Where is the sensor on a Kenmore electric 70 Series?
Could a defective timer be the cause?

Usually the sensor is located in the drum area. Some systems also have a sensor relay located in the control panel. The timer could also have defective contacts for the sensor circuit. Sorry I cannot be more clear on this but there are so many different setups over the years it is difficult to be extremely accurate as to locations. To test these circuits you will need a meter and know how to use it and also a wiring diagram and service manual to tell you what the readings on the meter mean as to the parts (weather bad or good). Most homeowners cannot work on these systems since it is electronic in design. Now replacing a belt or other mechanical is usually no problem. Sorry I cannot be of more assistance.