Nest Y1 always hot

Nest Gen2 thermostat, Y-G-Rc-W-C connected to HVAC.  

With Y connected, compressor always runs.  Simply disconnected Y for the winter to debug later, but now I'm trying to get it all to work correctly.

Measures 25V between C and Y, no matter if Nest is set to Heat, Cool, or Off.  It seems to always call for cooling.

Any suggestions?

Forgot to say, unit is simple Oil heat furnace, simple A/C, forced air.