| 摘要 |
<p>A mechanical four wheel drive tractor having a solenoid valve which is de-energized to effect engagement of drive to the front wheels in a predetermined operational mode. Normally, the valve is controlled by an operator switch but when the engine is overloaded and exhaust gas temperature rises, a thermal sensor breaks the circuit to the solenoid valve so that four wheel drive is automatically engaged. This action is restricted to a low gear range by a switch linked to the mechanism selecting between the low range and a high range. Restriction to a low gear range prevents unwanted automatic four wheel drive engagement from occuring in other gear ranges without regard to vehicle speed. Automatic engagement of four wheel drive reduces the overload on the transmission.</p> |
