Model #WED9400SW0, this Duet styled dryer would dry clothes properly, but the display would count down to around 8 minutes remaining, and then would just keep running until the customer shut the unit off manually. And to make the trouble shooting process just a little bit harder, this unit was only 3 months old, and installed in a garage without a vent. So much for the easy stuff.
Like many dryers that use electronic controls, this model uses a combination of temperature and moisture sensors to fairly accurately determine when the items in the drum are dry, and constantly updates the display with the time remaining. For years, we technicians have pointed out to our customers that the time remaining is only an approximation based on the information the control is getting from sensors. The previous version of this model would even add minutes if it suddenly thought it was going to take longer to dry.
The particular sensor used to determine actual dryness of the contacts is the moisture sensor. Often seen mounted to the front or rear dryer bulkheads, these metal strips provide resistance readings for the control board to interpret and determine how much longer to dry. They work on a pretty basic level. As clothes tumble across, an electrical connection is made between the two bars. Depending on the dampness of the clothes, the resistance between the bars will either be low (wet items), high (dry items) or anywhere in between.
If there is a problem with the sensor, or the circuit, the unit will continue to count down, but somewhere around the 8 minute remaining point (per the customer and Whirlpool) the unit will just keep running until it finally reaches the maximum 2.5 hour point. Which is exactly what the customer described as the problem.
Using my meter, I did a quick check of the moisture sensor at the harness to verify an open circuit between the two bars. Anything but an infinite resistance reading would indicate a failure or something stuck between the two. The reading I got was just right so I moved on to the next step. Checking between each outside wire to the middle wire I again took resistance readings. The middle wire is connected to ground and each outside wire is connected to one of the moisture bars. Between each set of wires is a varistor which is used for voltage suppression to protect the control board from static discharges. These too should give an infinite reading, but I found one of them with a very low resistance. Looks like it's time for a new harness.
After replacing the sensor harness on this dryer, it is now counting the remaining time properly and drying the clothes as expected. No more 2 hours cycles for the unit.