If you know how to wire a thermostat to turn the compressor on and off, you ALREADY know how to wire the tstat to control a Liquid Line Solenoid valve.

Pretend that the valve is the compressor contactor, and set your LPC to cut out at about 15, and cut back in at about 25.

When your tstat opens, (Shuts off) the Liquid Line Solenoid coil will deenergize, which closes the valve itself, which will cause the refrigerant in the evaporator to pump into the receiver and condenser, until the LPC "opens" (shuts off) the compressor contactor, turning off the compressor.

As the box temp rises, and the tstat "closes", it OPENS the valve, by powering up the solenoid coil, and the whole process starts all over.

Here is an abbreviated list of what you'd need to do:

  • Cut in a liquid line solenoid valve (rated greater than the entire system tonnage- REAL easy on a small system like you have.) right before the coil.
  • Install a thermostat (Close-on-rise, like a Penn A19) near the solenoid valve, preferably near the BACK (Inlet air side) of the evaporator.
  • Find a power source for the tstat and solenoid. (This can come from the evap fans, if no other source is available, although a separate source is much preferred.) It does NOT have to connect in any way, shape or form to the condensing unit.
  • Wire the "hot" to one side of the stat, then take this "hot" from the other side of the stat, and get it to the solenoid coil.
  • Wire the L2 or "neutral" (depending on the voltage. I'm assuming 115, but 230 is also common...) direct to the solenoid coil. (Other wire in coil, not already occupied by the "hot".)
  • Connect the ground whereever there is a place to connect it. (The stat and the coil should have grounding screws already in place.)
  • Turn all this **** ON.
  • Set the tsat to 35 degrees or so.
  • LOWER THE LPC SETTING TO 15-25, like I said above. {NOTE: The LPC is STILL turning the compressor on and off, but only on a "command" provided by a low suction pressure condition caused by the tstat turning the coil off!!!}
  • Get paid.
  • Go home.
  • Repent your sins.


The entire process sounds intimidating, until you've actually done it.

The parts are relatively inexpensive, and you don't have to CHANGE A DAMN THING, other than the setting on the low pressure switch.

I could do this whole thing, vacuum and all, in a real limited-access cooler, in about 2-3 hours, but I've done it a few times before...

Print this out, go unf*ck that thing, and make the customer happy.

You might want to discuss the bill with your boss before you hand it to the customer, as he may want to knock some off of it for all the excess time. That is OK. I'd worry if he didn't. Remember, he THREW your sorry ass into this mess while he was filling up the G-string of some worn out stripper near some airport. He should expect to cover some of that bill.

.Have fun