The construction and principle of operation of the conventional ZF-4HP24 automatic transmission used in a Land Rover Range Rover P38A off-road vehicle has been discussed and results of field testing of the automatic transmission (AT) have been presented. Within the field tests, automatic transmission fluid (ATF) pressures were measured at diagnostic take-off points in the AT hydraulic system. Moreover, the ATF viscosity was determined (at normal operating temperature) for fresh fluid and for ATF samples taken from the AT under tests after a long period of normal operation of the transmission. The field tests were carried out before and after servicing the AT, during which the AT fluid and fluid suction filter were replaced. The field tests revealed that before replacement of the fluid and fluid suction filter, the AT operated incorrectly, especially when the normal ATF operating temperature was achieved.