Hi Deborah,
There is no defrost timer in your refrigerator. The defrost operates by the control board (#5 at the
diagram).
You have to check the defrost heater and defrost thermostat (#10 & #14 at the
diagram) for continuity. The defrost thermostat must be ice cold at that time.
If there is nothing wrong with these two parts, replace the control board.
- The defrost heater
Part number: 12729128 
- The defrost thermostat
Part number: W10165425 
- The control board
Part number: 12868513 
Here are the breakdown diagrams and
Parts for Maytag ABB2222FEB2 Refrigerator - AppliancePartsPros.com
Gene.