The ZF 4HP 16 automatic transaxle consists primarily of the following components.

The converter consists of the impeller, the turbine wheel, the reaction member (stator) and the oil to transmit torque. The impeller, which is driven by the engine, causes the oil in the converter to flow in a circular pattern. This oil flow meets the turbine wheel, where is direction of flow is deflected. At the hub, the oil leaves the turbine and reaches the reaction member (stator), where it is once again deflected so that it reaches the impeller at the correct angle of flow.

The reversal effect generates movement in the stator, the reaction torque then amplifies the turbine torque.

The ratio between turbine torque and torque is referred to as torque multiplication.

The greater the difference is speed between the pump and turbine, the greater the torque multiplication; it is at its highest when the turbine is at a standstill. The higher the speed of the turbine, the lower the torque multiplication.

When the turbine speed reaches about 85%of the pump speed, torque multiplication=1, i.e. the turbine torque equivalent to pump torque.

The stator, which bears against the housing via the freewheel, is then rotating freely in the oil flow and the freewheel is over-come. From this point onwards, the converter acts as a straightforward fluid coupling.

The converter lock-up clutch is a device, which eliminates converter slip and thus helps to improve fuel consumption.

The previous control principle for converter lock-up clutch operation has been replaced by a controlling function on the 4 HP 16. The converter lock-up clutch is engaged and released in a controlled manner. During the controlled phase, a slight speed difference between the impeller and turbine wheel is established. This ensures that the engine’s rotating vibration is not phased on to the transaxle. The result is optimum shift quality.

An electronic pressure-regulating valve determines pressure regulation of the lock-up converter clutch's piston.

When open (conversion range), the oil pressure behind the converter lock-up clutch piston and in the turbine zone is equal. The direction of flow is through the turbine shaft and through the space behind the piston, to the turbine chamber.

To engage the lock-up clutch, the direction of flow is modified (reversed) via a valve in the hydraulic selector unit. At the same time, the space behind the lock-up clutch piston is vented. The oil pressure passes from the turbine chamber to the lock-up clutch piston and presses it against the converter’s cover. The turbine is thus blocked by way of the linings between the piston and cover, and permits rigid through drive with no slip (or reduced slip if controlled) to the mechanical stage of the transaxle.

The fluid pump is located between the torque converter and the transaxle case and is driven directly by the torque converter. The pump sucks the fluid through a filter and delivers it to the main pressure regulator valve of the control system. Excess fluid flows back to the pump. The fluid pump fulfills the following functions:

The ZF 4HP 16 automatic transaxle is equipped with a one sun gear, 4 planetary gears, planetary carrier, ring gear.

Each gear is located one directly behind the other and are linked together. In other words, front ring gear is permanently linked to rear planet carrier, front planet carrier is linked to rear ring gear.

The individual gear ratios are obtained by linking together the gear set elements in different ways by means of clutches and brakes.

On the 4HP 16, the power flow is directed into the planetary gear set via rear planet carrier or rear sun gear, or via both simultaneously, depending on the gear in question. The output is always via the front planet carrier.

The purpose of the shift elements is to perform shifts under load without the tractive flow being interrupted.

The shift elements consist of the following.

The shift elements are engaged hydraulically. The pressurized oil reaches the space between the cylinder and piston, as a result the discs are compressed. The clutch/brake is engaged when the oil pressure drops, the cup spring acting on the piston presses the piston back into its initial position. The clutch/brake is now released again.

Depending on the gear, the multi-disc clutches B and E supply the engine torque to the planetary gear train, with multi-disc brakes C, D and F directing the torque into the housing.

The dynamic pressure at clutches B and E is equal : i.e. the dynamic pressure in front of and behind the piston is equal. This equalizing effect is achieved in the following way.

The space between the baffle plate and piston is filled with unpressurized oil. A dynamic pressure dependent on the engine speed builds up. The space between pressure also builds up. However, there is simultaneously a static pressure, which causes the clutch to engage. If the static pressure is relieved, the cup spring is able to force the piston back into its original position.

The advantages of this dynamic pressure equalization are:

The parking lock is actuated via the selector lever when in position P. It protects the vehicle mechanically against rolling away.

The stop plate is actuated by the selector shaft, which is permanently connected to the selector lever via a pull cable. The parking lock pawl on the parking lock gear is welded onto the lateral shaft of the transaxle and this prevents the drive wheels from turning.

This blocks the driven wheels.

Valve body performs the following tasks:

The driver engages the travel position via the selector lever:

The Park/Neutral Position Switch is located on the selector shaft and informs the TCM of the current selector lever position P-R-N-D-3-2-1.

The selector lever position is transmitted to the TCM in encoded form along 4 lines. The encoding is such that malfunctions in the connecting lead are identified.

The Park/Neutral Position Switch is located on the selector shaft, which is connected to the selector lever via a pull cable. In addition, the Park/Neutral Position Switch controls the starter interlock, the reversing light and the selector lever position indicator on the instrument panel.

The vehicle A/T OSS is a magnetic inductive pickup that relays information relative to vehicle speed to the TCM.

Vehicle speed information is used by the TCM to control shift timing, line pressure, and TCC (lock-up clutch) apply and release.

The output speed sensor mounts in the case at the speed sensor rotor, which is pressed onto the spur gear. An air gap of 0.1mm~1.3mm(0.004~0.05in) is maintained between the sensor and the teeth on the spur gear teeth. The sensor consists of a permanent magnet surrounded by a coil of wire.

As the differential rotates, an AC signal is generated by the output speed sensor (OSS).

The A/T ISS is a magnetic inductive pickup that relays information relative to transaxle input speed to the TCM.

The TCM uses transaxle input speed information to control line pressure, TCC apply and release and transaxle shift patterns. This information is also uses to calculate the appropriate operating gear ratios and TCC slippage.

The input speed sensor mounts onto piston B that is inside of valve body.

An air gap of 1.8~2.2mm(0.07~0.086inch) is maintained between the sensor and the piston B.

The sensor consists of a permanent magnet surrounded by a coil of wire. As the piston B is driven by the turbine shaft, an AC signal induced in the input speed sensor.

Higher vehicle speeds induce a higher frequency and voltage measurement at the sensor.

Sensor resistance should measure between 825~835 ohms at 20°C (68°F). Sensor can measure from 1,000~8,000HZ.

The shift solenoids are two identical, normally open electronic exhaust work that control upshifts and downshifts in all forward gear ranges. These shift solenoids valves together in a combination of ON and OFF sequences to control the line pressure and shift mechanisms (clutches, brakes).

Solenoid 1 controls the high or low of the line pressure (flow to each clutch valve) by the operation type (ON/ OFF), i.e. solenoid 1is ON, line pressure will be low (87~116 psi (6~8bar)), solenoid 1 is OFF, line pressure will be high (232~261 psi (16~18bar)).

Solenoid 2 controls the oil flow to clutch valve E or lockup clutch valve by the ON/OFF signal.

The TCM monitors numerous inputs to determine the appropriate solenoid state combination and transaxle gear for the vehicle operating conditions.

The pressure control valve (EDS valve 3,4,5,6) is a precision electronic pressure regulator that controls the operation of the clutches, brakes and the lock-up clutch.

The valve reduces the system pressure with which the downstream solenoid valves and electrical pressure regulating valves are supplied. It is possible to use smaller solenoid valves as a result. The EDS require a constant input pressure.

The TFT sensor is a positive temperature coefficient thermistor (temperature sensitive resistor) that provides information to the TCM regarding transaxle fluid temperature. The temperature sensor is located in valve body. Calculated temperature is a factor used to determine the shift time and shift delay time.

The internal electrical resistance of the sensor varies in relation to the operating temperature of the transaxle fluid (see chart).

The TCM sends a 5 volt-reference signal to the temperature sensor and measures the voltage rise in the electrical circuit. A higher fluid temperature creates a higher resistance in the temperature sensor, thereby measuring a lower voltage signal.

The TCM measures this voltage as another input to help control line pressure, shift schedules and TCC apply.

When transaxle fluid temperature reaches 140°C (284°F) the TCM enters "hot mode." Above this temperature the TCM modifies transaxle shift schedules and TCC apply in an attempt to reduce fluid temperature by reducing transaxle heat generation. During hot mode the TCM applies the TCC at all times in fourth gear.

Also, the TCM commands the 2-3 and 3-4 shifts earlier to help reduce fluid heat generation. Hot mode may not be available on some applications.

The transaxle electrical connector is a very important part of the transaxle operating system. Any interference with the electrical connection can cause the transaxle to set Diagnostic Trouble Codes (DTCs) and/or affect proper operation.

The following items can affect the electrical connections:

Carefully limit twisting or wiggling the connector during removal. Bent pins can occur.

DO NOT pry the connector off with a screwdriver or other tool.

To reinstall the external wiring connector, first orient the pins by lining up arrows on each half of the connector.

Push the connector straight down into the transaxle without twisting or angling the mating parts.

The connector should click into place with a positive feel and/or noise.

The transaxle control module (TCM) is an electronic device which monitors inputs to control various transaxle functions including shift quality and transaxle sensors, switches, and components to process for use within its' control program. Based on this input information, the TCM controls various transaxle output functions and devices.

The data link connector (DLC) is a multiple cavity connector. The DLC provides the means to access serial data from the TCM to aid in powertrain diagnosis. The DLC allows the technician to use a scan tool to monitor various systems and display diagnostic trouble codes (DTCs). The DLC connector is located within the driver's compartment, directly below the steering column.