Discovery 2. Manual - part 144

AUTOMATIC GEARBOX - ZF4HP22 - 24

DESCRIPTION AND OPERATION

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The gearbox consists of a torque converter housing, an intermediate plate, a gearbox housing and a rear extension 
housing, bolted together in series. The rear of the gearbox is supported by a rubber mounting installed between a 
mounting bracket on the gearbox and the LH chassis rail. A heat shield is installed on the mounting to protect it from 
the exhaust.

Sectioned view of gearbox

1 Lock-up clutch
2 Impeller
3 Turbine
4 Forward drive clutch
5 Reverse drive clutch
6 Brake clutch
7 Brake clutch
8 Brake clutch
9 Epicyclic gear set

10 Epicyclic gear set
11 Clutch
12 Brake clutch
13 Output shaft
14 Freewheel (one way clutch)
15 Freewheel (one way clutch)
16 Freewheel (one way clutch)
17 Stator and one way clutch

Torque converter housing
The torque converter housing attaches the gearbox to the engine and contains the torque converter. Different torque 
converter housings are used to accommodate the difference between the V8 and Td5 engine interfaces. The torque 
converter is connected to the engine drive plate and transmits the drive from the engine to the gearbox input shaft. 
When engaged, a hydraulic lock-up clutch in the torque converter prevents slippage, to give a direct drive from the 
engine to the gearbox for improved driving response.

Intermediate plate
The intermediate plate supports the gearbox input shaft and provides the interface between the transmission fluid 
pump and the lubrication circuit. The pump attaches to the front of the intermediate plate and is driven by the impeller 
in the torque converter. The pump pressurises transmission fluid drawn from the sump on the gearbox housing. The 
pressurised fluid then circulates through the torque converter and gearbox housing components for cooling, 
lubrication and gear shift purposes. Ports around the outer periphery of the intermediate plate provide the inlet and 
outlet connections to the fluid cooler and a pressure take-off point for servicing.

AUTOMATIC GEARBOX - ZF4HP22 - 24

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DESCRIPTION AND OPERATION

Gearbox housing
The gearbox housing contains two epicyclic gear sets on input and output shafts. Hydraulic brake clutches on the 
shafts, control which elements of the gear sets are engaged, and their direction of rotation, to produce the P and N 
selections, four forward gear ratios and one reverse gear ratio.

Gear ratios

Gear

Ratio

1st

2.480 : 1

2nd

1.480 : 1

3rd

1.000 : 1

4th

0.728 : 1

Reverse

2.086 : 1

AUTOMATIC GEARBOX - ZF4HP22 - 24

DESCRIPTION AND OPERATION

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Valve block

1 Valve block
2 Pressure regulating solenoid valve (MV 4)
3 Shift control solenoid valve (MV 2)
4 Shift control solenoid valve (MV 1)
5 Lock-up solenoid valve (MV 3)
6 Output shaft speed sensor
7 Bolt
8 Sensor retaining clip

9 Manual valve

10 'O' ring
11 Filter
12 'O' ring
13 Suction pipe
14 Bolt
15 Bolt
16 Washer

AUTOMATIC GEARBOX - ZF4HP22 - 24

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DESCRIPTION AND OPERATION

The lock-up and brake clutches are operated by pressurised transmission fluid from the valve block in the sump. A 
manual valve and four solenoid valves, also known as Motorised Valves (MV), control the supply of pressurised 
transmission fluid from the valve block:

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The manual valve controls the supply in P, R, N and D.

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Solenoid valves MV 1 and MV 2 control the supplies that operate the brake clutches for shift control.

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Solenoid valve MV 3 controls the supply that operates the lock-up clutch.

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Solenoid valve MV 4 modulates the pressure of the supplies to the brake clutches, to control shift quality.

Operation of the manual valve is controlled by the selector lever assembly. In the gearbox, a selector shaft engages 
with the manual valve. The selector shaft is connected to the selector lever assembly via the selector cable and a 
selector lever on the left side of the gearbox. The selector shaft also operates a mechanism that locks the output shaft 
when P is selected.

Operation of the solenoid valves is controlled by the EAT ECU.

An output shaft speed sensor in the gearbox housing outputs a signal to the EAT ECU. The EAT ECU compares 
output shaft speed with engine speed to determine the engaged gear, and output shaft speed with vehicle speed to 
confirm the range selected on the transfer box.

A bayonet lock electrical connector in the gearbox casing, to the rear of the selector lever, connects the solenoid 
valves and the output shaft speed sensor to the vehicle wiring.

A pressed steel sump encloses the valve block and collects transmission fluid draining from the gearbox housing. A 
suction pipe and filter on the underside of the valve block connect to the inlet side of the fluid pump. A magnet is 
installed in the sump to collect any magnetic particles that may be present. A level plug and a drain plug are installed 
in the sump for servicing.

Rear extension housing
The rear extension housing provides the interface between the gearbox housing and the transfer box. A splined 
extension shaft, secured to the gearbox output shaft by a bolt, transmits the drive from the gearbox to the transfer 
box. A seal in the rear of the housing prevents leakage past the extension shaft. A breather pipe, attached to the left 
side of the rear extension housing, ventilates the interior of the gearbox and rear extension housings to atmosphere. 
The open end of the breather pipe is located in the engine compartment at the right front corner of the engine sump 
on gearboxes fitted to early vehicles and is clipped to the top of the gearbox on later vehicles.

Gearbox power flows
The following Figures show the power flow through the gearbox for each forward gear when D is selected, and for 
reverse. The key to the Item numbers on the Figures, and in parenthesis in the accompanying text, can be found on 
the 'Sectioned view of gearbox' Figure, above.

1st Gear (D selected)

Clutches (4) and (11) are engaged. The front planet gear carrier of gear set (9) locks against the gearbox housing 

through freewheel (15) when the engine powers the vehicle, and freewheels when the vehicle is coasting. Gear set 

(10) rotates as a solid unit with the front planet gear carrier. In 1st gear hold brake clutch (8) is applied to provide 

overrun braking.