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Thermo Switch or Evaporator Switch Question.

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  • Thermo Switch or Evaporator Switch Question.

    Thermo Switch or Evaporator Switch Question.

    Short Back Story.

    1990 Mazda B2200 AC all parts took apart, cleaned and put back together, new drier and compressor and using Duracool.

    Worked great, till my evaporator had a leak in it at the suction side connection.

    When I say worked great, I mean 95F ambient and heat index of 114F and vent temps reaching 36-38F
    It cycled beautifully. There is a thermo switch on the side of the evaporator case and its designed to cut the compressor off at 33.8F and I believe it did that just fine.

    I have two of these trucks and they both work the same.

    Now present day.

    I had an extra evaporator and I cleaned it up and had it ready to go with all new Expansion Valve in place. I put in it, charged it up as I have done many times and all was working well ACCEPT it was not getting as cold, or it would, it would just take longer because of the thermoswitch, turning it off at 46F and back on at 55F (vent temps).
    This was not what it had been doing, plus it was sort of stinking like a water funk.

    So after many adjustments and things that claimed to take smell away, I decided to just order a NEW evaporator and try again.

    Brand new evaporator in place, no stink, all parts in place and its doing the same thing.
    Instead of cutting off at 38F vent temp, it consistently turning off at 48F.
    I decided to see if it was the thermoswitch and used a wire to bypass it.

    When its bypassed, it gets the vents down to 35F, of course because its freezing down there.

    I can't understand why both of my (known good) ThermoSwitches are doing the exact same thing.

    It is not hard to install and I have placed them in the exact spot where they go and its done it to me twice. Its like its 10 degrees off in both directions.

    Any tips or ideas?

    Should I order a new one that has an adjustable dial?

    This is the current switch part number, its by Ranco

    Click image for larger version

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  • #2
    Mitch - I understand the issue completely. If mine, there are several options I might explore.

    1. Test your switches in cold water baths using a digital thermometer. These switches are contact switches designed to be "open" at about 35F and to "close" a few degrees above that to complete the circuit and engage the compressor.

    2. Pull the sensor line out a few inches so its in a less-cold region of the coils, or just sitting in the evaporator box.

    3. Install a toggle switch in place of the evaporator switch/de-icer switch and just use the AC push-in button to switch off the compressor if the temperatures get too cold or switch off every few minutes to allow any ice to melt and condensate to drain.

    4. Bypass/jump the evaporator switch/de-icer switch and just use the AC push-in button to switch off the compressor if the temperatures get too cold or switch off every few minutes to allow any ice to melt and condensate to drain.

    5. On my B2200 and 1998/2004 Frontiers, if the cabin gets too cold (yes - I'm in Arizona desert) I simply push in the AC switch to turn off the compressor for a couple of minutes, then push it in to re-engage the compressor as needed; I don't add in heated air to temper the air temperature. In your part of the country you don't get as hot as Arizona, but you have more humidity, so cycling it off manually is easy and should work for you. I do that on ll three of those trucks.

    6. Run a indicator light on the dashboard in parallel to the compressor wire so you can be positive that the compressor is not getting power when the cooling stops. I have such indicator lights rigged up on both my B2200 and my 1998 Frontier; last summer I used that light on the Frontier to diagnose AC clutch slippage on it, and removed a shim under the clutch drive plate to fix it (didn't even need to remove the AC belt or compressor to do that !!).

    7. Try an adjustable switch, especially if the stated range is lower than 32F.


    • #3
      1st check the evaporator temp with something, your vent temp may be in the 40's but the evaporator temp may be close to freezing. Air bypassing the evaporator, or heat leaking in from the heater are places to look. The switch reacts to evaporator temp not vent temp, so make sure the switch is the problem. Check it in a ice water bath like Cusser said. The important thing is to prevent the evaporator from freezing, not to control the vent setting.
      That is a real common switch design and looking in any A/C catalog will show several, with varying probe lengths, settings (fix or adjustable).