Inspired by a post several years ago by p_gill and more recently by fly, I'm planning to buy a manifold gauge set to measure the low and high pressure on my A/C to determine if the refrigerant is full. I have several family cars I maintain so this will be another preventive check for each of them. If a recharge is needed, I'll probably bring it to an A/C shop. But at least I'll know if that's necessary by taking the measurements myself.

However, I can't find the specs in the LR2 service manual for what the low and high side pressure should be for a full system. Some older posts by p_gill indicate 35-40 psi on the low side and 150-155 psi on the high side, but no reference back to a spec. Are those readings with the compressor running and at what engine RPM?

Anyone with more info than I found is invited to jump in.

 

If you overcharge the system then the low side pressure will be too high and you won't get sufficient cooling.

Take a look at this video


Here is what I recommend

Before you start looking at pressures get a temperature gauge that will measure the vent temperature

This is the one that I bought


And my S80 with the same 3.2 as my LR2 and the same AC system was able to get the vent temperature down to 55F when the outside temperature is over 100F
Note: the AC compressor on my S80 failed and this information is after replacing it.

If I waited longer or if it was a little cooler outside it would have gotten below 50F

https://www.matthewsvolvosite.com/fo...7687&start=110

Good luck

Paul

 

Hey Paul, thanks for this additional info. It made me realize that the pressures & temps are standard across vehicles and widely published in charts easily found online. My mindset has become so accustomed to the "Land Rover Way" that I missed the fact there are generic systems like A/C.

I have a digital thermometer gun that I will try on the vents before purchasing a set of manifold gauges. If the cooling temp is in range, then no point in going further.

 

Also, recharging the system is done by weight after drawing a vacuum on the system for an hour or so. There is a sticker in the engine bay that lists the amount or R134a needed from empty.

 

Understood, that's why I would bring it to the shop if it really needs a recharge. I might be tempted to top it off myself if it's just a little low, but a proper evacuation and recharge follows the steps you indicate.

I don't think it's worthwhile to buy the equipment needed as a DIY project for something I will rarely do, if ever. But I do think it's worthwhile to take measurements to determine if the service is needed.

 

Update and Learnings: I haven't purchased a set of manifold gauges yet. However, I found an A/C pressure measurement solution that I already have and did not discover until now.

Going into my OBD2 reader, I selected the live data for the PCM - A/C Pressure Sensor Voltage. This value corresponds to the pressure on the high side of the A/C system. (The LR2 does not use a sensor for the low side.) Note that this PID is in the PCM, not the HVAC module where I looked first. However, I recall that the HVAC module shows the A/C Evaporator temp which provides some correlated insights to the pressure on the low side (based on a quick read of some online posts, I don't have further explanatory details on that).

I could not find the exact formula for converting the voltage to psi for the LR2 specifically, and I don't know the PID value since that is not displayed on my reader. But an online search revealed formulas for other vehicles that are very similar to each other. When applied to my LR2 live data readings, the psi was spot on what I expected based on a standard chart of pressure ranges for R134a. Good stuff!

The formula is:
high side psi = (voltage * 93) - 16

For my quick test, the live data reading was 1.65v with the A/C turned off, and then jumped up to 1.85v when I turned the A/C on. This calculates out to a psi range of 137 to 156, which is about right for my ambient temp ~65 F.

This confirms that my pressure is good (at least on the high side). It's also something that can easily be checked if your A/C is not working. If the sensor is not showing any signal, you may simply have a bad sensor.

 

Update #2: Scotty Kilmer (Youtube auto advisor) recommends to avoid placing a manifold gauge on the high side unless absolutely necessary to diagnose a problem. He explains that the high side service port, if left undisturbed for many years, is prone to leak when the port is manipulated to connect the manifold gauge hose. This is not as much of a concern on the low side since the pressure is lower. So for a simple health check of the A/C, it's best to check the high side pressure from the OBD2 reader ... hands-off and not intrusive.

For a routine health check of the low side, measuring the temp of the air blowing from the vent is a good indicator (purely by inference). Typically, it will be under 45 F when working properly (rule of thumb is 30 - 50 degrees lower than the ambient temp). I measured mine at 42 F when the ambient temp was 66 F, which is OK.

However, I am stumped on the OBD2 reading of my Evaporator Temp in the HVAC module. It was 81 F with the A/C off, and fell to 77 F with the A/C on. This is not what I was expecting, which was for the evap temp to be a little cooler than the vent temp, like approx 38 F. I'm wondering if I'm not measuring what I thought I was, or if there is a bug in my Autel OBD2 reader. I'm noodling on this confusing measurement ...

 

The evaporator is intended to absorb heat from the cabin to transfer it to the cold refrigerant that is flowing through it. The blower pushes air (outside+cabin if recirculation is off, just cabin if it is on) through it to cool it and that's how you get cold air out of the vents. It makes sense for it to show a warmer temperature.

The OBD measurement is likely the temp at the outlet after it has absorbed the heat and transfered it to the refrigerant.

 

24 is indeed okay. Most home window A/C units produces a 20 F degree differential at best, for example. But most cars produce 30+ differential from pulling outside air into the cabin but not every car is the same for a multitude of reasons - also you have to take great care in measuring. Never go by "what my phone says the air temp is" or whatever, you have to measure the temperature of air coming INTO the vehicle at the inlet to get a proper differential reading. And with the hood open, vehicle running that cabin intake temperature can be pretty high. So depending on how you measured things, you may have a greater differential. So basically there is no proper "output temp" when it comes to any A/C system because its all based on differentials, a designed number of degrees it should drop when working properly. The hotter the intake, the hotter the output. Then you have the humidity factor. Humidity hampers efficiency.

 

I'm going to remove it (LR003200) for a bench test. It appears to be one of the few items that's actually easy to remove and replace.