Diagnostic Aids

Perform this test only when directed by QUICK TEST. This test is intended to diagnose:

Wiring harness circuit (OSS).
Faulty Powertrain Control Module (PCM).

Fig 1: Output Shaft Speed (OSS) Sensor Circuit (Bronco)

TEST PIN NO. 5 (OSS+) WIRE COLOR IDENTIFICATION

Application	Wire Color
Bronco	Dark Green/White

1) KOER Code 639 KOER Code 639 indicates input from Output Shaft Speed Sensor (OSS) to PCM is out of calibration. Possible causes for this fault are:

Faulty OSS sensor.
Circuit open or grounded.
Faulty Powertrain Control Module (PCM).

Turn ignition off. Disconnect OSS wiring harness connector. Disconnect 60-pin PCM connector. Inspect terminals, and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between OSS+ terminal at OSS wiring harness connector and test pin No. 5 at breakout box. Measure resistance between SIG RTN terminal at OSS wiring harness connector and test pin No. 46 at breakout box. For E Series 5.0L, measure resistance at SIG RTN terminal at OSS connector and test pin No. 29 at breakout box. On all models, if either resistance is more than 5 ohms, repair open circuit and repeat QUICK TEST . If both resistances are 5 ohms or less, go to next step

2) Check Circuit For Short To Power Or Ground Leave ignition off and OSS disconnected. Measure resistance between test pin No. 5 and test pins No. 37 and 57 at breakout box. Measure resistance between test pin No. 5 and test pins No. 40, 46 and 60 at breakout box. If any resistance is less than 10,000 ohms, repair short circuit and repeat QUICK TEST. If all resistances are 10,000 ohms or more, go to next step.

3) Measure OSS Resistance Leave ignition off and OSS disconnected. Measure resistance between sensor terminals. Resistance should be 450-750 ohms. If resistance is not correct, replace OSS and repeat QUICK TEST . If resistance is correct, check internal transmission components. If okay, replace PCM and repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 3) to step 10). No test procedures have been omitted.

10) KOEO Code 639 Code 639 indicates incorrect input signal from OSS. Possible causes for this fault are:

Faulty OSS.
Circuit open or grounded.
Faulty PCM.

Turn ignition off. Disconnect transmission 12-pin connector. Measure resistance between OSS (-) and OSS (+) terminal at sensor wiring harness connector. See Fig 2 . If resistance is not 200-600 ohms, check wiring for damage and repair as necessary. If wiring is okay, replace OSS sensor. If resistance is 200-600 ohms, go to next step.

Fig 2: Transmission 12-Pin Connector Terminal ID (Bronco)

11) Check OSS Circuit For Short To Power Leave ignition off and transmission 12-pin connector removed. Disconnect 60-pin PCM connector. Inspect terminals, and repair if damaged. Turn ignition on. Measure voltage between OSS (+) terminal at sensor wiring harness connector and chassis ground. Measure voltage between OSS (-) terminal at sensor wiring harness connector and chassis ground. If both voltages are less than 0.5 volt, turn ignition off and go to next step. If either voltage is 0.5 volt or more, repair short circuit and repeat QUICK TEST .

12) Leave ignition off. Leave transmission connector and PCM disconnected. Measure resistance between OSS (+) terminal at sensor wiring harness connector and chassis ground. Measure resistance between OSS (-) terminal at sensor wiring harness connector and chassis ground. Measure resistance between OSS (+) terminal and OSS (-) terminal at 12-pin connector. If any resistance is less than 10,000 ohms, repair short circuit and repeat QUICK TEST. If any resistance is 10,000 ohms or more, go to next step.

13) Check OSS Circuit Continuity Leave ignition off and transmission 12-pin connector removed. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between TSS (+) terminal at 12-pin connector and test pin No. 24 at breakout box. Measure resistance between OSS (-) terminal at 12-pin connector and test pin No. 44 at breakout box. If either resistance is more than 5 ohms, repair open circuit and repeat QUICK TEST . If both resistances are 5 ohms or less, go to next step.

14) Check PCM For Internal Shorts Turn ignition off. Connect PCM to breakout box. Leave transmission wiring harness connector removed. Measure resistance between test pin No. 24 and pins No. 37, 40, 44, 57 and 60 at breakout box. If any resistance is less than 500 ohms, replace PCM and repeat QUICK TEST. If each resistance is 500 ohms or more, go to next step.

15) Check OSS Output Turn ignition off. Leave PCM connected to breakout box. Reconnect transmission 12-pin connector. Set DVOM on AC voltage scale. Start engine. Measure voltage between breakout box test pins No. 24 and 44 while varying engine speed. If AC voltage varies more than 0.5 volt, replace PCM and repeat QUICK TEST . If AC voltage does not vary more than 0.5 volt, replace OSS and repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 15) to step 90). No test procedures have been omitted.

90) Continuous Memory Code 639 Code 639 indicates PCM detected incorrect output signal from OSS during previous 40-80 warm-up cycles. Possible causes for this fault are:

Faulty OSS.
Faulty transmission.
Circuit open or grounded.
Faulty PCM.

Record and clear continuous memory codes. While test driving vehicle, stop, ensure transmission gear selector is in Drive, and accelerate heavily to 35 MPH. After test drive, perform KOEO Self-Test and record all codes. If Continuous Memory Code 639 does not repeat, problem cannot be duplicated at this time. See SUMMARY in this article. If Continuous Memory Code 639 is repeated, go to next step.

91) Check OSS Circuit Continuity Turn ignition off. Disconnect 60-pin PCM connector. Inspect terminals, and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Disconnect OSS wiring harness connector. Measure resistance between OSS+ terminal at OSS connector and test pin No. 5 at breakout box. Measure resistance at SIG RTN terminal at OSS connector and test pin No. 46 at breakout box. For E Series 5.0L, measure resistance at SIG RTN terminal at OSS connector and test pin No. 29 at breakout box. On all models, if either resistance is more than 5 ohms, repair short circuit and repeat QUICK TEST. If all resistances are 5 ohms or less, go to next step.

92) Check Circuit For Short To Power Or Ground Turn ignition off. Leave OSS disconnected. Measure resistance between test pin No. 5 and test pins No. 37 and 57 at breakout box. Measure resistance between test pin No. 5 and test pins No. 40, 46 and 60 at breakout box. If either resistance is less than 500 ohms, repair short circuit and repeat step 90). If both resistances are 500 ohms or more, go to next step.

93) Measure Sensor Resistance Turn ignition off. Leave sensor disconnected. Measure resistance between sensor terminals. OSS terminal resistance should be 450-750 ohms. If resistance is correct, replace PCM and repeat step 90) to verify code elimination. If resistance is not correct, replace OSS sensor and repeat step 90) to verify code elimination.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 93) to step 95). No test procedures have been omitted.

95) Continuous Memory Code 639 Continuous Memory Code 639 indicates an error in OSS sensor output during previous 80 warm-up cycles. Possible causes for this fault are:

Intermittent fault in OSS sensor.
Intermittent short or open in wiring harness.
Transmission placed in gear when fluid level is low.

Turn ignition off. Visually inspect OSS wires and connectors for damage. Disconnect 60-pin PCM connector. Inspect terminals, and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV). Connect PCM to breakout box. Connect DVOM between test pins No. 24 and 44 at break-out box. Start engine. Shake and bend OSS wires between transmission and PCM. Lightly tap circuit connectors. Voltage should remain stable or change gradually. If voltage drops or surges abruptly, fault in circuit is indicated. If fault is indicated, isolate and repair as necessary. If no faults are found, go to next step.

96) Check PCM & Harness Connectors Turn ignition off. Disconnect all connectors related to OSS circuit. Inspect circuits and connectors for damage. Repair as necessary and repeat QUICK TEST . If fault cannot be found, problem is intermittent and cannot be duplicated at this time. See SUMMARY .