[dpugh717]

Ever since inadvertantly "allowing" the local electric company to install a control unit so they could cycle the compressor on the AC unit I have had trouble. The contactor will not always engage. The control unit is gone, but the the problem is still there. I have put in 2 new contactors, and still the same issue. It will click on, but will not engage. Any thoughts?

 

[mdshunk]

Uh-huh... check the voltage at the coil of the contactor. If you don't have every bit of your 24 volts, the contactor will not reliably pull in.

 

[HVAC Doc]

Who removed the control unit? If it wasn't wired back up properly it would do like MD said or they could of shorted something out. Make sure your two wires (if straight a/c) are properly ran to the contactor coil. Depending on the brand and model, they could route through pressure/temp safety switches as well and you might need to check those with an ohm meter if it has them and make sure none of them are tripped as well.

 

[dpugh717]

Checked the voltage, only had 23 across the contactor. Thought maybe a new wire might do the trick. Got to working on another project and left it alone, forgetting to turn off the TStat. A few hours later, it is working just fine. This has happened before. It seems to quit if I take it out of a "routine" that is, move the TStat up or down a degree or two, shut the unit off due to cooler weather, etc. Now what?

 

[HVAC Doc]

Check through your safety switches with a meter next time it does it. See what safety is tripped so you know what course of action to take.

 

[dpugh717]

Finally called in a HVAC company, as I was getting frustrated. Diagnosis was the time-delay circut board that controls the compressor was bad. I was told that it wasn't needed, and the tech just disconnected it. Does that sound correct? The unit appears to be worknig fine, so I have to assume he's right.

 

[mdshunk]

Sure, the "time dalay" (rightfully, anti short cycle timer) does break the Y wire normally. That's one of the safties that HVACDoc told you to check. Many electronic thermostats have their own delay built in now anyhow, so the delay in the condensing unit is often redundant.

 

[HVAC Doc]

You are ok having it disconnected. As MDshunk mentioned, they are 99% of the time redundant. The only time I feel it is really really needed ( and should be replaced if faulty) is when you either have an old mercury bulb stat or and early digital with no time delay. Stat time delay is actually better as most of them installed in the unit are only 3 minutes where almost all digital stats are 5 minutes.