Test FD - Brake On/Off Switch (Boo)

NOTE: Perform this test when Service Code 74 or 75 is displayed during QUICK TEST.

NOTE: To prevent replacement of good components, be aware that the following non-EEC related areas may be at fault: brakelight bulb, brakelight switch, or brakelight fuse. This test is intended to diagnose a faulty BOO circuit or ECA.

Fig 1: Mustang BOO Circuits

1) Code 74: Verify Brake Pedal Was Depressed Code 74 indicates that when the brake pedal was depressed during the KOER Self-Test, the BOO signal did not cycle high. Possible causes for this fault are: brake pedal not depressed during self-test, open brakelight circuit (Batt + side of BOO splice), short to ground, faulty ECA. If brake was NOT depressed during KOER Self-Test, repeat test and depress and release brake pedal ONLY once during test. Depress and release brake pedal AFTER dynamic response code but before brief WOT. If pedal was depressed, go to next step.

2) Check For BOO Circuit Cycling Turn key off and wait 10 seconds. Disconnect ECA 60-pin connector. Inspect connector for damaged pins, corrosion, and loose wires. Install breakout box, leaving ECA disconnected. Set DVOM on 20-volt scale. Measure voltage between test pins No. 2 and 40 while applying and releasing brake. If voltage cycles, switch circuit is okay, replace ECA and repeat QUICK TEST. If voltage does not cycle, go to next step.

3) Checking BOO For Shorts To Ground Turn key off. Leave breakout box installed and ECA disconnected. Set DVOM on 200-k/ohm scale. Disconnect BOO circuit harness connector. Measure resistance between test pins No. 2 and 40 at the breakout box. If resistance is greater than 10 k/ohms, repair brakelight circuit as necessary. If resistance is less than 10 k/ohms, repair short, remove breakout box, reconnect all components and repeat KOER Self-Test.

NOTE: There is a break in the step numbering sequence at this point, skipping from step 3 to step 5. Break is due to previous chart references to these testing procedures. No test procedures have been omitted.

5) Code 75: Check For BOO Circuit Cycling Code 75 indicates that while brake pedal was released during KOER Self-Test, BOO signal was high. Possible causes for this fault are: brake pedal depressed during entire self-test, open BOO/brakelight circuit (between ECA and brakelight ground), short to POWER, or faulty ECA. Turn key off and wait 10 seconds. Disconnect ECA 60-pin connector. Inspect for and repair any damaged wiring. Install breakout box, leaving ECA disconnected. Set DVOM on 20-volt scale. Measure voltage between test pins No. 2 and 40 while depressing and releasing brake pedal. If voltage cycles, replace ECA, remove breakout box and repeat QUICK TEST.

6) Check BOO Circuit For Short to Power Turn key off. With breakout box installed and ECA disconnected, set DVOM on 20-volt scale. Disconnect BOO circuit harness connector. Measure voltage between test pin No. 2 and an engine block ground. If voltage is greater than 1.0 volt, repair short circuit, Remove breakout box, reconnect all components and repeat KOER Self-Test. If not, go to next step.

7) Check Continuity of BOO Circuit Turn key off. Install breakout box, leaving ECA disconnected. Set DVOM on 20O-ohm scale. Disconnect BOO circuit harness connector. Measure resistance between test pin No. 2 at the breakout box and BOO circuit at the harness connector. If resistance is less than 5 ohms, check and repair brakelight system as necessary. If resistance is greater than 5 ohms, repair open circuit. Remove breakout box, reconnect all components and repeat KOER Self-Test.

NOTE: There is a break in the step numbering sequence at this point, skipping from step 7 to step 10. Break is due to previous chart references to these testing procedures. No test procedures have been omitted.

10) Code 74: Verify Brake Pedal Was Depressed Code 74 indicates that when brake pedal was depressed and released during KOER Self-Test, the BOO signal did not cycle high and low. Possible causes for this fault are: brake pedal was not depressed and released during the KOER Self-Test, brake pedal was depressed during entire KOER Self-Test, open in BOO/brakelight circuit, short to GROUND or POWER, or faulty ECA. If brake was NOT depressed during KOER Self-Test, repeat test and depress and release brake pedal ONLY once during test. Depress and release brake pedal AFTER dynamic response code but before brief WOT. If pedal was depressed, go to next step. If pedal was depressed correctly during self-test, go to next step.

11) Check Operation of Brake Lights Turn key on. Depress brake pedal and check brakelights. If brakelights operate normally, go to next step. If brakelights are never on, go to step 13. If brakelights are always on, go to step 14.

12) Check For BOO Circuit Cycling Turn key off and wait 10 seconds. Disconnect ECA 60-pin connector. Inspect connector for damaged pins, corrosion, and loose wires. Install breakout box, leaving ECA disconnected. Set DVOM on 20-volt scale. Measure voltage between test pin No. 2 or test pin No. 5 (as applicable per vehicle) and test pin No. 40 while applying and releasing brake. If voltage cycles, switch circuit is okay, replace ECA, remove breakout box, reconnect all components and repeat QUICK TEST. If voltage does not cycle, repair open BOO circuit between ECA and BOO connection to brakelight circuit. Repeat QUICK TEST.

13) Check For Brake Light Power to Harness Connector Turn key off. Disconnect BOO circuit harness connector. Set DVOM on 20-volt scale. Measure voltage between BOO circuit at harness connector and chassis ground while depressing brake pedal. If voltage is greater than 10 volts, repair open circuit between BOO connection to brakelight circuit and brakelight ground. If voltage is not greater than 10 volts, verify brake switch operation. If okay, repair open circuit between Batt (+) supply to circuit and BOO connection to brakelight circuit. Reconnect harness connector and repeat QUICK TEST.

14) Check BOO Circuit For Short to Power at Harness Connector Turn key off and wait 10 seconds. Disconnect BOO circuit harness connector. Set DVOM on 20-volt scale. With KOEO, measure voltage between BOO circuit at harness connector and chassis ground with brake pedal released. If voltage is less than 1.0 volt, go to next step. If not, verify operation of brake switch. If okay, repair short to power. Reconnect harness connector and repeat QUICK TEST.

15) Check For Short to Power in ECA Turn key off. Reconnect BOO circuit harness connector. Disconnect ECA. With KOEO, check to see if brakelights are on. If so, repair short to power between ECA and harness connector. Repeat QUICK TEST. If not, replace ECA and repeat QUICK TEST.