AC and heat not working. Any Ideas as to why?

[jetjock]

If your coolant valve isn't closing off the air mix servo probably isn't moving. That could also cause issues with the heat.

Are you trying to fix heat or AC? One is simple. The other, not so much...

 

[BigKO]

Originally I was fixing both. AC still does not work, heat does. I had to replace the heater valve switch with a known working one. This solved the heat problem.
I am now working on the AC.

I then went on to ask about the diagrams listed in one of my recent posts, where it talks about the AC Amplifier. Due to condition A in the diagram (open circuit) it says that the heater valve will remain always open, even if the temps are turned down in cabin. (which it is) I was wondering if the AC amplifier being a likely cause of this, or if I should look elsewhere.

I also asked (which grim jack gave a good hint) , if the auxiliary fans (two small ones on drivers side) were related to the AC and if they wouldn't be turning on because of a related issue.

I am now trying to diagnose the problem with the AC.
I think it may be either the amplifier or the pressure switches. From the diagram in my second to last post, I am beginning to believe the amplifier has an open circuit. I also am unsure if the diagram is referring to the amplifier internals (open circuit situation) or to the entire AC circuit. However at the end of the diagram it says to replace the AC amplifier.

I am looking for likely sources, or opinions on my thoughts so far. Thanks

 

[jetjock]

Heat can be complicated, AC is simple. Your coolant valve not shutting off when the temp is set to 65 indicates a problem but doesn't have anything to do with the AC being inop.

As Nick said it's mostly about the AC amplifier (not the system amplifier which is what you're referring to) and the dual pressure switches. Try this: jumper the diag block and, with your foot off the gas and engine off, turn on the AC. Make sure the AC button illuminates. Does the compressor clutch engage? Does it click?

Next look at your check engine light. Is a code 51 being displayed? Alternately you can warm the engine up and see if the rpm increases when the AC is turned on. That should also cause a 51 to be displayed on the CEL.

If any of these things happen and assuming the system has sufficient refrigerant the problem lies in the AC amp or compressor circuit.

Lastly, assuming the wiring has not been mucked with, the aux fans will only come on when refrigerant high side pressure or coolant temp is excessive...

 

[BigKO]

Thank you those are good things to try. I will get on that and update when done. I have not attempted to jumper the diagnostic block, but AC compressor does not engage. But, I will do as stated. Second, the refrigerant has recently been filled, because it was completely empty. Leaks were fixed at the same time.

Additionally, what is the difference between the AC amplifier, and system amplifier? I though the AC amplifier was inside of the control unit? Am I mistaken?

 

[jetjock]

You're mistaken. The system amplifier is inside the head unit. It controls cabin temperature using the ambient and in-car sensors by manipulating fan speed and the blend dampers in an attempt to match cabin temp to desired temp when the system is in auto. It's temperature driven and has nothing to do with compressor clutch operation.

The AC amp is a small box located located up behind the glove compartment. It's purpose is to control clutch operation by accepting a simple off/on signal from the head unit. As long as the few other sensors connected to the amp are happy (dual pressure switch, evaporator limit sensor, and coolant temp) it will turn the clutch on.

When the AC amp receives the turn on signal from the head unit the amp also sends a signal to the engine ECU that causes idle to increase. It's this signal that generates the code 51 when in diag mode. By using this function one can verify if the head unit and AC amp are working and on which side of the amplifier the problem with clutch operation is located.

See here: http://www.cygnusx1.net/Supra/Library/TEWD/MK3/manual.aspx?S=Main&P=143

Be sure when doing this you have your foot off the gas and the car is in P or N if an auto. Of course, if the code comes and goes when you push the AC button on and off it's obviously caused by head unit to AC amp communication and not something else...

 

[BigKO]

Great information jetjock. That really helped my understanding of the systems functionality. I should be testing this soon (after my university's finals week).