Diagnostic Aids

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

• Powertrain Control Module (PCM).
• Wiring harness circuits (TCIL, TCS and TCSM).

To prevent replacing good components, be aware the following non-EEC related areas may be at fault:

• Basic engine condition (valves, vacuum leaks, valve timing, etc.).
• Charging system.
• Transmission (fluid, friction elements and cooling).

Transmission Control Switch (TCS) allows driver to disengage overdrive. TCS is located on shift plate. Mustang is equipped with a Transmission Control Switch Module (TCSM). The TCSM informs PCM of drivers request to disengage overdrive and also controls the Transmission Control Indicator Light (TCIL). TCIL is located in instrument cluster.

1. 2) Code 632 Or 653
   
   Code 632 or 653 indicates that the TCS was not cycled between engine ID Code and WOT check during KOER SELF-TEST. Possible causes for this fault are:
   • Faulty TCS or switch was not cycled during SELF-TEST.
   • Shorted wiring harness.
   • Faulty Powertrain Control Module (PCM).
   • Open in wiring harness or fuse.

   If TCS was cycled during KOER SELF-TEST, go to step 4). If TCS was not cycled during KOER SELF-TEST, Repeat KOER SELF-TEST  while cycling TCS.

2. 3) Code 623 Or 631
   
   Code 623 or 631 indicates transmission control indicator light circuit fault during KOEO SELF-TEST. Possible causes for this fault are:
   • Burned out bulb.
   • Open or shorted wiring harness.

   If TCS was cycled during KOEO SELF-TEST, go to step 5). If TCS was not cycled during KOEO SELF-TEST, Repeat KOER SELF-TEST  while cycling TCS.

3. 4) Check TCS Circuit
   
   Turn ignition off. Disconnect 60-pin PCM 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. Turn ignition on. Measure voltage between test pin No. 41 and test pin No. 40 or 60 at breakout box while cycling TCS several times. If voltage cycles replace PCM. Remove breakout box, reconnect all components, and repeat QUICK TEST  . If voltage does not cycle, go to next step if diagnosing a trouble code or step 20).
4. 5) Check Circuits For Short To Ground
   
   Turn ignition off. Leave PCM disconnected from breakout box. Disconnect TCS connector. Inspect connector for damage and repair as necessary. Measure resistance between test pin No. 41 and test pin 40 or 60. Also, measure resistance between test pin No. 55 and test pin No. 40 or 60. If each reading is more than 10,000 ohms, go to next step (Code 623 or 631) or step 8) (Code 632 or 653). If any reading is 10,000 ohms or less, repair short circuit. Repeat QUICK TEST  . If code is still present, go to step 7).
5. 6) Check Power Through TCIL Circuit
   
   Turn ignition off. Leave PCM disconnected from breakout box. Measure voltage between test pin No. 55 and test pin No. 40 or 60. If voltage is more than 10.5 volts, replace PCM. Remove breakout box, reconnect all components, and repeat QUICK TEST  . If voltage is 10.5 volts or less, go to next step.
6. 7) Check Output Driver Voltage Signal
   
   Leave PCM disconnected from breakout box. Turn ignition on. Measure voltage between test pin No. 55 and test pin No. 40 or 60. If voltage is 2 volts or more, go to next step. If voltage is less than 2 volts, check bulb and fuse. Replace as necessary. If bulb and fuse are okay, repair open circuit. Remove breakout box, reconnect all components, and repeat QUICK TEST  .
7. 8) Check Transmission Control Switch Wiring Harness
   
   Turn ignition off. Leave PCM disconnected from breakout box. Ensure TCS is disconnected. Measure resistance of Purple/Orange wire between TCS and No. 1 fuse (15-amp) at instrument panel fuse block. Measure resistance of TCS circuit between test pin No. 41 at breakout box and TCS circuit terminal at TCS wiring harness connector. If any reading is 5 ohms or more, repair open circuit. Remove breakout box, reconnect all components, and repeat QUICK TEST  . If each reading is less than 5 ohms, go to next step.
8. 9) Check For Short To Power
   
   Turn ignition off. Leave PCM disconnected from breakout box. Ensure TCS is disconnected. Measure resistance between test pin No. 41 and test pin No. 37 or 57. Also, measure resistance between test pin No. 55 and test pin No. 37 or 57. If each reading is more than 10,000 ohms, replace TCS switch. Remove breakout box, reconnect all components, and repeat QUICK TEST  . If any reading is 10,000 ohms or less, repair short circuit. Remove breakout box, reconnect all components, and repeat QUICK TEST  .
   NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 9) to step 20). No test procedures have been omitted.
9. 20) Check Transmission Control Switch Module (TCSM)
   
   Turn ignition off. Leave PCM disconnected from breakout box. Remove TCSM to access wiring harness connector. TCSM is located behind right side of instrument panel. Turn ignition on. Measure voltage between Orange/Black wire and Black/White wire of TCSM connector while cycling Transmission Control Switch (TCS) several times. If voltage does not cycle, go to next step. If voltage cycles, repair open circuit between TCSM connector and PCM. If voltage cycles but Transmission Control Indicator Light (TCIL) does not turn off and on, go to step 25).
10. 21) Check TCSM Wiring
    
    Turn ignition off. Leave PCM disconnected from breakout box. Leave TCSM connected. Turn ignition on. Measure voltage between Red/Light Green wire and Black/White wire at TCSM connector. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, repair open circuit between ignition switch and TCSM connector. Remove breakout box, reconnect all components, and repeat QUICK TEST  .
11. 22) Check TCSM Input From TCS
    
    Ensure ignition is on and PCM is disconnected from breakout box. Measure voltage between Tan/White wire and Black/White wire of TCSM connector while cycling TCS several times. If voltage cycles, replace TCSM. Remove breakout box, reconnect all components, and repeat QUICK TEST  . If voltage does not cycle, check TCS circuit. Repair as necessary.
12. 23) Transmission Control Indicator Light (TCIL) Always On
    
    Turn ignition on. Cycle Transmission Control Switch (TCS) and observe TCIL. If TCIL turns off and on, problem may be intermittent. Check all TCIL related wiring and repair as necessary. Recheck TCIL operation. If TCIL does not turn off and on, go to next step.
13. 24) Check Transmission Control Switch Module (TCSM) For Short Circuit
    
    Turn ignition off. Disconnect TCSM connector. TCSM is located behind right side of instrument panel. Turn ignition on. If TCIL is still on, repair short to ground in White/Light Green wire between TCIL and TCSM. Remove breakout box, reconnect all components, and repeat QUICK TEST  . If TCIL is off, replace TCSM. Remove breakout box, reconnect all components, and repeat QUICK TEST  .
14. 25) Transmission Control Indicator Light (TCIL) Inoperative
    
    Road test vehicle to ensure overdrive is functioning correctly. If overdrive is functioning correctly, repeat QUICK TEST  . If Code 631 is present, return to step 3). If Code 631 is not present, go to next step.
15. 26) Check Voltage At Fuse Panel
    
    Turn ignition on. Remove and inspect fuse No. 15 (10-amp) at instrument panel fuse block. If fuse is blown, replace fuse and recheck TCIL operation. If fuse is okay, go to next step.
16. 27) Check Wiring To TCSM
    
    Turn ignition off. Disconnect TCSM connector. TCSM is located behind right side of instrument panel. Connect one end of a jumper wire to ground and connect other end of jumper to White/Light Green wire at TCSM connector. Turn ignition on. If TCIL is on, problem may be intermittent. Check all TCIL related wiring and repair as necessary. Recheck TCIL operation. If TCIL light is off, check TCIL bulb. Replace bulb as necessary and recheck TCIL operation. If bulb is okay, repair open circuit between fuse block and TCSM connector. Recheck TCIL operation.