The V-belt driven cooling fan increases in fan speed in proportion to the increase in engine speed. For this reason, the fan speed of the cooling fan with the temperature control fluid coupling control the fan speed by sensing the temperature of the air flowing through the radiator. The temperature control fluid coupling contains a fluid clutch with silicone oil. The transfer of revolution to the fan via the V-belt is controlled by the adjustment of the amount of oil in the operating chamber. When the temperature is low, the rate of fan revolution is decreased helping with engine warm-up and preventing noise. When engine temperature is high, the fan revolution rate is increased to supply an adequate volume of air to the radiator, thereby increasing the cooling effect.

2. Operation

Air Temperature (HOT) during low speed driving

The fluid coupling shaft revolution is transferred to the fan as is.

Air temperature (HOT) during high speed driving

Fan revolution resistance increases and the fluid coupling slips to make the fan revolve at a rate slightly slower than the fluid coupling shaft.

Air temperature(WARM) during high speed driving

The bimetal plate switches the oil route and decreases the operation oil amount. This increases slippage of the operating chamber, resulting in an even further decreased rate of revolution.

Air temperature (COLD) during high speed driving

The oil route is switched and operation oil level further decreases. At this time slippage is greatest and the fan revolution rate is the least.

Electronically Controlled Hydraulic Cooling Fan System The electronically controlled hydraulic cooling fan system uses a hydraulic motor to turn the fan. The computer adjusts the amount of oil flowing to the hydraulic motor, so the fan revolves without stages and the fan speed can always be adjusted to achieve the most appropriate air volume. Compared to the electric fan, the motor is smaller and lighter in weight, and has the ability to supply a greater air volume. However, the oil pump and the control system are more complicated.

EngineCompression Pressure

1. Check compression pressure

Allow the engine to warm up and stop. Remove all spark plugs and crank the engine with the throttle valve fully opened in order to measure the compression pressure of all cylinders.

HINT:

Disconnect connectors of all injectors so that fuel cannot be injected.

Remove the igniter or disconnect connectors of the igniter so that a spark is not generated.

The fully charged battery should be used to obtain an engine speed of over 25rpm.

NOTICE:

This inspection must be performed in as short a time as

possible.

Example1NZ-FE engine (NZE12#)

Compression pressure1,471 kPa ( 15.kgf/cm2)

Minimum pressure1,07kPa ( 11.kgf/cm2)

Difference between each cylinder98 kPa ( 1.kgf/

cm2) or less

SERVICE HINT:

If the compression pressure is low, pour a small amount

of engine oil in the spark plug hole. Measure the compression pressure again.

When the compression pressure risesA piston ring or cylinder bore may be worn or damaged.

When the compression pressure stays lowA valve may be sticking, a valve seat may be improper, or there may be leakage from the gasket.

1. Transfer

The layout of the center differential, front differential, transfer drive gear and transfer driven gear is almost the same as with the center differential mechanical locking type, but viscous coupling is built into the transfer block to limit the differential action of the center differential. The center differential assembly consists mainly of the center differential right case, center differential intermediate case, center differential left case, center differential pinion gears, and center differential right side gear.

2. Power transmission

(1) During straight-ahead driving The transmission of engine power from the center differential drive gear to the center differential right and left side gears is the same as that in the center differential mechanical locking type. The power from the center differential right side gear is transmitted via the transaxle mode select sleeve to the transfer drive gear, transfer driven gear and rear propeller shaft. It is also transmitted to the viscous coupling outer plate. Since the No.2 intermediate shaft is inserted into the center differential left side gear, power is also transmitted to the viscous coupling inner plate.

(2) During cornering Since the viscous coupling acts to reduce the difference in the number of revolutions between the center differential right and left side gears, the differential action of the center differential is limited. The differential limiting function is not so strong as to obstruct smooth turning. (2/4)

3. Center differential (viscous coupling)

(1) Construction The center differential gear is the bevel gear type and the center differential limit control mechanism is the viscous coupling type. The viscous coupling is built into the transfer ring gear mounting. The inner plates of the viscous coupling are joined to the center differential left side gear and the front differential case, while the outer plates of the viscous coupling are joined to the ring gear mount case and the center differential right side gear. Since the viscous coupling is built into the transfer ring gear mounting case the construction has been changed from the three-axis center differential shaft, based on the mechanical locking transaxle, to the four-axis type. A viscous coupling is a kind of fluid clutch which transmits torque by the viscous resistance of oil. It uses this viscous resistance to control the operation of the center differential, creating an effect in the center differential like that created by an LSD (Limited Slip Differential).

(2) Operation The center differential with viscous coupling operates when rotational speed differences between the inner plates (front) and outer plates (rear) of the viscous coupling occur, transmitting power and limiting the differential action of the center differential. Torque is transmitted by the viscous resistance of the silicone oil.

<1>When plates rotate at identical speeds No viscous resistance is generated.

<2>When plates rotate at different speeds When the plates begin to turn at different speeds, the silicone oil particles are pulled away from each other and a resistance is generated. This resistance reduces the difference in speed between the two plates.

<3>Transmitted torque characteristics

During normal operation The amount of viscous resistance due to differences in the rotational speeds of the outer and inner plates increases or decreases in proportion to the extent of the difference in speed.

During humping In the viscous coupling continues to operate with the outer and inner plates rotating at large different speeds, the temperature and internal pressure inside the viscous coupling increases. The inner plate, which can move in an axial direction, is pulled toward the side where pressure is lower. As a result, the inner plate contacts the outer plate directly to create a large resistance. This condition is called humping. Since there exists no speed difference between the inner and outer plates during humping, the temperature and internal pressure inside the viscous coupling decreases. The compressed bubbles expand again and separate the inner plate from the outer plate.