Conventionally, four wheel drive vehicles that have a drive power distribution control device that optimally distributes the drive power transmitted to the wheels, based on the lateral G measured by a lateral G sensor are known. If the lateral G sensor in this vehicle is faulty, it is not possible to appropriately distribute the drive power, so sometimes a breakdown detection device is provided. When a breakdown is detected by the breakdown detection device, a predetermined fail safe operation is carried out (see for example, Japanese Patent Publication No. H7-40040). This fail safe operation can include suspending control of distribution of drive power and running as a two wheel drive vehicle, or controlling the engine to reduce the output. Also, a warning light is lit to alert the driver that a fail safe operation has been carried out.
A breakdown detection device that determines that the lateral G sensor has a breakdown when the absolute value of the difference between the actual lateral G and the estimated lateral G calculated from various parameters expressing the vehicle condition is equal to or greater than a predetermined value (for example, see Japanese Patent Application Laid-open No. H10-142256) is known. At this time, when running with the wheels gripping the road surface and rotating normally, the yaw rate increases and decreases together with the lateral G, so it is also possible to calculate the estimated lateral G based on the yaw rate measured by a yaw rate sensor.
However, a problem that can arise when running on a road surface with a low coefficient of friction due to snow or ice is that if the vehicle spins by oversteering while turning, the actual lateral G reduces but the yaw rate increases. At this time it is easy for load to be applied to the outer wheels, but the inner wheels slip, so the rotation speed of the outer wheels becomes equal to or less than the rotation speed of the inner wheels. When the yaw rate does not follow the increase and decrease of the lateral G in this way, it is not possible to accurately calculate the estimated lateral G. Therefore, even if the lateral G sensor is normal, the difference between the actual lateral G and the estimated lateral G increases, and there is the possibility of incorrectly determining that the lateral G sensor has a breakdown. Also, when it has been incorrectly determined that the lateral G sensor has a breakdown, the fail safe operation will implemented incorrectly. If the fail safe operation has been carried out, the driver may drive to a place where the lateral G sensor can be repaired, and start the operation to repair the breakdown, even though the lateral G sensor is normal. This will cause the driver and repair person unnecessary time and effort, which is a problem from the point of view of serviceability of the vehicle.