I have a problem where my heat pump contactor isn't pulling it. It's a 17 year old Weatherking unit. I saw that I was getting some voltage at the coil so I replaced the contactor. It still didn't pull in, so I checked the voltage again and then I noticed that I was actually getting ~14.5V on both sides of the coil, not 24v. I checked the R & C contact points in the thermostat and verified that I have 24V there. If I manually press in the contactor, the compressor and fan run. What would cause a voltage drop like that? I know there's a transformer at the air handler, that I assume provides the 24V to the thermostat. I assumed that the thermostat then sends the 24v to the coil when calling for cooling. I did notice a second transformer in the outdoor unit, that leads to a control board. This control board does have a wire leading to one side of the contactor coil. Could that transformer be the problem?

I would think the outside transformer supplies power to outside electronics.
Transformer usually supplies 24 volts to thermostat then out to Contactor, the board could add delay and other control or diagnostics. Maybe a relay on board has bad contacts not allowing contacto to see 24 volts.
What voltage do you get from outside transformer?
I don't believe outside transformer powers contactor, just interrupts it.
How many control wires from inside, most wer 2 conductors, have seen as many as 5 to allow diagnostics from inside.

I haven't measured the outside transformer. Will do asap. Will also check on the amount of control wires from inside. I will also try disconnecting the wire that runs from the coil to the control board and see if that has any effect.

Ok. I have four control wires with the following voltage during a call to cool:
Brown: 12.2V
Black: 11.4v
Yellow: 15.5v
Blue: 2.2v

I transformer on the outside unit is getting 120v from the line side of the contactor. It's outputting 13V to the control board via two different wires and 11.3V to the contactor coil via another wire.

My thermostat is OK because I have two in my house and the others been working fine, so I swapped them out as a test and still no luck.

120 v in and only 11.3 to 13V coming out, I`m not a weatherking man,but all the contactors I`ve worked on needed 24V to make it pull in,, so not being there,not able to check it out for myself,1st guess replace the transformer

Most/all I have worked on have been 24 Volts as well.
The 120 Volts is odd, do you have a neutral.
Verify 240 volts out of contactor when overridden. If there is no neutranl and you are getting 120 volts, sounds like you lost 1 leg.
Reset breaker?

It's a single-pole contactor with one wire running between one of the two line sides of the contactor and feeding the transfomer 120v. The weird thing is, my thermostat is getting 24V, but none of the control wires leading to the outdoor unit are sending 24v.

I took a look inside my air handler this morning. The transformer in the air handler is outputting 24v.

There's a control board inside the air handler with what looks like a relay attached. This relay has four terminals. Two of the terminals have a constant 120v. The other two terminals have nominal voltage until a call to cool, at which time these terminals output 11.5v and 14.5v, respectively. These 11.5v and 14.5v wires go back to the thermostat, one of them routing through the air handlers overflow switch (not tripped) along the way.

Could this little black box/relay be my problem? I couldn't tell if it is soldered into the circuit board or can be pulled.

Can you tell if Both units it have a neutral?
Sounds like you are saying it has 2 hots and neutral, plus ground going outside and inside?
Can you read the coil voltage on outside contactor(120 or 240)?

I know the outside contactor has two black 120v lines coming in and a 3rd ground wire that is grounded to the cabinet. So, no... I don't see a neutral.

I understand you have 2 120 volt hots at contactor, but do you have 240 volts between the 2?

That, I haven't checked, but if I press down on the contactor, it does fire up the unit. I did some more troubleshooting before I left for work this morning and discovered I don't know what I am doing when measuring voltages. I was measuring the voltage coming from the control wires to ground (the unit's cabinet). So now, I just measured the yellow wire with one lead of the meter and the brown (common) wire with the other lead and got 25V. So I'm guessing the control wires are OK coming from inside. When I measure the voltage at the contactor coil, am I supposed to be measuring two wires connected together like that, because I've been measuring the voltage of each wire individually, with my ground lead on the meter touching metal?

You should not be measuring off ground, through the transformer, they should be isolated from ground. Wonder if 1 is touching ground.
You need to measure and have 24 volts between the 2 wires on the coil of the contactor.
When measuring voltage in and out of contactor, you need to measure between both hots, not ground.

OK, so I have two yellow wires running into the coil on one side of my contactor and on the other side of the coil is a brown wire.
Am I supposed to get 24v between the two yellow wires on the one side?

I'll measure later today, but if I am getting 24v between those two wires, should I suspect that something on the brown wire side is keeping the 24v from completing the circuit? I think that brown wire runs through a low pressure switch.

BTW, Stratmando, thanks a ton for helping me with this!

Ah ha! Sure enough I had 24v between the two yellow wires. So I bypassed the low pressure switch that's on the brown wire on other side of by contactor coil. That did it.. contactor pulled in. So it's my low pressure switch. Now I have to ask... will it wreck my system to run it while under low pressure? I plan to replace the whole system in 6 months anyway (take advantage of tax credit).

You can run the system with low pressure but it will do you no good as it will not cool.

But can you say you have 240 volts between both hots?

Yes, 240V between the two.

I took the switch out of the equation, nutting the wires leading to it together. So far, I'm getting cool air, so I'm guessing it was just a bad switch.

You think you got it? Hopefully.

Running a system on low refrigerant pressure will take a compressor out every time. If you bypass a safety with out checking to see if it was bad will only bite you in the end. Cool air for a few hours vs $2000 compressor change out or worse for a system change out.