Automotive Air Conditioning Information Forum

Regardless of whether the compressor is the variable-displacement or fixed-displacement type, in this car it has a clutch, which de-energizes or remains energized when it shouldn't. The question is, why does it do that?

We know that the compressor clutch is getting voltage. But then the voltage turns off prematurely, or it doesn't turn off at all and frosts the suction line (and probably the evaporator). We've covered a number of other possibilities, including the relay, which has been replaced with a new one.

All of this still tends to point toward a faulty thermistor, even though it was changed once already. When you try another one, let us know whether that fixes it. If not, double-check to ensure that the probe is positioned very close to the coils on the correct side of the evaporator and that nothing is insulating it from getting a good temperature reading. Sometimes, thermistors are shipped from the factory with a plastic protector on the probe's tip that's easy to overlook. It should be removed before installation.

JohnHere wrote:The ac pressures at idle with blower at max and windows open are: 45 psi low and 260 psi high. Unfortunately we didnt get any readings at above idle speed.

You didn't mention the ambient temperature at which the pressures were recorded. But these readings appear to be elevated. Once the compressor is running properly, re-record the ambient temperature and the HP/LP readings at around 1,800 RPM and post them here.

snorky wrote:a 30 minute vacuum after a system opening is not long enough to pull out moisture and leftover flushing vapors esp in temps below 85 degrees and often times the vac pumps used are too weak to pull deep enough giving higher pressures and weaker performance plus contaminates the oil and desiccant resulting in shorter system life. Place the sensor towards the center of the evap or the slight left side, with the ac off and the temp set at cold do you get any heat from the vent indicating the blend door is stuck or its seals have failed? when the low line was frosting did you get proper vent temps? even some aftermarket resistor packs and fan motors will greatly reduce air flow across the evap, improper pressures/sensors/readings can make variable compressors behave badly even as little as a oz over or under the limits of oil and or gas will cause problems on newer stuff.

Just to clarify a bit more, the evaporator was delivered from another shop as they didnt have any stock at the time. And it took about five hours after flushing the condenser and lines before the evaporator arrived. So the flushing chemicals must have evaporated by then.

The system was also purged with a gas which i think is nitrogen before being vacuumed.

With the ac switch off and the ac fan running i get only ambient temps with the knob at max cold. All foam seals in the ac/heater have been replaced.

JohnHere wrote:Regardless of whether the compressor is the variable-displacement or fixed-displacement type, in this car it has a clutch, which de-energizes or remains energized when it shouldn't. The question is, why does it do that?

We know that the compressor clutch is getting voltage. But then the voltage turns off prematurely, or it doesn't turn off at all and frosts the suction line (and probably the evaporator). We've covered a number of other possibilities, including the relay, which has been replaced with a new one.

All of this still tends to point toward a faulty thermistor, even though it was changed once already. When you try another one, let us know whether that fixes it. If not, double-check to ensure that the probe is positioned very close to the coils on the correct side of the evaporator and that nothing is insulating it from getting a good temperature reading. Sometimes, thermistors are shipped from the factory with a plastic protector on the probe's tip that's easy to overlook. It should be removed before installation.

JohnHere wrote:The ac pressures at idle with blower at max and windows open are: 45 psi low and 260 psi high. Unfortunately we didnt get any readings at above idle speed.

You didn't mention the ambient temperature at which the pressures were recorded. But these readings appear to be elevated. Once the compressor is running properly, re-record the ambient temperature and the HP/LP readings at around 1,800 RPM and post them here.

The compressor is fixed displacement btw.

We tried a thermistor from another sentra yesterday and the results are still the same. My thermistor was also installed in the other sentra and it works perfectly. My (uneducated) guess is that the replacement evaporator of my car requires a different location of the thermistor from the original one. But where exactly, i do not know.

To explain my first post more: the thermistor was placed in different locations in the evaporator, those different locations caused the compressor to cycle at different vent temps.

With the probe in the upper right side, the ac would cycle when the vent temps reach 25 degrees celsius.

With the probe at the middle of the evaporator, the temps would reach as low as 18 degrees celsius... the temps would keep going down to about 10 or 9 degrees celsius before the ac shuts off. The lines have already started to frost by then.

When the probe was placed at the lower left side which is the original location on an original evaporator, the ac would cycle when vent temps reach 23 degrees celsius.

These are the ac pressures of my car and the car that we borrowed the thermistor from (taken just yesterday, ambient temps 28 degrees celsius)

My car at idle:45 psi low and 260 psi high
Other car at idle: about 40 psi low and i think 270 psi high (forgot to list it down, sorry)

My car at 2000 rpm: about 35 psi low and 270-280 psi high
Other car at 2000 rpm: a bit lower than 30 psi low (28-29) and 290 psi high.

The only difference between the cars is mine is manual transmission and the other is an auto. My radiator fans are one speed only, and the other has a two speed that operates at high speed when ac is on and another at one speed only. The other cars fans blow harder than mine.

UPDATE: I took the car to another shop for a second opinion, the theory of the guy in this shop is the replacement evaporator is made of thicker materials than the original one, reducing the heat transfer.

He suggested replacing the evaporator again with an original. He is selling me a used one in good condition for half the price of the replacement one. Im going to try this out and will update as soon as its done.

The thermistor seems to check out okay then.

Whether the condenser is causing the compressor cycling problem, as the second tech theorizes, I don't know. One thing's for sure, though, the high-side pressure is too high. At 28C (83F) ambient, the high side should be around 200 PSI, not the 260-270 PSI range you're seeing now, and vent temps should ideally be in the 5-7C (41-45F) range.

If the fans are working properly, blowing in the right direction, with all shrouds and air seals in place, try lightly spraying the condenser with a garden hose while watching the gauges. Dropping pressures at that point would indicate a condensing problem.

As suggested earlier, you might also have some moisture and flushing-solvent residue remaining in the system due to the rather short evacuation time. I always try for the longest evacuation possible...at least a couple of hours and sometimes even overnight. However, that might not be feasible in a busy commercial shop.

The tech meant evaporator. Anyway, i have an update:

I brought the car to the other shop to try out his original used evaporator... The first thing the guy from the other shop noticed when he took off my replacement evaporator is the placement of the TXV sensing bulb. He told me its pointing in the wrong direction and it is not insulated properly. The insulation is mere cork tape, it should be a rubber boot like thingy (i forgot what he called it).

So i took previous advice to have the system vacuumed overnight and left the car there, overnight. The guy called this morning to tell me its done. This shop didnt have a thermometer to check vent temps, but the ac seems colder before the compressor shuts off. Not as cold as i was hoping but definitely colder than when i started.

I had the guy hook the gauges again to check the pressures: ambient temp was about 35 degrees celsius and very humid, the pressures with car at idle and fan at max speed are 220 psi high and 30 psi low. The pressures are a bit lower now.

Miggy wrote:I brought the car to the other shop ....This shop didnt have a thermometer to check vent temps

Not having a thermometer to check the vent temperatures??? So weird !

Anyway, maybe his got lost or broke a few days ago....the real test is driving it in the heat and seeing if you're comfortable.

Miggy wrote:The tech meant evaporator.

My bad. I was looking at the pressures and thinking "condenser." You did indeed say "evaporator."

Miggy wrote:The first thing the guy from the other shop noticed when he took off my replacement evaporator is the placement of the TXV sensing bulb. He told me its pointing in the wrong direction and it is not insulated properly. The insulation is mere cork tape, it should be a rubber boot like thingy (i forgot what he called it

That's correct...the sensing bulb direction and insulation do make a difference. Press-tight black tacky tape usually works for that purpose. Sounds like he has a better alternative in mind, though.

Miggy wrote:I had the guy hook the gauges again to check the pressures: ambient temp was about 35 degrees celsius and very humid, the pressures with car at idle and fan at max speed are 220 psi high and 30 psi low. The pressures are a bit lower now.

These pressures look good for a 35C (95F) ambient. If the compressor now cycles normally, let us know about your "comfort factor," as Cusser mentioned.

Sorry for the late update, i was testing the ac of the car.

@cusser. Yeah, his thermometer broke days before i brought the car. Ive been testing the car since the evaporator was replaced with the "good used original". All has been good so far. Only the air feels very humid every time the compressor cycles. Probably due to the humid weather here.

@johnhere. The sensing bulb of the TXV was pointing upside down according to the ac guy. He used stainless steel clamps to mount the bulb, and the insulation he used for the sensing bulb is like a foam boot that wraps tightly around the bulb which he claims is the same as factory original.

Ive been researching about the evaporator for the past few days, and ive come to the conclusion that my replacement evaporator shouldnt be too far off in design as the original one as visually they look the same, and a lot of other people have replaced their evaporators with the same brand cheap replacement as i bought with good results.

With the information i gathered doing various google searches, my theory is the previous ac guy might have added too much oil to the ac, thus insulating the walls of the evaporator reducing heat transfer.

Its very interesting that small errors or mistakes in the installation would make a big impact on the cooking. I think i want to take a course on ac repair on something cause its actually kinda fun. Not fun for my wallet though, as its a lot lighter now after this ordeal