What was found
AFTER TAKEOFF, LANDING GEAR WOULD NOT RETRACT TO UP POSITION. PERFORMED QRH PROCEDURES FOR OVERRIDE SWITCH, STILL NO RETRACTION. REMOVED LEFT MAIN LANDING GEAR SHOCK ABSORBER PROXIMITY SWITCH, FOUND TO BE INSTALLED INCORRECTLY. BENCH CHECK AND REINSTALLED IAW MM 320101, PERFORMED SEVERAL GEAR CYCLES AND OPS CHECKS GOOD.