Tn75s air conditioning relay

[jammer48]

Keep at it, you'll get there. For openers, your static pressure should closer to 75 - 80 PSI on a hot day. Your pressures indicate an overcharge condition. How much refrigerant did you put in? Just guessing I'd say a TN cab system will take just over two pounds. The high pressure will vary more based on a number of variables, but the low side even on a hot day shouldn't be over 25 PSI. In fact I've seen systems work perfectly well between 15 and 20 on the low side. Wide open throttle. I don't bother with reading pressures at low RPM.

Why didn't you change the drier before you went to all this trouble? Replacing that is pretty much a given whenever you open up a system that hasn't been apart in a long time.

Thanks for your reply Harry. I didn't change anything yet till I find out what I needed to order. This is about week 3 of me doing all this. The tractor had a/c two years ago but developed a very slow leak. I can't locate it and I am not sure if it still exists since it holds pressure now. I went around and tightened all the fittings. In terms of how much freon is in the system, at this point I am embarassed to say I don't know. I have ordered an evacuation pump since i wasn't aware until to day that the vacuum pump will not remove refrigerant. I already had a recover tank.

 

[LouNY]

I wouldn't be surprised to see 100PSI static on a 95F ambient temperature.
I'd be more happy to see 90PSI static as usually the tractor is going to be a bit less then ambient.
And 45-50 wouldn't be too high on a 95F ambient.

Here is a good article:

Normal AC pressures R-134a: Pressure gauge readings​

Normal Pressure Readings for R-134a Refrigerant Car AC Systems​

As an automotive AC expert, I can tell you that knowing the normal pressure readings for a R-134a system is essential if you’re diagnosing a problem with your car’s air conditioning system. If you don’t know what the pressures should be, you can’t possibly determine whether your system is functioning properly, or whether you’ve got a problem that needs to be fixed.

When I’m checking an AC system, I always refer to a few critical conditions before evaluating whether the pressure readings are within the normal range. Here’s exactly what you need to do.

What Are the Conditions for Checking Car AC Normal Pressure Readings?​

When you’re testing the AC system, it must be operating under these conditions for accurate readings

• The engine should be running at 1,500 RPM
• Air conditioning turned on and set to maximum
• The compressor clutch (if equipped) must be fully engaged

If you don’t meet these conditions, your car’s AC normal pressure readings won’t be valid.

Just as important, you must account for ambient temperature. AC pressures rise and fall in relation to the outside temperature, so any pressure reading is meaningless without that context.

My Rule of Thumb for Car AC Normal Pressure Readings (R-134a)​

Over the years of working on AC systems, here’s the simple rule I rely on:

• In most typical conditions (70–85°F ambient temperatures), expect the low side to read in the 27–32 psi. range with the compressor engaged. This pressure corresponds to the refrigerant boiling at around 32°F to 40°F at the evaporator, which is necessary for proper cabin cooling without freezing the evaporator coil. However, the exact pressure will vary slightly based on:

• Ambient temperature
• The ambient humidity
• System load (fan speed, engine RPM)

As Ambient Temperature Increases— The low-side pressure may creep slightly higher—up to 45–55 psi—especially on very hot days (90°F to 105°F).

• The high side pressure is usually 2.2 to 2.5 times the ambient temperature (in °F). For example, if the ambient temperature is 90°F (32°C), the ideal high-side pressure should be around:

2.2 x 90 = 198 psi
2.5 x 90 = 225 psi

So, the target high-side pressure range would be 198-225 psi when the ambient temperature is 90°F (32°C).

Using this rule of thumb, you can quickly determine if your AC system’s low and high-side pressures are operating within normal ranges based on the current ambient temperature.


Generally speaking, you want around 27-32 psi on the low side and 200 on the high side.

Why is 27-psi the optimal pressure for the low side?​

Because, on an R-134a AC system, a pressure of 27-psi. on your gauge means the refrigerant will produce about 32°F at the evaporator, as long as the orifice tube/expansion valve is operating properly and there’s no air in the system (air in the system can artificially inflate the pressure to 27-psi).

Car AC Normal Pressure Readings by Ambient Temperature​

If you want a fast cheat sheet, this is the reference chart I personally use when diagnosing AC systems:

65°F Ambient temperature: Low side pressure 25-35 psi High side pressure 135-155 psi
70°F Ambient temperature: Low side pressure 35-40 psi High side pressure 145-160 psi
75°F Ambient temperature: Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Ambient temperature: Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Ambient temperature: Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Ambient temperature: Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Ambient temperature: Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Ambient temperature: Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Ambient temperature: Low side pressure 50.55 psi High side pressure 340.345 psi

Quick Tip:
If the high side is running higher than this, check the airflow over the condenser first. Blocked condensers are one of the most common causes of pressure issues that fall outside car AC normal pressure readings.

What If Your Pressures Don’t Match This Chart?​

When the car AC normal pressure readings are out of range, that’s a clear red flag. It could mean:


• Low refrigerant (low pressures both sides)
• Overcharged system (high pressures both sides)
• Bad compressor (low high-side pressure)
• Clogged orifice tube/expansion valve (low low-side, high high-side)
• Condenser airflow restriction (high high-side pressure)

I highly recommend checking out my full guide on diagnosing abnormal AC pressure readings to pinpoint the issue quickly.See this article on abnormal pressure readings and the most common causes.

© 2019 Rick Muscoplat

Posted on April 4, 2024 by Rick Muscoplat

 

[Oz_Kioti]

No both are extremely dirty... I am working with basicly one arm... shoulder surgery
in 10 days... trying to get this all done before surgery.

The above was in answer to the question: "Are you sure the evaporator and condensor are clean and clear? Can you see light through them?"

In the case of the AC in my Kioti being annoyingly erratic - producing nice cold air one day, then no cooling at all the next (or even the same afternoon) - we, that is the air-con guy, found an incorrect factory installation of the upper cabin frame sealing strip. The foam strip is designed to prevent ingress of air into the space between the hood liner and the fibreglass roof, such that the ONLY air which enters that space is via the cabin air filters - one on each side.

In the 4+ years since I've owned it since new, dusty ambient air had been sucked into the space (bypassing the cabin filters altogether) to the point where the evaporator ended up like this:

 

[LouNY]

Would be hard to get much cool air from that.

 

[nyone]

Cold on both sides of the evaporator tells me you dont have any air flow across it. No gauges, pressures or calculations needed.

 

[jammer48]

Would be hard to get much cool air from that.

Mine looks like that... I bought this used off our local NH dealer I suspect no one has ever been up here since it was new... I can't put it all back together till I get the cork tape coming in the mail.

 

[grsthegreat]

Yeiks…you have to drop it into the cab. That sucks. On my kioti the roof hinges open.

Everytime ive opened it the space is clean of any dirt. I replace the filters often though. But there not cheap.

But my AC keeps me so nice and tosty.

A month or so ago the compressor stopped working, luckily was an easy fix…a shorted wire coming off of ac fuse.