Continuity Of Fuel Pump Monitor (FPM) Circuit Turn ignition off. Disconnect PCM 60-pin connector. Inspect and repair any damaged terminals. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Disconnect fuel pump relay. Measure resistance between test pin No. 8 at breakout box and POWER-TO-PUMP circuit at fuel pump relay wiring harness connector. If resistance is less than 5 ohms, go to step 25) on dual HO2S, or replace PCM and repeat QUICK TEST on single HO2S. If resistance is 5 ohms or more, repair open circuit and repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 13) to step 15). No test procedures have been omitted.

15) Check Inertia Fuel Shutoff (IFS) Switch Turn ignition off. Disconnect IFS switch. See INERTIA FUEL SHUTOFF SWITCH LOCATION table. Ensure switch is reset. Measure resistance between IFS switch terminals. If resistance is less than 5 ohms, check for open POWER-TO-PUMP circuit, poor fuel pump ground or faulty fuel pump. If resistance is 5 ohms or more, reset or replace IFS switch. Repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 15) to step 20). No test procedures have been omitted.

20) Code 543 Code 543 indicates fuel pump secondary circuit failure between B+ circuit and FPM connection to POWER-TO-PUMP circuit. Following are possible causes:

No Start:

Open circuit between B+ and FPM connection to POWER-TO-PUMP circuit.
Fuel pump relay contacts are always open.

Engine Starts:

HO2S short to power (dual HO2S models).
Faulty PCM.

Attempt to start engine. If engine starts, go to step 25) on dual HO2S, or replace PCM and repeat QUICK TEST on single HO2S. If engine does not start, go to next step.

21) Check For B+ To Fuel Pump Relay Turn ignition off. Disconnect fuel pump relay. Measure voltage between B+ terminal at fuel pump relay wiring harness connector and negative battery terminal. If voltage is 10.5 volts or more, go to next step. If voltage is less than 10.5 volts, check fuse/fusible link. If fuse/fusible link is okay, repair open B+ circuit. Repeat QUICK TEST.

22) Check POWER-TO-PUMP Circuit Continuity Turn ignition off. Leave fuel pump relay disconnected. Measure resistance between POWER-TO-PUMP terminal at fuel pump relay wiring harness connector and negative battery terminal. If resistance is less than 10 ohms, replace fuel pump relay and repeat QUICK TEST. If resistance is 10 ohms or more, repair open POWER-TO-PUMP circuit between FPM junction and fuel pump relay. Repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 22) to step 25). No test procedures have been omitted.

Due to internal circuitry of PCM, a left/front HO2S signal short to power could produce a Code 542 or 543.

25) Check Left/Front HO2S Sensor For Short To Power Turn ignition off. Disconnect left or front HO2S sensor. Measure resistance between HO2S SIGNAL terminal and KEY POWER terminal at HO2S sensor connector. See Fig 1 . If resistance is 10,00 0 ohms or more, go to next step. If resistance is less than 10,000 ohms, replace HO2S sensor. Clear KAM, and repeat QUICK TEST.

Fig 1: Identifying HO2S Sensor Connector Terminals (3.0L)

26) Check HO2S Circuit For Short To Power Turn ignition off. Leave left or front HO2S disconnected. Disconnect PCM. Inspect and repair any damaged terminals. Turn ignition on. Measure voltage between HO2S SIGNAL at HO2S wiring harness connector and chassis ground. See Fig 2 . If voltage is less than 2 volts, replace PCM and repeat QUICK TEST. If voltage is 2 volts or more, repair short circuit. Clear KAM, and repeat QUICK TEST.

Fig 2: HO2S Sensor Wiring Harness Connector (3.0L)

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 26) to step 90). No test procedures have been omitted.

90) Continuous Memory Code 542 Code 542 indicates one of the following conditions:

Faulty ground circuit at fuel pump.
FPM or POWER-TO-PUMP circuit short to power.
IFS switch not reset.
Fuel pump relay contacts stuck closed.
Open circuit in or between fuel pump and FPM circuit at PCM.
Left or front HO2S circuit short to power (dual HO2S system).
FUEL PUMP circuit is activated at a time that is not programmed into PCM strategy.
Engine stall due to excessive load.

Start engine. Check for engine miss or intermittent fuel pump deactivation while performing following:

Shake and bend POWER-TO-PUMP circuit wiring harness connector between fuel pump relay and fuel pump.
Shake and bend fuel pump ground circuit between fuel pump and ground.
Lightly tap fuel pump to simulate road shock.
Lightly tap IFS switch to simulate road shock.

Turn ignition off. Check harness connectors for corrosion and damage. Isolate and repair any faults. Clear continuous memory, and repeat QUICK TEST. If no faults are found, go to next step.

91) Check FPM Circuit Turn ignition off. Disconnect PCM 60-pin connector. Inspect terminals and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Connect test light between test pin No. 8 and test pin No. 37 at breakout box. With test light lit, perform wiggle test on FPM circuit between fuel pump and PCM. Light will go out if fault is found, indicating an open circuit. If a fault is found, repair as necessary and repeat QUICK TEST. If no fault is found, go to next step.

92) Check For Short To Power Connect a test light between breakout box test pins No. 8 and 40. Test light should be off. Observe test light while bending and shaking FPM circuit and POWER-TO-PUMP circuit. Lightly tap fuel pump relay to simulate road shock. Fault is indicated if test light comes on. Isolate and repair fault as necessary, and repeat QUICK TEST. If no fault is found, go to step 96) on dual HO2S or step 99) on single HO2S.

93) Continuous Memory Code 543: Check For Continuous Memory Code 556 If Code 556 is present, go to step 95). If Code 556 is not displayed, go to next step.

94) Check EEC-IV Harness A Continuous Memory Code 543, without an accompanying Continuous Memory Code 556, indicates one of the following conditions has occurred during vehicle operation:

Open B+ circuit between B+ and fuel pump relay.
Fuel pump relay contacts open.
Open POWER-TO-PUMP circuit between fuel pump relay to FPM splice (if applicable).
Left/front HO2S circuit short to power (dual HO2S).

Start engine. Check for engine miss or intermittent fuel pump deactivation while performing the following:

Shake and bend B+ circuit wiring harness between power source and fuel pump relay.
Shake and bend POWER-TO-PUMP circuit wiring harness connector between fuel pump relay and FPM splice (if applicable).
Lightly tap fuel pump relay to simulate road shock.

Turn ignition off. Inspect all fuel pump relay and B+ connectors for damage and corrosion. If fault is found, repair as necessary and repeat QUICK TEST. If no fault is found, Continuous Memory Code 543 may have been set without Continuous Memory Code 556 even though a fault has occurred in fuel pump primary circuit. Go to next step for fuel pump primary circuit check.

95) Continuous Memory Code 556: Check EEC-IV Harness A Continuous Memory Code 556 indicates primary fuel pump circuit has failed during vehicle operation. Possible causes for this fault are:

Open VPWR circuit between EEC power relay and fuel pump relay.
Open coil in fuel pump relay.
Open fuel pump circuit (pin No. 22).
Faulty IFS switch.

Start engine. Check for engine miss or intermittent fuel pump deactivation while performing the following:

Shake and bend VPWR circuit wiring harness between EEC and fuel pump relay.
Shake and bend FP circuit wiring harness connector between fuel pump relay and PCM.
Lightly tap fuel pump relay to simulate road shock.
Lightly tap IFS switch to simulate road shock (if equipped).

Turn ignition off. Disconnect fuel pump relay connector and PCM 60-pin connector. Inspect terminals. If fault is found, isolate and repair as necessary. Repeat QUICK TEST. If no faults are found but Code 543 is present, go to next step on dual HO2S, or go to step 99) on all others.

Due to internal circuitry of PCM, an intermittent left/front HO2S signal short to power could produce a Continuous Memory Code 542 or 543.

96) Check Left/Front HO2S Circuit For Short To Power Turn ignition off. Breakout box should be installed with PCM disconnected. Connect test light between left/front HO2S test pin and pin No. 40 at breakout box. See CIRCUIT TEST H for left/front HO2S test pin identification. Test light should be off. Observe test light while bending and shaking left/front HO2S circuit from HO2S sensor to PCM. Lightly tap HO2S to simulate road shock. Fault is indicated if test light comes on. If fault is indicated, isolate and repair as necessary, and repeat QUICK TEST. If no fault is indicated, go to step 99).

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 96) to step 99). No test procedures have been omitted.

CAUTION: Following road test is an optional procedure. Follow all applicable safety procedures and traffic laws. This road test requires a driver and an assistant. Assistant should make measurements, observe changes and record notes. If this test is not performed, go to TESTS W/O CODES - EEC-IV article in the ENGINE PERFORMANCE section for other possible causes.

99) Road Test Vehicle Purpose of this test is to identify faults by monitoring certain controlled parameters while trying to recreate a drive-ability or MIL symptom. To prepare for road test:

Install fuel pressure gauge and MAP/BARO tester (if available).
Disconnect PCM 60-pin connector, install breakout box, and reconnect PCM to breakout box.
Connect "T" vacuum gauge into manifold vacuum line.
Have DVOM, writing materials and appropriate schematics and pin voltage charts available.

With ignition on and negative lead of DVOM connected to negative battery terminal, ensure following signals are correct:

POWERS: KAPWR (pin No. 1) is greater than 10.5 volts, VPWR (pins No. 37 and 57) is greater than 10.5 volts, and VREF (pin No. 26) is 4-6 volts.
GROUNDS: PWR GND (pins No. 40 and 60), SIG RTN (pin No. 46) and IGN GND (pin No. 16) are 0.0-0.5 volt.
OPTIONAL GROUNDS: HO2S GND (pin No. 49), CSE GND (pin No. 20) and MAF RTN (pin No. 9 or 15) are 0.0-0.5 volt.

Test lights and DVOM are useful during diagnosis. For example: with Continuous Memory Code 556 (fuel pump primary circuit failure) and a surge or stall symptom, connect a test light to fuel pump relay between VPWR and ground. Connect DVOM between FP circuit at fuel pump relay and breakout box test pin No. 1. Under normal driving conditions, test light will be on and DVOM will read battery voltage. If vehicle stalls, PCM will open FP circuit and DVOM voltage will be low. If fault is in fuel pump circuit, test light and DVOM status will change as fault occurs. If test light at VPWR circuit goes out, fault is in VPWR circuit to fuel pump relay. If test light and DVOM status do not change and Continuous Memory Code 556 is set again, replace fuel pump relay. If only FP voltage goes low, fault is in FP circuit or PCM. To diagnose FP circuit and PCM, connect DVOM between breakout box test pins No. 1 and 22. If voltage goes low as symptom occurs, replace PCM. If voltage stays high as symptom occurs, fault is in FP circuit. For fuel pump secondary circuit Codes 542 and 543, circuits B+, POWER-TO-PUMP and FPM can be diagnosed using same procedure. Drive vehicle and attempt to induce symptom. Information provided by vehicle operator may help when trying to recreate symptom. When symptom occurs, assistant should observe and record changes in voltage signals. Information about symptom and operating condition value of voltage signal and any other information available should be recorded for analysis. If unable to duplicate symptom during road test, verify EEC-IV values are within acceptable range. After test is completed, analyze results to locate and repair fault causing symptom. If problem cannot be identified, go to TESTS W/O CODES - EEC-IV article in the ENGINE PERFORMANCE section for other possible causes of symptom.