1994 W900L AC Theromostat Switch

After thinking about it for a bit, you could test the thermostatic switch with a pulp thermometer and a test light. Stick the probe of the thermometer in the evaporator next to the probe for the switch.

Disconnect the switch and see which side is always hot on the plug. Then reconnect it and stick your test light on the other terminal.

If the evaporator is warmer than about 32°, your test light should be lit. Start the system up and watch the thermometer go down. The test light should go out very close to 32°. If it doesn't, then your thermostatic switch is bad.

Depending on where that switch is in the circuit, you may have multiple problems if you are having to jump the trinary switch to get power to the thermostatic switch.

From the AC relay, power flows in a series circuit through the thermostatic switch and the trinary switch on its way to the compressor clutch.

If the trinary switch is first in line, then there should be sufficient refrigerant pressure to keep the low-pressure part of the switch closed and the pressure shouldn't be high enough so that the high side of the switch opens. If everything is good at the trinary switch it then sends power through the thermostatic switch and then on to the compressor.

The positions of the trinary switch and the thermostatic switch can be swapped. Either way will work.

The third function of the trinary switch is to turn the fan on when the pressure gets above a set point.

As far as the refrigerant pressures go, refer to a temperature pressure chart for R134a. But basically, on an 80° day with the AC set to recirculate and the fan solenoid unplugged so the fan stays on, you should see about 40 pounds on the low side and 175 to 200 on the high side.

 

The override is for the fan and the terminals on the switch are not like the trinary switches I am used to. Most trinary switches have four wires...

1. Input wire from the fan solenoid. [override]
2. Ground

When the pressure goes above 250 lbs or so, the switch between 1 & 2 closes, grounding the fan solenoid and turning the fan on.

3. Hot from thermostatic switch
4. Compressor

If the refrigerant pressure is above 20 lbs and below 350 lbs, the switch between 3 & 4 is closed, sending power out terminal 4 to the compressor.

This makes me think that KW uses a hot wire to the fan solenoid insteading of grounding it.

So... the ones labeled power and override are for the fan. There is probably voltage on the override terminal and you're sending it through to the thermostat terminal through the closed switch to the compressor terminal and on to the compressor.

If there is no voltage on the wire connected to the thermo terminal, you need to check that there is power on both terminals of the thermostatic switch. If none have power, you have a problem further up the line... a blown fuse, a bad relay or a bad AC dash switch.

If only one has power, the thermostatic switch is bad.

Now for the other problem... fluttering gauges. The gauges should be fairly steady and not flutter. What kind of gauge set do you have?

Is it only the low side, the high side, or both? If they are fluttering real bad (like 1/4 scale), your compressor probably took a hit of liquid on the suction side and busted a reed valve.

 

Since we are on the subject on the amount of freon, if the dryer has a eye glass, can you still go by it, referring to stop adding freon once the bubbles clear thru the eye glass ?

 

Not with R134a unless you put a sight glass right at the expansion valve. There will still be foamy looking liquid at the dryer when it's fully charged.

 

Like I said, when it quits, check for power on both sides of the thermoswitch. After any frost melts off the evaporator, you should have power on both sides of the switch again.

Also verify that there really is no power at the compressor clutch when it doesn't want to come back on. It may be that the clutch is worn out and the air gap is too wide to pull it back in. Simetimes tapping the clutch with a screwdriver handle will get it to engage.

Harbor Freight gauges are ok, albeit the fittings are a bit leaky. I carry a set on the truck with me. If both sides are fluctuating like you say, the compressor is probably toast.

 

You can also estimate when the system is fully charged by placing your hand on the outlet pipe of the evaporator. When you feel it suddenly become very cold, it is properly flooded and has enough.