AC relay

As far as I see it is not ECCS control module (ECM) located below the center of the dash that grounds that pink wire.

Is fuse #13 in the left side of dash okay and voltage making it to the ECCS where that fuse feeds the light green/red wire?
If so, I know the black/yellow wire at the ECCS will not be hot until that AC relay is energized, so we can ignore that one for now. That leaves either the ECCS itself or the triple pressure switch and its feeds, etc.
At this point simply looking at the wiring diagram I cannot tell just how that ECCS and triple switch are supposed to operate, so give me a few minutes to look for any information on it in the repair manual. In the meantime, can you scan for trouble codes from all the computer modules and let me know if there is any codes set and if so, what they are.
Here is a start to understanding it, "Triple-pressure protects A/C system from high pressure build-up due to restriction, overcharge or compressor malfunction. If excessively low or high system pressure is sensed, switch electrically turns on radiator cooling fan and/or turns off compressor to reduce system pressure"
If the ac system pressure is okay, compressor clutch does not disengage, see A/C COMPRESSOR CLUTCH CIRCUIT which I will get as well as If A/C refrigerant pressure is correct, check thermo control amplifier operation. See THERMO CONTROL AMPLIFIER CIRCUIT under SYSTEM TESTS. If thermo control amplifier operation is okay, see A/C COMPRESSOR CLUTCH CIRCUIT, BRB

Knock sensor would have nothing to do with AC.

Must be that ECCS then.

Hi,

I attached the wiring schematic for a 2000 Frontier Manual A/C system. I had to cut the schematic in half to make it readable for you. However, I did overlap the pics so you can follow from one to the next.

Let me know if this is what you needed or if there is something I can help with. Also, here is a link that you may find helpful:

https://www.2carpros.com/articles/how-to-check-wiring

Let me know if you have other questions

Take care,

Joe

See pics below.

You are very welcome. I look forward to hearing from you. I'm interested in knowing if you get it fixed.

Take care,

Joe