3 wire thermostat help

I have an old white rogers mercury thermostat with 3 wires coming from an old spacepak system (cooling only). Wire colors are green, white and red. Based on the install manual from 1969 it appears white is actually yellow and there’s a jumper between 1&3. Is there a way to make this work? Do I need a plug in transformer? Any help would be appreciated.

Welcome to the Wyze community @orestes107!
Are the wires labeled anything on the thermostat? What does your control board look like? Please post pictures if possible.

First, take pic of your current thermostat wire coming out the wall and the backing plate.

Second, take pic of your board where thermostat wires connected to your furnace.

If you have 5 wires bundle you may just use free wire to connect to C

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On the thermostat they’re labeled W,R,G. The unit is in the attic and I can try to get pictures later. But if I can get it to work with a plug in transformer I’m guessing it’s only 3 wire bundle? If that’s the case could I just use a plug in transformer? If so, what colors do I tell the app and how do I wire it?

So I got a 24V adapter and hooked it up to RC and C. Then hooked up G and W and taped off the red wire. Thermostat fired up but showed missing wires (the ones from the AC). I skipped past that and tried to test it but it didn’t fire anything up. Any thoughts?

Since U have AC only .
Your W is Y.

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Tell the wyze you have the following wires: R, G, Y
(your white wire is Y)
Do not use the 24V adapter / transformer, it may not work with your air conditioner. Instead, use the C adapter included with the wyze. You will need to open up your air handler and take a picture of the thermostat wiring on that side and post it here if it is not immediately clear to you which wire is which and how to hook up the C adapter.