Transmission - Operation
OPERATION
CAUTION:
A unique transmission fluid has been developed for the 8HP45 transmission. This fluid is NOT compatible with ATF+4 or any other current Chrysler transmission fluid.
The 8HP45 is an electronic eight-speed automatic transmission. The Transmission Control Module Assembly (TCMA), which is integrated into the valve body, provides fully synchronized clutch-to-clutch shifting through four planetary gear sets. The TCMA includes a mounting plate that holds the Transmission Control Module (TCM) and a molded wiring harness for connection to various transmission sensors and solenoids. The valve body assembly contains all the sensors and solenoids required for operation, completely inside the transmission. Eight speeds allow the engine to maintain its optimal rpm range, increasing fuel economy and performance.
Transmission control is performed by the TCM based on hard-wired and CAN bus signals from sensors and modules. The TCM receives driveability data from the Powertrain Control Module (PCM) and other modules over the CAN-C bus. It also receives shift lever position information from the electronic shifter over a dedicated transmission CAN bus. The TCM processes this input data and controls operation of the torque converter clutch, park lock system, solenoid valves, and pressure regulating valve.
The input and output speed sensors are Hall-effect sensors that measure shaft rotational speed. The input speed sensor is located at the top, near the center, of the of the TCMA and reads input shaft speed from the magnetic ring on the P2 carrier. The output speed sensor is located at the back of the TCMA and reads output shaft speed from the P4 carrier
FILTER SERVICE The 8HP45 has a conventional fluid sump design, however, the filter is integrated into the oil pan resulting in a lower profile for improved vehicle packaging. The oil pan gasket is reusable providing it is not damaged during removal.
FLUID CHECK AND FILL A fluid fill tube and indicator are not provided. All work is performed under the vehicle while raised on a hoist. In the event of a transmission shift quality concern, a fluid leak, or in conjunction with a transmission repair, the transmission fluid level must be validated and topped off as necessary. The procedure involves the use of a scan tool to monitor transmission fluid temperature. Specific service procedures are necessary to check and fill the transmission with fluid Service and Repair.
CLUTCHES
The 8HP45 transmission uses two multi-plate holding clutches (A and B) and three multi-plate driving (rotating) clutches (C, D, and E). Gear shifts from one - eight and eight - one are synchronous shifts, meaning one of the clutches must continue to transmit torque at lower pressure until the other clutch is able to accept the input torque.
Clutches A and B are holding clutches. Clutch A is applied by hydraulic pressure and released by a return spring, similar to the driving clutches. Clutch B operation is unique because it has fluid chambers on both sides of the apply piston, and therefore it is applied and released hydraulically. Regulation of fluid pressure on each side of the piston allows precise positioning of clutch B for apply, release, and controlled slip conditions.
Clutches C, D, and E operate in a similar manner. Regulated pressure is available at the clutch C shift solenoid and the clutch valve. When the shift solenoid is de-energized, fluid flows through the shift solenoid and applies pressure to the clutch valve. During the shift, pressure moves the clutch valve, which begins to apply the clutch. Pressure from the holding valve regulates the opening of the clutch valve, softening the shift. Above a set threshold, the holding valve seats and allows the clutch valve to open completely.
EMERGENCY RUNNING FUNCTION - LIMP IN MODE
In the event of a complete transmission electrical failure (loss of power to the TCM), the transmission enters default limp-in mode. When the TCM loses power, all solenoids are de-energized. Maximum pressure locks the transmission in 6th gear, and a diagnostic trouble code (DTC) is stored in memory. If the vehicle is in a forward gear range when the fault occurs, the transmission defaults to 6th gear.
If the vehicle is in park, reverse, or neutral, or if the engine is turned off when the fault occurs, the transmission will remain in park because the park lock release system cannot release the parking pawl. No hydraulic pressure is supplied to the driving clutches. In this situation, the manual park release lever must be used to disengage the park pawl.
The limp-in function remains active until the DTC is rectified or the stored DTC is erased with the appropriate scan tool.
Oil Pump Operation
The oil pump is located just behind the torque converter, between the pump housing and cover assemblies. The torque converter drives the pump assembly using a chain and sprockets. The oil pump is a double-stroke vane pump. The pump has dual chambers, two inlet and two outlet ports, allowing it to produce the fluid volume necessary for all operating conditions. The pump draws fluid through a filter and pressurizes the fluid as the pump rotates. After the fluid is pressurized, it exits the pump through two exhaust ports that feed the system pressure valve. The system pressure valve maintains fluid pressure and allows excess pressure to be returned to the pump. This reduces cavitation and noise. A slipping clutch can often be determined by comparing which internal units are applied in the various gear ranges. The Clutch Application chart provides a basis for analyzing road test results.
CLUTCH APPLICATION
SOLENOID CHART
TP= Touch Point Pressure
± = Variable Actuation
POWERFLOW COMPONENTS
Figure 1 - POWERFLOW COMPONENTS
Use this POWER FLOW COMPONENTS graphic and chart to identify the individual components of the specific gear powerflow explanations below:
FIRST GEAR POWERFLOW
Clutch A (1) holds the P1/P2 sun gear (17) and clutch B (2) holds the P1 annulus (3). Because two members of the same gear set are held, the entire P1 gear set is stationary. The stationary P1 carrier (18) is connected to the P4 annulus (9), locking the annulus. The input shaft (19) drives the C clutch (13), and the C clutch (13) drives the P4 sun gear (10). The P4 sun gear (10) drives the P4 carrier (11), whose pinions walk around the held P4 annulus (9).
SECOND GEAR POWERFLOW
Clutch A (1) holds the P1/P2 sun gear (17) and clutch B (2) holds the P1 annulus (3). As with 1st gear, the entire P1 gear set is stationary. The stationary P1 carrier (18) is connected to the P4 annulus (9), locking the annulus. The input shaft (19) drives the P2 carrier (16). The P2 carrier (16) drives the P2 annulus (4). The P2 annulus (4) drives the E clutch (14) through the P3 sun gear (5) connection. The E clutch (14) then drives the P4 sun gear (10). The P4 sun gear (10) drives the P4 carrier (11), whose pinions walk around the held P4 annulus (9).
THIRD GEAR POWERFLOW
Clutch B (2) holds the P1 (3) annulus. The input shaft (19) drives the C clutch (12), which drives the P4 annulus (9). The C clutch (12) also drives the E clutch (14), which drives the P2 annulus (4). Because the P2 carrier (16) and the P2 annulus (4) are both driven at input shaft speed, the P2 gear set is locked at input shaft speed. The P1/P2 sun gear (17) drives the P1 carrier (18), and the P1 carrier drives the P4 annulus (9).
FOURTH GEAR POWERFLOW
Clutch B (2) holds the P1 annulus (3). With the E (14) and D (12) clutches applied, all components of the P3 and P4 gear sets are locked together to rotate at the same speed. The input shaft (19) drives the P2 carrier (16) which drives the P1/P2 sun gear (17). The P1/P2 sun gear drives the P1 carrier (18). The P1 carrier drives the P4 gear set through the P4 annulus (9).
FIFTH GEAR POWERFLOW
Clutch B (2) holds the P1 annulus (3). The input shaft (19) always drives the P2 carrier (16). Because the C clutch (13) is applied, the P3 annulus (15) and P4 sun gear (10 ) are also driven at input shaft speed. The D clutch (12) is applied to connect the P3 carrier (6) with the P4 carrier (11). The P2 carrier (16) drives the P1/P2 sun gear (17), which drives the P1 carrier (18). The P1 carrier walks around the held P1 annulus (3) and drives the P4 annulus (9). Gear reduction is achieved between the P4 sun gear (10) rotating at input shaft speed and the P4 annulus (9) rotating at a reduced speed.
SIXTH GEAR POWERFLOW
Clutch C (13) drives the P3 annulus (15) at input shaft speed and E clutch (14) drives the P3 sun gear (5) at input shaft speed. Because two components of the same gear set are driven at the same speed, the entire gear set is locked in rotation. The D clutch (12) connects the P3 carrier (6) to the P4 carrier (11) and the output shaft.
SEVENTH GEAR POWERFLOW
Clutch A (1) holds the P1/P2 sun gear (17) stationary. The input shaft drives the P2 carrier which drives the P2 annulus, increasing the speed of the P3 sun gear (5) and creating overdrive. The C clutch (13) drives the P3 annulus (15) at input shaft speed. The P3 sun gear (5) spins at a faster speed than the P3 annulus (15) and the P3 carrier (6) is the output of the gear set. This results in a reduction that offsets the overdrive of the P2 gear set. The D clutch (12) connects the P3 carrier (6) to the P4 carrier (11) and the output shaft.
EIGHTH GEAR POWERFLOW
Clutch A (1) holds the P1/P2 sun gear (17) stationary. As with 7th gear, the P2 gear set creates an overdrive ratio. However, the E clutch (14) is now applied, which locks together the P3 gear set and eliminates any torque multiplication from the P3 gear set. The overdrive created by P2 is transferred by the D clutch (7) to the output shaft.
REVERSE GEAR POWERFLOW
Clutches A (1) and B (2) hold the P1 gear set to hold the P1/P2 sun gear (17). The input shaft drives the P2 carrier, which produces an overdrive ratio from the P2 gear set, driving the P3 sun gear (5) in engine direction. The P3 carrier (6) is connected to the output shaft by the D clutch (12). This holds the carrier, reversing the direction and multiplying torque. The P3 annulus (15) is connected to the P4 sun gear (10), which drives the P4 gear set opposite of engine direction and multiplies torque again.