Hey guys, I recently switched my old r-12 ac components to the r143a standard from a 93 maxima. The problem i'm having is that I'm experiencing the evaporator freezing up because my car wasn't equipped with a Thermo Amp to keep this from happening by turning off the compressor. I have the Thermo Amp but would like to know if I would be able to install it on an 89?

 

what is a thermo amp?

 

Thermo Control Amp Freeze Protection
From what I gather, it is a device that protects the evaporator in Nissan and some other automotive AC systems from freezing up. The device is a transistor switch component driven by voltage or current derived from an integral thermistor.

Colder/Freezing temperatures cause the circuit to open and through the ECM and Air Conditioner Relay stop the compressor.

The unit used by Nissan works as follows:
As the temperature is decreasing the circuit opens around 38 to 37° F to turn off the compressor. There is a one to two degree hysteresis in the unit to prevent rapid cycling of the compressor. The device closes the circuit when the temperature reaches 39 to 41°F.

 

you mean pressure switch...similar to orifice tubes?

 

No it's called the thero-amplifier in the 93-94 models, the 89-92 models didn't come with this but did come with a suction throttle valve to prevent freezing of the evaporator.

From "A/C Authority.com"
"Typically used on older A/C system designs, the STV valve has been replaced because of fixed orifice tubes and cycling compressors.
The STV Valve created a backpressure. It actually controls the amount of compressor suction on the evaporator. When liquid refrigerant (freon) enters the evaporator, it boils and changes state to vapor refrigerant. That vapor is �pulled� from the evaporator by the A/C compressor suction. If there is too much suction on the evaporator, the evaporator pressure and temperature will become too low, causing it to freeze the condensation from the hot air. If the compressor suction is too little, the evaporator pressure and temperature will not be low enough to maintain proper cooling performance in the vehicle.
The STV Valve or Suction Throttling Valve, located at the evaporator outlet, controls the amount of compressor suction and therefore refrigerant flow through the evaporator. - regulated device, used on some Ford and General Motors systems, that controls refrigerant flow to prevent evaporator core freeze-up."

Used on the Maxima up untill 1992. They then changed to R134 and a CC system.


http://www.courtesyparts.com/maxima-...1648_1666.html

****check part #27675x******* (Maxima AMPLIFIER-THERMO )

 

i dont see why not, at worse you will need to swap out evap cores.

 

i call expansion valve.

 

Hey 300Zmax, I have the evaporator already installed with the thermo-amp, but I don't see any connections for it on my 89.

 

your gonna hvae to make you own. i know exactly what your talking about...also my gxe is a 92 and it has this thermo-amp.
after my trip to georgia the a/c has gotten colder and this device was actully working.

what you may have to do is get the wire harness off of a maxima that has this feature. i think the fsm has a diagram of this.

 

good luck w/ the wiring...i guess it's not the exp valve.

 

The expansion valve is on the high pressure side of the evaporator, it controls the flow of liquid refrigerant into the evaporator, and STV is on the compressor/suction side of the evaporator, and it controls the gas pressure in the evaporator. But maintaining a minimum gas pressure, the evaporator can never go below freezing. The alternative setup to an STV is to use a temperature sensor on the evaporator to turn off the compressor when it gets too cold. Nissan call this setup a 'thermo amp' If you have neither, your evaporator can freeze up.

 

I am going to change everyting from R-12 to R-134 and after remembering reading this a while ago, I decided to research on it. This is what I got so far looking at the wiring diagrams.

There is one wire that goes from the push control unit (manual AC) to the ECM. But it goes through two components before it reaches to the ECM. And it changes from R-12 on my 89 to R-134 on a 94 (not sure if it differs for years as well)

R-12: Push Control -> Ambient Sensor -> Dual Pressure Switch -> ECM

R-134a: Push Control -> Thermo Control Amp -> Dual Pressure Switch -> ECM

So my understanding would be to bypass the ambient sensor and then wire the Thermo Control Amp with that wire.

But has anyone tried this with success? If anyone hasn't, then my max will be the guinea pig on this test. I would think an 89 ECM getting a reading from a 94 AC might cause some sort of issue?

 

Very interested in the results.

 

Ok so far I have everything replaced, including cutting and replacing the necessary harness connectors that change on the compressor and dual pressure switch and ready to be charged (I have not wired the amp though, yet). But after reading a lot of info about ACs, it seems that a new drier needs to be used whenever the AC has been serviced or the compressor may have problems later on. So I am not taking the risk and just placed an order on it along with the compressor oil.

On the meantime I'm posting a couple of pics comparing the R-12 (left) and R-134a (right) evaporators. And I pointed out the thermo control amp as a couple (and probably others) not have known.




The low preesure end diameter on the R-12 evaporator is smaller than the diameter of the R-134a.

Also I found out that doing this may cure a noise that I had on my AC.
http://maxima.theowensfamily.com/tsb/NTB91-048.pdf

 

Ok I finally got the AC system charged and just like Amillyon mentioned, after 20 minutes of usage with fan level on 3 and max AC, the evaporator was starting to freeze up by looking at the ends where the hoses go. When I turned off the AC and left the fan running, a stream of water was coming out of the drain valve.

So I will work on the wiring when I have the time on the weekend. I will need to get a new fan control panel too since it has not been working properly since I can remember.

 

Better late than never. Today I took the time to bypass the ambient sensor and wired up the thermo amp. And it does toggles the compressor on and off when the evaporator gets cold enough.

 

That's not the ambient sensor, you also aren't bypassing it (or not supposed to if that really is what you're doing). It's the intake sensor, and the the thermo amp acts as a pass through between the sensor and the A/C system.

 

The intake sensor is only available for the auto AC system and I have the manual AC. And I did bypass the ambient sensor in place of the thermo amp.

Before:


After


I just cut it off (it wasn't working anyways) and soldered the two ends together.

 

Ahh, I was under the impression you had ACC because you mentioned the ambient "sensor". Manual system uses ambient "switch".