88 TR AC Clutch 12V power wire

Have you performed the resistance checks of the input devices to the HVAC ECU shown on page H30 of the TR WSM?

 

Wow. I didn’t even know my TR had a HVAC ECU. Is it solid state or does it have tubes?

 

There are quite a few connections (beyond just the high-low pressure switch) involved in the HVAC ECU "deciding" to turn on the AC Compressor Clutch Coil to that signal actually reaching the AC Compressor, but the OP needs to determine if:

1. The HVAC ECU is trying to turn the AC Compressor Clutch "on", but the signal doesn't get there. When the HVAC ECU wants to turn the AC Compressor Clutch "on", it puts +12V on its (Roman Numeral) pin II output (two CN orange-black wires).

or

2. The HVAC ECU never tries to turn the AC Compressor Clutch "on" because it's dead or it's getting a wacky input signal so it never tries to turn it "on" (these are the resistance checks on page H30).

The OP's passenger cabin blower working is a good sign that +12V power is present to run the HVAC ECU -- the three M brown wires on its (Roman Numeral) pin III should be +12V whenever the key is "on".

 

Yes, key "off" (but having so many seemingly wrong seems weird). All of those resistance measurement use pin #1 which should be a good ground -- measure the resistance from pin #1 (the BN white-black wire) in the 6-pin (unplugged) HVAC ECU connector to the negative battery terminal = should be near 0 Ohms. If not = that needs to be fixed.

 

may be the meter only needs a new battery?

 

And, with everything plugged in, do you also have no power (no +12V) on (Roman Numeral) pin II (two CN orange-black wires) of the 4-pin HVAC connector when you request AC "on"?

 

That's a very good sign that the HVAC ECU is OK, and the problem is just a bad connection in the path from Pin II to the AC Compressor Clutch. Next make the same voltage measurement between the CN (orange-black) wire (and ground) at the high-pressure switch:

1. if it also is +12V, measure between the VM (green-brown) wire (and ground) at the high-pressure switch -- it should also be +12V if the high-low pressure switch is really working .

2. If it isn't, you need to check the connections in the k and b white connectors of the fuse-relay panel. (By "check" - just unplug, visually inspect, and replug.)

If the VM wire at the high-low pressure switch is +12V (but not +12V at the AC compressor), check the a and j white connectors at the fuse-relay panel and the C11 9-pin square connector under the coolant expansion tank in the engine bay.

If you need a copy of Fig 9 for the late US TR HVAC system, use this link:

https://www.dropbox.com/s/gne5xw608ba818b/509_88 US TR Wiring Diagram Book.PDF?dl=0

 

That's always the problem with electrical Gremlins -- just getting in to investigate, stuff gets wiggled around enough to alter things. Even though the connectors didn't look obviously "burned", the contact area is very small so unplugging and replugging them "wipes" the contact surfaces which can be very beneficial. You can't fix it when it's not broken, but now you are armed with enough knowledge to know where to look in the food chain from Pin II of the HVAC ECU to the AC Compressor Clutch Coil if it recurs. (You should still be thanking the Gods that your HVAC ECU was OK .)

 

Is your a/c refrigerante low as to shut the pressure switch to out?
that is correct operation if you need fill so compressor does not run 100% to die