Diagnostic Trouble Code (DTC) P1780 indicates the Transmission Control Switch (TCS) was not cycled during Key On Engine Running (KOER) Self-Test.

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1996 PCED On Board Diagnostics II A



SECTION 6A: Pinpoint Tests





TB: Transmission Control Switch (TCS)/Transmission Control Indicator Lamp (TCIL)



← TB: Introduction


TB1 CHECK TEST VALIDITY


Diagnostic Trouble Code (DTC) P1780 indicates the Transmission Control Switch (TCS) was not cycled during Key On Engine Running (KOER) Self-Test.

Possible causes:

• TCS damaged or not cycled during Self-Test.
  
• Shorted or open harness.
  
• Damaged PCM.
  

Was TCS cycled during Self-Test?


Yes


No


GO to TB2.


RERUN KOER Self-Test to cycle TCS.


TB2 CHECK TRANSMISSION CONTROL SWITCH (TCS) CIRCUIT FOR VOLTAGE

• Key off.
  
• Disconnect PCM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.
  
• Install breakout box, leave PCM disconnected.
  
• Key on, engine off.
  
• Measure the voltage between Test Pin 29 and Test Pins 24 and 77 at the breakout box while cycling the TCS several times.
  

Does the voltage cycle?


Yes


No


REPLACE PCM. REMOVE breakout box. RERUN Quick Test.


GO to TB3.


TB3 CHECK HARNESS CIRCUIT(S) FOR SHORT TO GROUND

• Key off.
  
• Breakout box installed, PCM disconnected.
  
• Disconnect TCS. Inspect both ends for damaged or pushed out pins, moisture, corrosion, loose wires, etc. Service as necessary.
  
• Measure the resistance between Test Pin 29 and Test Pins 24 and 77 at the breakout box.
  

Is the resistance greater than 10,000 ohms?


Yes


No


GO to TB4.


SERVICE short circuit. RERUN Quick Test.


TB4 CHECK CONTINUITY OF THE TCS HARNESS

• Key off.
  
• Breakout box installed, leave PCM disconnected.
  
• Measure the resistance between TCS keypower at the fuse panel (ohmmeter positive probe) and power side of the TCS vehicle harness connector (ohmmeter negative probe).
  
• Measure resistance between Test Pin 29 at the breakout box and signal side of the transmission control switch vehicle harness connector.
  

Are both resistances less than 5.0 ohms?


Yes


No


GO to TB5.


SERVICE open circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test.


TB5 CHECK HARNESS CIRCUIT FOR SHORTS TO POWER

• Key off.
  
• Breakout box installed, leave PCM disconnected.
  
• Measure the resistance between Test Pin 29 and Test Pins 71 and 97 at the breakout box.
  

Is the resistance greater than 10,000 ohms?


Yes


No


REPLACE damaged Transmission Control Switch. REMOVE breakout box. RECONNECT all components. RERUN Quick Test.


SERVICE short circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test.


TB6 CYCLE TCIL

• Key on, engine off.
  
• Cycle the Transmission Control Switch (TCS).
  

Does the TCIL turn on and off?


Yes


No


Concern may be intermittent. GO to Pinpoint Test Step Z1.


GO to TB7.


TB7 CHECK FOR SHORT TO GROUND IN THE TCIL CIRCUIT

• Key off.
  
• Disconnect PCM. Inspect both ends for damaged or pushed out pins, moisture, corrosion, loose wires, etc. Service as necessary.
  
• Key on, engine off.
  

Does the TCIL remain on?


Yes


No


SERVICE short to ground between bulb and Test Pin (79). VERIFY a symptom no longer exists.


TCIL turns off when PCM is disconnected. REPLACE PCM. RERUN Quick Test