question for HVAC guys

If yer smacking the control box to get it to work, it's most likely a bad blower relay. Not many motors have oil caps, and you'd have to pull the blower wheel off to get to the one on the shaft end. The last few years they had oil plugs, they were yellow plastic. Have you checked to see if yer getting the heat elements on with no blower? Or are you getting nothing, no blower or heat elements. Does your test meter have microfarads? The switch would have MF or UF along with volts, amps, and ohms. If so, test your capacitor on the blower motor(unhook wires and discharge by grounding first). Most capacitors have the value stamped on them, it's most likely 12.5 -20 mf. If the capacitor is swollen or leaking, replace it, they are only $5-$10. Another capacitor test is to manually spin the blower wheel with the stat on, if it takes off, yer capacitor is bad. BTW, the reason you weren't getting a reading at the stat was there is no common if the stat only has 4 wires, you'd have to find common inside the AHU and meter it there.