General Description

The 45RFE electronic transmission is a conventional transmission in that it uses hydraulically applied clutches to shift a planetary gear train. However, the electronic control system replaces many of the mechanical and hydraulic components used in conventional transmission valve bodies.
The 45RFE electronic transmission is a fully electronically controlled transmission. The Transmission Control Module (TCM) is similar to (But not the same as) the one used in the 41TE and 42LE (FWD) transmissions, therefore many similarities exist in function and diagnosis.
The 45RFE has an overrunning clutch (used in 1st gear), an electronically controlled torque converter clutch, 3 planetary gearsets, and six clutch packs. The clutches are called 2nd Clutch (2C), 4th Clutch (4C), Low/Reverse Clutch (LR), Reverse Clutch (RC), Underdrive Clutch (UD), and Overdrive Clutch (OD).
Although the 45RFE is considered a 4speed transmission, it really has 5 forward gear ratios. 2 ND gear (1.67:1) and 2 d prime (1.50:1) gear are so close in ratio that they are not considered to be different gear ratios, although both are used as 2 ND gear under certain conditions. During most upshift and downshift maneuvers, 2 ND gear will always be used. 2 ND prime gear is only used for a heavy throttle 4-2 downshift. All gear ratios in the 45RFE are achieved by applying two elements (clutches). During a shift, one element is released and another is applied, resulting in a different ratio. This is called a clutch to clutch shift. In order to perform a 4-2 downshift, two elements would have to be released and two different elements applied. The 2ND prime gear ratio allows a clutch to clutch 4-2' (2ND prime) downshift.
The oil pump in the 45RFE is a dual stage positive displacement gear type pump. At idle and low engine speeds, both stages are working. Once the engine speed reaches a point where one side of the pump can supply the necessary system requirements, the second stage is vented. This pump configuration gives the pressure and flow of a large displacement pump at low speeds, and the economy of a small displacement pump at higher engine speeds. The oil pump housing also contains some of the valves that are found in the valve body in a 41TE or 42LE transmission. The Converter Clutch Switch Valve, Converter Clutch Regulator Valve, Torque Converter Limit Valve, and the Pressure Regulator Valve, are all found in the oil pump housing.
The electronic control system consists of a Transmission Control Module (TCM), a Transmission Range Sensor (TRS), an Input Speed Sensor (ISS), an Output Speed Sensor (OSS), a Line Pressure Sensor (LPS), a Transmission Temperature Sensor (TTS), five pressure switches, and seven solenoids. Each clutch pack has a corresponding solenoid and pressure switch except for the reverse clutch which is controlled by the manual valve. The other two solenoids are called the Multi Select (MS) solenoid and the Pressure Control Solenoid (PCS).
The PCS is used to control line pressure. The 45RFE controls line pressure based on inputs to the TCM. The line pressure is torque based (line pressure increases with torque) most of the time, however it is set to a predetermined value just prior to a shift and reverts back to torque based after the shift.
The MS solenoid is used to control the LR clutch during P-R and N-R garage shifts and to control the OD clutch when the manual Valve is in the "D" position as reported by the TRS. If the manual valve is slightly out of position, the TRS will indicate a temporary zone (T3 or T4). In this case the OD clutch will be controlled by the OD solenoid. Note that if the TRS indicates a temporary zone, this is a valid PRNDL code and will not set a DTC 28. If the PRNDL code consistently indicates a temporary zone while the shift lever is in the "D" position, this would indicate some sort of mechanical problem in the shift linkage as opposed to an electrical TRS problem.