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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:

Fig 1: BOO Switch Circuit
G93I40541
TEST PIN WIRE COLOR IDENTIFICATION

Application Wire Color
No. 2 (BOO) Light Green
  1. 1) DTC 536: Verify Brake Pedal Was Depressed  DTC 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 brake light circuit.
    • Short to ground or power.
    • Faulty brake light switch.
    • 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. 2) Check Operation Of Brake Lights  With ignition on, check operation of brake lights. If brake lights operate normally, go to next step. If brake lights do not operate, go to step 4). If brake lights are always on,go to step 5).
  3. 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 breakout box, leaving PCM disconnected. Measure voltage between BOO test pin (No. 2) and test pin No. 40 while applying and releasing brake. If voltage cycles, replace PCM. Remove breakout box and repeat QUICK TEST. If voltage does not cycle, repair open circuit in BOO switch circuit between PCM and BOO switch connection to brake light circuit. Repeat QUICK TEST.
  4. 4) Check For Power To Brake Switch  Ensure related fuses and brake light bulbs are in good condition. Turn ignition off. Disconnect brake light switch (located on brake pedal). Measure voltage between BAT (+) input to brake light switch and ground. If voltage is greater than 10 volts, check condition of brake light switch. If brake light switch is okay, repair open circuit between brake light switch and brake light ground. Repeat QUICK TEST. If voltage is less than 10 volts, repair open BAT (+) circuit to brake light switch and repeat QUICK TEST.
  5. 5) Verify Brake Switch Is Not Always Closed  Turn ignition off. Disconnect brake light switch (located on brake pedal). Turn ignition on. If brake lights are still on, go to next step. If brake lights are not on, verify correct installation of brake light switch. If installation is okay, replace brake light switch and repeat QUICK TEST.
  6. 6) Check For Short To Power In PCM  Turn ignition off. Disconnect PCM. Turn ignition on. Check brake lights. If brake lights are on, go to next step. If brake lights are not on, replace PCM and repeat QUICK TEST.
  7. 7) Check For Short To Power In Shift Lock Actuator  Turn ignition off. Ensure PCM and brake light switch are disconnected. Disconnect shift lock actuator, cruise control module and ABS module (if equipped). Turn ignition on. If brake lights are still on, repair short to power in BOO or stoplight circuit and repeat QUICK TEST. If brake lights are not on, repair short circuit in shift lock actuator circuit, cruise control system circuit or ABS circuit. Repeat QUICK TEST.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 7) to step 90). No test procedures have been omitted.
  8. 90) DTC 536: Check For Proper Brake Light Switch Installation  Continuous memory DTC 536 indicates a BOO circuit failure. If BOO input does not cycle after a predetermined number of transitions from 0 mph to a specific speed, the BOO input is assumed to be damaged and continuous memory DTC 536 is set. Possible causes of failure are:
    • Incorrect brake light switch installation.
    • Open brake light/BOO circuit.
    • Brake Light/BOO circuit short.
    • Damaged switch or ground circuit.

      If switch is correctly installed and in good condition, go to next step. If switch or harness is damaged, service as needed, clear continuous memory repeat QUICK TEST.

  9. 91) Inspect Brake Light Ground  Inspect brake light ground connection and harness connector. Repair as needed, clear continuous memory and repeat QUICK TEST. If connections are okay, go to next step.
  10. 92) Inspect Brake Light/BOO Circuits For Shorts  With KOEO and brake pedal released, perform wiggle test of brake light/BOO circuit harness and connectors while observing brake lights. If brake lights illuminate, inspect and repair circuits as needed, clear continuous memory and repeat QUICK TEST. If brake lights do not illuminate, go to next step.
  11. 93) Inspect Brake Light Circuit Continuity  With ignition off, depress brake pedal and hold. Perform wiggle test of brake light circuits while observing brake lights. Lightly tap brake light switch while observing brake lights. If brake lights intermittently go out, inspect and repair circuits as needed, clear continuous memory and repeat QUICK TEST. If brake lights remain illuminated, go to next step.
  12. 94) Inspect BOO Circuit Continuity  With ignition off, ensure brake pedal is released. Disconnect PCM 60-pin connector. Inspect terminals, and repair if damaged. Install breakout box, leaving PCM disconnected. Measure resistance between BOO test pin No. 2 and brake light circuit at switch while performing wiggle test on harness and connector. If resistance intermittently increases above 5 ohms, inspect and repair open circuit. Repeat QUICK TEST. If resistance is within specification, go to next step for further diagnosis.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 94) to step 99). No test procedures have been omitted.
  13. 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, complete the following:
    • Install fuel pressure gauge and if available, a MAP/BARO tester.
    • 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:

    • POWER: 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: a test light could be connected at brake light switch between battery and ground and another test light between switch bulb circuit and ground. Test Light to battery circuit should always be illuminated and other test light should only illuminate when brake light is depressed.

With DVOM connected between test pins No. 2 and 40 at breakout box, check voltage. If voltage is 6-7 volts with brake pedal released, possible open circuit between PCM and brake light ground could exist.

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