Wrong on both assumptions.

The coolant sensor is a termistor and it is measured by The Control Panel. That triggers the relay.
The a/c high side pressure is a simple On/Off relay circiuit.

So, there is the relay sensor side and the relay load side.

The first test is to unplug the blu coolant temp termistor w/key ON. That defaults to high fan.

The second test is to jumper the pigtail at a/c high side pressure sw and that triggers relay directly for low fan. { Make sure you are jumping the pigtailed sw, not the other spade connector one]

So, if you do the two sensor test and no fan, the next test is to bring/jumper battery positive 12v to the Resistor . On one side you should get low fan , the other high.
If yes , then you have a feed problem. [ usually the fuses I had you check.]

One fuse is for load [ relay contacts] and one is relay coil side. [ sensor activated]
So, the next test is to do the sensor test again and listen closely for relay activation [ clicking] That test coil side of relay. If yes , then sensors circuit is working and the feed is bad on the load/fan side. That can be then tested by taking the relays out and jumping the sockets at 30 and 87. That will give direct fans w/o sensor circuits.
You can also test at 30 of the sockets for fuse 12v feed.......

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A Dalton