HVAC BLower Motor Control Module

[Ground_Loop]

I have an older Carrier Infinity HVAC system.  The thermostat communicates with the compressor and air handler on a simple digital network.  The digital controller on the air handler then communicates with the blower motor controller to provide run signals and return some basic motor run status.  I have had the motor control module go out a couple times, I think related to power droops and or lightning strikes.  Fortunately, I have been able to diagnose the problem and get replacements off Ebay.  The motor appears to be a 3-phase permanent magnet motor with no feedback, pictured below.  Anyway my question, do any of you know the signaling to one of these?  Based on the simplicity, just TX and RX to the motor controller, I'm guessing its just a PWM signal for three different speeds and a returned pulse train indicating activity.  I could probably cobble up a break out connector and drag my scope under the house to check, but that would be messy.

[ajb]

There are a couple of 'standard' control connectors/pinouts, but the motor control unit is typically lightly customized for the air handler manufacturer.  It could have anything from one or more fixed speeds selected by on/off control lines all the way up to a full digital control interface.  There may not actually be feedback from the motor control to the air handler control at all -- if the air handler is giving you some sort of error, it may be detecting a problem with the motor indirectly based on the state of the rest of the system. 

The motor manufacturer might have some documentation for their standard motor configurations, but it will be hard to know for sure if it matches your air handler's configuration without some reverse engineering.  Examining the air handler control might be useful.  There are some motors that can be configured in the field, used as generic replacements, once you know what kind of interface you need.  Control voltages may be 24V or 240V, so be careful if you choose to probe at it.