2006 Johnson SD 30 HP 4 Stroke Outboards Service Manual, PN 5006592, Page 243Get this manual
POWER TRIM AND TILT OPERATION
Thermal Valve
The PTT system incorporates thermal valve for protection of the internal components if excessive downward force is exerted on the lower unit with the motor in tilted position or, in the case of an impact in reverse gear, the stern clampswivel brackets and the transomShould the propeller strike an underwater object while in reverse gear, build up of pressure will be induced in the lower cylinder chamber and the brackets andor the transom may sustain damageTo prevent this, the thermal valve will open to relieve the oil pressure, thereby softening the impactInternal PTT circuits are protected, as the thermal valve will open to reduce oil pressure caused by either hot climate or abnormally heavy usage-