my freezer is not defrosting. I checked the defrost timer by turning it and it goes thru cycle, it sounds like a clock ticking.
The timer could be ticking but not moving the cam.
You have to mark a spot on the frame/cover and on the cam, then check it an hour or so later to see if it is rotating. Note that the timer cam may hang up at some other point in it's rotation.

Rotate the timer cam till the compressor and the fans go off.
You are now in a defrost cycle.
Check if the defrost element comes on.
Be careful you do not want to burn your fingers.
If it does heat up then probably your timer is toast.

If not the problem could be the defrost element or defrost thermostat.
Could also be the timer perhaps it's internal contacts are shot.

Following are a couple good sites with basic fridge repair help on checking the above components.
http://www.applianceaid.com/refrigerator.html
Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com