When a Variable Blower Runs Too Slow
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First, you have to check that all of the pins have been set up properly at the air handler (set to the proper kilowatts, tonnage, etc.). You also have to check whether you have nominal, low, or high airflow or if you’re running in enhanced mode.
Make sure you have your yellow wire at the correct Y terminal; in some cases, there are multiple Y terminals (Y1 and Y/Y2), and the Y/Y2 terminal is the one that will allow the blower to run at its proper speed. Then, you have to check to make sure there is a blower call between G and common. Without a G call, the blower will run, but it won’t run as it’s supposed to. So, you would use your voltmeter and check for 24v between G and C. Finally, you need to make sure there’s a call on the dehumidification terminal, which allows the blower to reach a higher speed. (Check for 24v between C and DH.)
If the blower is oscillating and the wiring looks good, then you may have a blower module problem.