Transmission Shift Solenoids
Transmission shift solenoids are used by electronically (computer) controlled transmissions. The design causes them to either open or close allowing pressurized transmission fluid to flow through a given circuit. This fluid will be used to apply bands and clutches that when held and released cause an automatic transmission to shift to the desired gears at the appropriate time.
They are a simple device consisting of a spring loaded plunger wrapped with a coil of wire that when energized draws the plunger inwards either opening or closing a desired fluid circuit. When the power is removed from the solenoid the plunger is then pushed by its spring outward to its normal position.
Shift solenoids are controlled by either the ground or the power side of the circuit. On some transmissions (usually foreign) the solenoid is controlled by a 12 v signal from the TCM (transmission control module). Other transmissions control the solenoid from the ground (usually domestic) by pulsing the ground circuit on and off. This facilitates smooth application of desired gears.
They are easily diagnosed with a scanner, a test light, and an ohmmeter. Usually the scanner will lead you in the right direction right from the start. You will usually find a code somewhere in the P0750 range. Most scanners will even give you a definition of the code. If the scanner were to describe a low power code then you would use the proper test light (always be careful with TCM’s) to check the power feed to the solenoid.
If the scanner described a high power circuit code you would check the ground side of the solenoid and follow the flow chart…. Transmissions will use two or three of these and turn one or both of these on at different times to achieve different gears. Not all transmissions are the same and have different on/off patterns to achieve the same results.
If there were two solenoids the transmission may activate solenoid A for first gear, both solenoids for second gear, and only solenoid B for third. Solenoids are actuators (devices the transmission control unit uses to mechanically achieve a desired result) controlled by the TCM based upon inputs from engine and transmission sensors. These inputs are placed into a preset pattern of parameters that tell the TCM which solenoid to activate and when to activate it.
