Need AC help

[1998Exp]

Oh FFS, I went through the service pinpoint test for AC and it says it's the EATC controls: " REPLACE A/C, heater control switch. "

As annoying as it is to now need to try replacing the EATC unit, I'd have gladly paid the $20 for all this info a week ago!

I wouldn't rush to replace the EATC before making sure that it's indeed the culprit. According to your latest schematic, the EATC provides the current to drive the compressor clutch. It may not be able to, or there may be some problem along the way. If I were you, I'd unplug the low pressure switch, unplug the relay, and improvise a direct connection from the EATC output (the P - I guess pink - wire) to the clutch positive terminal (black/yellow wire), then put the EATC in MAX A/C mode. If the clutch works on direct battery power, but not on this arrangement, you pretty much eliminated everything but the EATC.

 

[Eric1971]

I think I found it, and it explains my inconsistent test results from the low-pressure switch all the way around to the clutch. I pulled the EATC and when I apply 12V to the EATC's clutch output (Purple) everything engages consistently. After a hint in my Google searches, I dismantled the EATC and there was a shot solder connection on the mainboard relay "normally on" pin. Very clear. I touched it up and so far the AC is working, but the post I found that led me there also said his solder fix lasted for 3 days before it went bad again, suspecting a partial internal short in the clutch itself causing the system to draw enough amps to melt that solder point. So now I want to figure out the right way to test around the entire path for any shorts just to be safe.

 

[1998Exp]

I think I found it, and it explains my inconsistent test results from the low-pressure switch all the way around to the clutch. I pulled the EATC and when I apply 12V to the EATC's clutch output (Purple) everything engages consistently. After a hint in my Google searches, I dismantled the EATC and there was a shot solder connection on the mainboard relay "normally on" pin. Very clear. I touched it up and so far the AC is working, but the post I found that led me there also said his solder fix lasted for 3 days before it went bad again, suspecting a partial internal short in the clutch itself causing the system to draw enough amps to melt that solder point. So now I want to figure out the right way to test around the entire path for any shorts just to be safe.

I doubt you have a short to ground and the fuse isn't blowing - it should be 10A; at least that's the rating on mine. You can measure the resistance to ground from the EATC connector and should see about 3-4 Ohms. Some DVMs probes are fairly high resistance, so first take a reading with the probes shorted together and subtract that.
The more likely reason for the solder melting would have been a worn relay contact. With the typical 3 Amp or so that the clutch coil draws, just 0.5 Ohm of resistance there will dissipate about 5 Watts and slowly melt the solder, while still passing enough current to operate the clutch - until it melts completely and disconnects. A crack in the solder could have the same effect.

 

[Eric1971]

Resistance shows 3.6 Ohms, perfect. At this point I guess I'll just see if the sol;der fix "sticks". I also temporarily hooked up a tiny LED to the clutch connector so that I can see when it's engaged from the driver seat.