4wd will always engage when you move the lever. It's a mechanical operation. The flashing green light is only a symptom of either the front diff actualtor not being fully operated (poor vacum), or the switch the actuator operates, not working. 99% of the time, it's poor vacuum that causes either flashing, or inoperarating 4wd light. Actuator should move about 1/2 inch = 12 or so mm. If it's only moving 5mm, it's not operating the switch, hence no light.