I have a heat pump that is not energizing when the tstat calls for cool (or hot). I'm getting 24V to the circuitboard (yellow in from tstat to common), but not across the control terminals of the contactor. Manually pushing the contactor down starts the fan and compressor and the system appears to work fine.

It seems likely that I have a problem with (a) the coil, (b) the circuit board, or (c) the "other little black box" (reversing valve?). Am I on the correct path, and if so, could someone please suggest an appropriate troubleshooting procedure to track down the faulty component.

Thanks,
-Tim

What model is the heat pump? The little black box? Is it next to the contactor? It may be a time delay relay... Do you have 24 volts on any terminals outside?

It could be many different problems... Newer heatpumps have several safety devices, I would start with making sure it's not the loss of charge switch. Again I need to know if you have 24 volts anywhere on the heatpump?

OK, I found this similar post and it looks exactly like the problem I am having (you also worked on this one hvac):
May 2, 2007, 06:38 PM
Cnyle
Compressor Contactor chattering?
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Of course, my contactor is not chattering, but rather not engaging at all.

I'll be checking the wiring to the inside unit for continuity in the morning if necessary, but as I mentioned before, there are 24V at the circuit board, so I think the problem is in one of the components in the control area. I'll also jump the control leads on the contactor to make sure the coil is pulling.

As far as the model, I'll have to get back to you on that tomorrow.

Please help me understand the components in the control area and their general function / relationship. If I'm standing at the front of the unit, this area is a box with weatherstripping to the right of the fan / condenser housing. Part of the access panel forms the top of the box by sealing against the gasket. On the far left side of this box is the contactor (sitting roughly above the main capacitor). On the right side, from back to front is a coil, the circuit board (mounted vertically on the right wall of the box), and this other black box (mounted vertically on the front wall of the box). It sounds like this latter control is a time-delay relay based on the above thread.

Let me know if this description is not useful and I can send a pic instead.

Thanks for your help.
-Tim

Check the high pressure switch. Red button bottom of the heatpump just above or bellow the freon lines(out side of the unit) push it in, it's a manual reset for the high pressure switch. Good luck let us know what happens.