P0785 – Shift/timing solenoid -circuit malfunction
Last Updated 2016-01-17
Ed.D, PhD Philosophy, BSME
|Code||Fault Location||Probable Cause|
|P0785|| Shift/timing solenoid -circuit malfunction |
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|Wiring, shift/timing solenoid, ECM/PCM!TCM|
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Table of Contents
- What Does Code P0785 Mean?
- What are the common causes of code P0785 ?
- What are the symptoms of code P0785 ?
- Codes Related to P0785
- Get Help with P0785
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.
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.
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
- 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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I’m experiencing “P0785 Shift/Timing Selenoid Malfunction” from OBDII scan. I’ve replaced both shift solenoids even though they tested 21-23 ohms, which I read was normal. The fluid looked & smelled good & clean,when the truck is cold it shifts good but once it gets hot,it starts giving problems again.
1997 chevy silverado 1500
Having the exact same problem. Bought solenoids and ignition switch/cable. Need to perform some electrical tests before I start pulling parts off.
Did you determine/fix your issue?
Having this same problem change solenoids and still no change and also change key switch
Same problem. I replaced both the 23 and the 34 solinoid but i just read that the code is for the 32 down shift solinoid. Going to try that next. Hope it takes care of the problem
On my 1998 GMC SIERRA K1500 w/4L60-E, I’m experiencing “P0785 Shift/Timing Selenoid Malfunction” from OBDII scan. I’ve replaced both shift solenoids even though they tested 21-23 ohms, which I read was normal. The fluid looked & smelled good & clean, very few metal shavings in tranny pan. I cleared the code but after 2 miles or so, P0785 popped up again, via SES light. I can clear code & tranny shifts perfectly each time. Any suggestions on what to do next? Can this be a PCM/VCM issue? Thank You.
Maybe your radiator loosened pressure. Or over heated your oil and makes a false reading
Have you confirmed that the circuits running between the PCM and the transmission are good? Try testing them from the PCM connector to verify you’re getting the same resistance value. Chances are good you may have a corroded connector somewhere in the circuit that’s causing the fault. If the circuit is good and the solenoids are good, then PCM might be at fault. PCM failure isn’t impossible, but it’s a last resort.