abunai

I have an early 1990 with manual A/C.
A/C not as cold as before.
Compressor is good. Replaced the TXV and drier.
Recharged with R12.
The early models came with a STV on the evaparator.
This part is no longer carried by the dealer.
Cannot find it from the aftermarket.
Systems made after 1991 did away with the STV.
Has anyone replaced the 1990 evap with the 1991 evap????
Would anything else need to be changed???
Is there a retro fit kit????
Possibly a pipe that replaces the STV????

Called the dealer here. They were no help at all. Didn't even know what a STV was.

Also when I have the blower switch to 4, or high, the compressor turns off. All lower speed work fine.
Looking at the wire diagram, can't see any reason why this would happen.
Anyone have any ideas ?????

mrkanda

iTrader: (3)

For us A/C noobs what is an STV?

James92SE

iTrader: (31)

I would almost guarantee it's actually just the selector not engaging crisply anymore due to wear over the years. It's merely just the "play" that has developed in the selector over the years. You'll notice on "1", or "2", etc. that there is actually a millimeter of play where the lever can be jiggled within each speed, and for some reason, the climate control is temperamental about that play now.

So, long story short, while the selector is on "4", move it ever so slightly/fidget with it while it's still on "4", and if it's anything like my cars I bet the compressor will then turn on.

I had this problem for a good year or two and thought my A/C was actually going out until I figured out it was just the selector itself getting picky in its old age.

abunai

Suction throttle valve.
It's functions is to keep the low side of the system at 30psi. Also keeps the evaporator from freezing up. It also makes it so the compressor will always stay on.
Newer systems don't have it. They cycle the compressor to keep the evaporator from freezing up.

Birken Vogt

What are the pressure readings on it? Birken

abunai

I changed the TXV and drier.
Only TXV they had was for 134 not R12.
I charged it by outlet temp. 30 to 35.
Getting 30 low and 150 high at idle.
At 1500rpm readings are 20 low and 225 high.

sonicii

Never heard of a STV, every car airconditioner I have seen uses a thermostat and cycles the compressor. The TX valve prevents the evaporator from icing up.

You will need a TX valve to suit R12, the R134A units run higher pressure. Either that or you will need to change over to R134A.

LvR

So you wanna keep the compressor running AND prevent the evaporator from freezing up AND keep the low side at 30psi? ...................an a vehicle's aircon, all at the same time? ............ good luck!

STV even on an industrial fridge or freezer is a heat-load sensing and evaporator ice-up protecting device - its job is to prevent the evaporator from freezing up by governing the amount of compressed gas from the compressor entering the evaporator ............ and as a result, because of the governing action, it WILL cause the compressor to switch off/cycle on high pressure if the thermostat still has not "made" at the selected temp ...............as such those 3 variables cannot possibly be controlled all at the same time

Birken Vogt

It is important to know the temperatures at which the pressure readings were taken too, before making any judgement. Birken

abunai

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.
American cars used them untill about 1974.
It's a geart system.
Much more reliable and efficent than a CC(cycling clutch) or CCOT(cycling clutch oriface tube) systems we have now.
I had an old 1971 Chevy Malibu that would frost the windows with this system.
Maxima did the same untill the STV went dead.

Nissan did have a retro fit kit to change to R134 for the Maxima.
Dealer doesn't know anything about it.

My A/C is working OK. Not as cold as before, but I think it's because the TXV is for R134 and not R12.
Part store didn't have a listing for the R12 TXV.
I'm probably starving the system to get the lowside to 30psi.
If I fill it to factory, low side goes uo to 50psi.
Could be the STV has something to do with it, as well as the R134 TXV.
Outside temp is 85'.

Anyone know who sells an adjustable TXV that would fit ????
I used to get them from napa back in the 80' for Honda's.
Car used to be a rolling icebox.
Compressor didn't last long though.

Birken Vogt

OK, it was not clear to me that at factory fill the suction pressure goes too high. Now I see. You can't run a low charge and expect it to work well at all. You need that reservoir of liquid in the bottom of the condenser and in the accumulator to supply the evaporator and if you charge until the suction line is the pressure you want, without the STV operational, you get low capacity as you surely know now.

Guess what the solution is? Just block open the STV and cycle the clutch. It accomplishes the same result. You will need a clutch cycling switch installed in there somewhere. The clutch might not last as long but will it outlive the car? It might. The STV and the clutch cycling switch accomplish the same result, that is, limiting the capacity of the compressor to keep the suction pressure high enough to keep the evap from freezing. See this page.

You probably already know this but the way they do it now (it accomplishes the same result as an STV) is to use a variable displacement compressor that senses suction pressure and adjusts the compressor output accordingly. Maybe you could find a variable displacement compressor from a later model car that would fit?

Also I don't think the 134a TXV has much, if anything, to do with the problem. The TXV works by varying its opening based on evap temperature and is basically just a valve. It should not care what is going through it. If anything it will be a little bit larger for 134a than R12 because of the larger volume needed.

Birken

abunai

I thought of blocking open the STV. Looked into it when it was out.
This one is non serviceable.
If I do find a tube to replace the STV, I will have to add a CC switch to the system.
Whole system is original. It looked like even the drier was original.
Kind of amazing for a car this old.
I'm sure if I change to a CC system the clutch will fail.
Not much I can do about it.
Variable displacement compressor is an interesting idea.
System is working, so I'll leave it for now.
I'm going to see if I can find a 1993 or later system to see if the Evap will fit.
That would solve a lot of problems.

LvR

Perhaps these Corvette guys can return your life to bearable again?

sonicii

Interesting my maxima came as R12 and has no such valve, only a TX valve, my previous 1986 nissan was also the same. Sounds like its function is similar to a TX valve anyway, it just works on the suction side of the Evaporator instead of the pressure side like the TXV..

Birken Vogt

The clutch might fail but I am sure they are available to buy at least.

If you run it low on refrigerant the whole compressor might fail and contaminate the system because the compressor depends on proper refrigerant level in the evaporator to return oil to itself.

<br />
<br />

Not really. Its function is to control the temperature at which the liquid evaporates inside the evaporator. The other means to do this is the low pressure cycling clutch or the variable displacement compressor. They all maintain a constant pressure inside the evap, more or less, which leads to a constant temperature of vaporization of the liquid refrigerant. The TXV modulates the amount of refrigerant and thereby the amount of heat absorbed as the air flows through. It responds, more or less, to the temperature of the air and the blower speed setting, and then the low pressure control system (CC, variable comp., or STV) responds to the pressure changes caused by the TXV.

Birken

sonicii

Ok, correct me if I am wrong, but does my system work just using a TXV and then simply uses an air temperature sensor on the outlet of the evaporator to switches off the compressor clutch if the evaporator gets too cold? and then an additional cabin temperature sensor to keep the compressor off if the cabin temp is too cool?

abunai

To turn the compressor clutch on and off, on older systems, a sensor monitors the evap outlet temp.
If it senses that the temp is low enough to freeze the moisture on the surface of the evap, it cut off the compressor.
When the temp warms up the sensor turns the comp back on.
There are also pressure sswitches in the system that will torn the compressor off to protect the compressor from damage.

Up untill 1992 all Maxima's came with a STV.
Nissan did have a retro fit kit that removed the STV from the system.
I think it was done so the system would be able to use R134.
I'm thinking that if a Maxima, with a STV, went to the dealer for repairs, it would have had the kit installed.

I know mines was never at the dealer for A/C repairs. Owned it new.

Anyone have a 1992 or later evap????
I'm like to see a picture of it.
I should have taken a picture of mine, but didn't think of it at the time.

Lyon

The problem with the fan only working on the slower speeds is a fan resistor. Roughly 30 bucks from the parts stores.
The other problem is you have the wrong TX valve in that system. R12 is so expensive now. You should do a retro kit to 134A. Just make sure you have a good recovery unit and vacuum pump to pull out any traces of the R12. This way you can use the original drier as well and save some money. Good luck.

Birken Vogt

The most succinct way I can think of is that the variable displacement compressor, cycling clutch switch, or STV cause the evaporator to maintain a pressure such that the refrigerant entering does not freeze. (The temperature of vaporizing refrigerant depends entirely on its pressure.) The TXV senses the temperature of the evaporator and if it gets too warm, allows more liquid refrigerant to enter and cool it down.

The cabin air sensor varies fan speed to maintain temperature, and if it needs cooling, simply sends power to the clutch to engage the compressor, but if there is a cycling clutch switch that will momentarily shut the compressor down as needed. The climate control doesn't need to know this and doesn't care.

I may be ignorant, but I don't think you could use a thermostatic switch on a TXV system. Both would be doing different things in response to the same input and how would it stabilize? Rather, the compressor switch is set to maintain refrigerant pressure inside the evaporator slightly above the freezing point of water outside of it, so there is no possibility of ice no matter how much refrigerant is coming in.

This could work on an orifice tube system, though. But why use a temperature sensor (which is usually a bulb of refrigerant connected to a pressure switch) when a pressure switch connected directly to the system will do?

Birken

abunai

On most aftermarket systems, this is how they did it.
Seems like it wouldn't work well, but suprisingly it works very well.
Older Hondas used to have a system like this.
I used to change the TXV for one that would have a smaller orifice and it would feel like an icebox in the car.
The thermostatic switch looks like the same one used on GM systems when CC system first came out.
Really basic.
A capillary tube, two conectors in a small box.
Two wires controlled the ground for the clutch relay.
They were adjustable.

Birken Vogt

Could it be that the cycling clutch switch sensed temperature right after the TXV, at the evap inlet, where I would think that it would be measuring pressure more or less due to the temperature of the evaporating liquid? And then the TXV itself sensed at the evap outlet, where it would be sensing superheat?

Birken