Hi hvac guys; this is my problem. I have a new t-stat installed there is 240v power at contact l1 -l2 single pole) . There is 24v across contacts. So when all the power is turned on , I turn the t-stat down below the room temp. (room-85--t-stat down to 60) wait 5- min or so . Nothing starts. Air-h nothing . Cond unit nothing. The contacts do not pull in. But : If I push them in manually the comp and comp fan at the cond unit runs. Let go and cond stops. Also the air handler blower will not start at all. Even when the cond. Is started manually. Im hoping you may have an answer to this trouble shoot . Thank you. Chag.

If you can manually close the contactor and everything works the contactor points are good. The question is whether the coil is good.

Disconnect the low voltage wiring and do a continuity check of the low voltage leads to the coil of the contactor. You should have voltage between the low voltage wiring and ground. With low voltage wiring disconnected you should also have voltage between wires. With low voltage wires connected to contactor your meter should show no voltage or almost no voltage between terminals. That would be because the voltage is going through the coil and should be equal. If coil is good the terminals should show no voltage between terminals but each should show voltage to ground. If coil is faulty there would be no current going through coil so terminals would show difference in voltage.

I suspect you've wired something wrong. If you are getting 24 volts at the contactor, you haven't blown a fuse or transformer. Double check the wiring on the stat & configuration of the stat if it is one which can be configured for different applications.

24v across contacts? Do you mean you get a 240v reading when checking from one side of contact to the other? Or you are getting 24v to the coil of the contactor? If the air handler is not coming on, I do think you may have blown a low voltage fuse, or transformer on the airhandler when wiring the new stat. This would cause the air handler to not come on and the outdoor unit to not come on. Check for power to the indoor air handler. Check for 24v between R and C at the air handler control where the t-stat wires hook. If you don't get 24v there, you have a fuse or transformer issue in the air handler.