SA227 pilot reports diverting to repair a malfunctioning main gear uplock. Upon moving the speed levers to low at 40 knots; the aircraft makes a hard left turn of the runway. No damage is apparent and the reporter is able to taxi to the hanger.

Narrative

I departed for my morning flight and after departure I noted a right main landing gear light flashing intermittently. The gear was also making a rhythmic thumping; trying to catch the up-lock. I tried cycling the gear and the light and thumping continued. I contacted our Dispatcher and described the problem. After speaking with the Assistant Chief Pilot and Maintenance it was decided that I would divert to ZZZ to fix the problem. The gear lowered normally and I determined that emergency equipment was not needed. I made a normal approach and landing. Our company policy is to move the speed levers to low at 40 knots; which engages the nose wheel steering. As I did that the airplane made an immediate 90 degree left turn and exited the runway. I was able to avoid striking the lights and the runway sign; which passed between my nose wheel and the propeller. When the plane came to a complete stop I was asked by the Tower if I could move the plane. I cautiously tried moving the plane and found that I could taxi normally. I taxied back to our hangar and checked the plane for damage. I found none. Our maintenance department checked the plane and found the up-lock on the right main gear was broken and replaced several items for the nose wheel steering system. The Metroliner is known for 'hardovers' such as this one. They have been very rare at my company since implementing the procedure of waiting until 40 knots to engage the nose wheel steering.