What does it 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. When shift solenoid malfunctions, the problem is generally not the electrical part of the solenoid; it is debris obstructing the mechanical function of the solenoid or the flow of the fluid through the transmission valve body.
If the transmission fluid is very dirty, you should change the fluid and filter and when the pan is down look for excessive debris or metal particles, which could be an indication that there is a mechanical failure and that the transmission will need to be rebuilt or replaced.
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. The 1–2 SS (shift solenoid) valve controls the fluid flow action on the 1–2 and the 3–4 shift valves. The 1–2 SS valve is a normally open exhaust valve that is used with the 2–3 SS in order to allow different shifting combinations.
When the TCM or PCM detects the wrong gear ratio, then DTC P0751 sets or detects an improper voltage drop when it tries to operate the solenoid valve.