Diagnostic Aids

Perform this test when directed by QUICK TEST. This test is intended to diagnose a faulty BOO switch circuit or PCM. To prevent replacement of good components, be aware following non-EEC related areas may be at fault:

• Brakelight bulb.
• Brakelight switch or brakelight fuse.

Fig 1: BOO Switch Circuit (Bronco)

1) Code 74/536: Verify Brake Pedal Was Depressed  Code 74/536 indicates that when brake pedal is applied during KOER SELF-TEST, BOO signal did not cycle high and low. Possible causes for this fault are:

• Brake pedal not applied during self-test.
• Brake pedal applied during entire self-test.
• Open brakelight circuit.
• Short to ground or power.
• Faulty Powertrain Control Module (PCM).

If brake was not applied during KOER SELF-TEST, repeat test. Depress and release brake pedal only once during test. If pedal was depressed, go to next step.

2) Check Operation Of Brakelights  With ignition on, check operation of brakelights. If brakelights operate normally, go to next step. If brakelights do not operate, go to step 4). If brakelights are always on,go to step 5).

3) Check For BOO Switch Circuit Cycling  Turn ignition off. Wait 10 seconds. Disconnect PCM 60-pin connector. Inspect terminals, and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure voltage between test pin No. 2 and test pin No. 40 while applying and releasing brake. If voltage cycles, replace PCM and repeat QUICK TEST  . If voltage does not cycle, repair open circuit in BOO switch circuit between PCM and BOO switch connection to brakelight circuit. Repeat QUICK TEST.

4) Check For Power To Brake Switch  Ensure related fuses and brakelight bulbs are in good condition. Turn ignition off. Disconnect brakelight switch (located on brake pedal). Measure voltage between BATT (+) input to brakelight switch and ground. If voltage is greater than 10 volts, check condition of brakelight switch. If brakelight switch is okay, repair open circuit between brakelight switch and brakelight ground. Repeat QUICK TEST. If voltage is less than 10 volts, repair open BATT (+) circuit to brakelight switch and repeat QUICK TEST.

5) Verify Brake Switch Is Not Always Closed  Turn ignition off. Disconnect brakelight switch (located on brake pedal). Turn ignition on. If brakelights are still on, go to next step. If brakelights are not on, verify correct installation of brakelight switch. If installation is okay, replace brakelight switch and repeat QUICK TEST  .

6) Check For Short To Power In PCM  Turn ignition off. Disconnect PCM. Turn ignition on. Check brakelights. If brakelights are on, go to next step. If brakelights are not on, replace PCM and repeat QUICK TEST.

7) Check For Short To Power In Shift Lock Actuator  Turn ignition off. Ensure PCM and brakelight switch are disconnected. Disconnect shift lock actuator, cruise control module and ABS module (if equipped). Turn ignition on. If brakelights are still on, repair short to power in BOO or STOPLAMP circuit and repeat QUICK TEST. If brakelights are not on, repair short circuit in shift lock actuator circuit, cruise control system circuit or ABS circuit. Repeat QUICK TEST  .

90) Continuous Memory Code 536: Check For Proper Brakelight Switch Installation  Code 536 indicates a BOO circuit failure. Possible causes for this fault are:

• Brakelight switch installed improperly.
• Open brakelight circuit.
• Short to ground or power.
• Faulty brakelight switch.
• Faulty brakelight switch ground connection.

Check brakelight switch for proper installation (alignment with brake pedal, corrosion or frayed wires). If brakelight switch is okay, go to next step. If brakelight switch is not okay, repair as necessary. Clear codes and repeat QUICK TEST  .

91) Check Brakelight Switch Ground  Check brakelight switch ground connection. Also, check brakelight connector wires for corrosion or damage. Repair as necessary. Clear codes and repeat QUICK TEST. If connector and wires are okay, go to next step.

92) Check Brakelight BOO Circuits For Short To Power  Turn ignition on. Observe brakelights. With brake pedal released, wiggle brakelight BOO circuit wires and connectors. If brakelights flash, isolate short to power and repair as necessary. Clear codes and repeat QUICK TEST. If brakelights do not flash, go to next step.

93) Check Brakelight Circuit Continuity  Turn ignition off. Depress and hold-down brake pedal. Observe brakelights and wiggle brakelight circuit wires and connectors. Also, lightly tap on brakelight switch (to simulate road shock). If brakelights flash or go off, isolate open in brakelight circuit and repair as necessary. Clear codes and repeat QUICK TEST  . If brakelights stay on (normal operation), go to next step.

94) Check BOO Circuit Continuity  Turn ignition off. Release brake pedal. Disconnect PCM 60-pin 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. Connect DVOM between BOO test pin (No. 2 or 5) at breakout box and BOO terminal at brakelight switch. Observe DVOM and wiggle BOO circuit wires and connectors. If resistance at anytime was more than 5 ohms, isolate open in BOO circuit and repair as necessary. Remove breakout box, reconnect all components, and repeat QUICK TEST. If resistance was 5 ohms or less at all times, fault is intermittent and cannot be duplicated at this time. Testing is complete.