This article is written by my good buddy Bill Frisbie. Service manager and crack technician at AirFx, a Trane dealer in lovely Inverness Florida. Thanks Bill!
One of the most intimidating things in the field is to walk up to a piece of equipment that you have either never worked on, or you just don’t understand. One of the most misdiagnosed systems is the Trane Hyperion air handler, more especially the Tam7, or Aam7 (American Standard). The most common issue they've had is the sensors that connect to the EVC board, and one attaches to the suction line, much like a sensing bulb, and the other is after the EEV and entering the coil. The black lead goes to the gas sensor (suction) and the orange goes to the ET sensor (coil entering.)
In order for the system to function correctly, these sensors must be able to accurately read those temperatures, and there is a chart that helps us check those values on site.
Another common fault has to do with the way the float safety is tied into the system. When the float trips (opens), the loss of voltage should occur between the thermostat and Y1 before entering the board. If you break the circuit between the board and the condenser, the condenser will not run, however, the EVC does not know the outside is not running. The EVC continues to monitor evaporator temperatures, it thinks that the superheat is incorrect, and can cause a fault. To fix this, simply confirm the wiring is correct, and of course make sure the drain is clear.
The system also has test pins located at the bottom front of the EVC board, that you can test open and closed, to make sure the stepper motor is working. Although they can be bad or plugged, it’s been very rare in my personal experience, to have a bad EEV valve body. Our company installs 10-12 systems per week on average, and I’ve replaced 2 in 7 years.
As far as the actual EVC board, I may personally have 5 changed in 7 years. There is also a chart to check the stepper motor, and can tell you if it’s bad, but that’s also pretty rare.
A more common issue would be the wire harness that connects the board to the coil. I’ve had several where the harness has a wire that does not make contact through the plug, or an installer has drilled in a bad area to attach something to the cabinet, and damaged the wire buried between the inner and outer panel. One fairly common issue is also having the dc voltage control fail. Its located left of the door switches and has 240 v entering, and 13.8 VDC leaving to the AFC board. Every failure though is because a bug has gone between the plate and board, and caused a short.
I’ve come up with a simple image that our technicians have as a resource on site, so they can confirm the boards have communication and control voltage where it needs to be.
Included is a PDF of the Tam7 service facts sheet that can help as a guide to help with troubleshooting in the field. Once you get the hang of it, its really a solid piece of equipment, and fairly easy to repair.