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

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 step 2).

2) Check Operation Of Brakelights  With ignition on, check operation of brakelights. If brakelights operate normally, go to step 3). 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 BOO test pin (No. 2 or 5) 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 andbrakelight bulbs are in good condition. Turn ignition off. Disconnectbrakelight 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 step 6). 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 step 7). 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.