Is my thermostat transformer voltage too low?
Is there a C wire at the thermostat?  You should measure between R and C for a proper voltage reading.

Y connects back to the other side of the transformer through the coil of the contactor, in the condensing unit.  G connects back to the other side of the transformer through the coil in the fan relay, in the air handler.  C connects directly back to the other side of the transformer. 

The only way to get an accurate measurement of the transformer voltage, is to measure between R and C.