P0785 – Shift/timing solenoid -circuit malfunction

John Kershaw

By John Kershaw (Contact Me)
Last Updated 2016-01-17
Ed.D, PhD Philosophy, BSME

Trouble CodeFault LocationProbable Cause
P0785 Shift/timing solenoid -circuit malfunction Wiring, shift/timing solenoid, ECM/PCM!TCM

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What Does Code P0785 Mean?

Shift solenoids control the pressure force which in turn controls the position of the shift valve. They are called ON/OFF solenoids and can be normally open to fluid flow or normally closed to block fluid flow. They are commanded on or off by the PCM or TCM. The resistance of most ON/OFF shift solenoids is 10-15 ohms. A linear (straight line) solenoid can be varied by changing the amount of on time to precisely control the fluid flow through the solenoid valve. The variable power or ground applied to the linear solenoids is PWM (pulse-width modulated) which allows the PCM/TCM precise control over the shifting and the fluid pressure. The resistance of most linear (PWM) shift solenoids is about half of ON/OFF type at about 4 to 6 ohms. A PWM signal is a digital signal, usually 0 volts and 12 volts, which is cycling at a fixed frequency. Varying the length of time that the signal is on provides a signal that can vary the on and off time of an output. The ratio of on time relative to the period of the cycle is referred to as duty cycle. Most TCC solenoids are PWM linear types.

DTC P0785 generally involves the 3-2 Shift Solenoid (SS) valve assembly, which is used in many different 4-speed automatic transmissions and transaxles, like the GM 4L65-E and 4T65-E. This solenoid unlike the one in the above animation is a normally-closed, 3-port, on/off device that controls the 3-2 downshift, where the typical shift solenoid only has 2 ports.  The 3-2 solenoid is in valve body inside the transmission. The solenoid receives ignition voltage usually from a fused ignition circuit. The solenoid is operated or controlled by the PCM/TCM providing a ground path. See Figure 1.

NOTE: Whenever diagnosing a 700 series DTC (Diagnostic Trouble Code) with a transmission, transfer case or other driveline components, check for a DTC under chassis and body systems, not just under “P” or powertrain/engine or emission control type codes. Module communications DTCs are “U” codes. These are most often found when looking for DTCs under chassis or body systems. Chassis related codes are labeled “C” and body system related codes are labeled “B” codes and these can cause drivetrain issues if they affect a sensor that is also used by the heating and air conditioning system.

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Figure 1 TCC Solenoid Circuit Wiring Diagram

What are the common causes of code P0785 ?

  • Defective shift solenoid
  • Hydraulic blockages inside the internal transmission fluid passages
  • Mechanical internal transmission failures
  • Low or contaminated transmission fluid
  • Shift solenoid valve harness is open or shorted
  • Shift solenoid valve circuit poor electrical connection
  • Defective PCM/TCM

What are the symptoms of code P0785 ?

  • MIL (Malfunction Indicator Lamp [Check Engine & Service Engine Soon])lights on
  • Failure to shift from (or into) any particular gear
  • Harsh shifting
  • Slippage
  • Transmission overheating
  • Decreased fuel efficiency
  • TCM/PCM may also enter “limp-in” mode when this P0785 is stored.

Codes Related to P0785

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