Range for A/C low side pressure

[mclasser]

I hooked up one of those chintzy gauges from a DIY A/C recharge canister on my Accord for fun. Not sure how reliable it is but it read 32 PSI; ambient temp was 85F with ~50% humidity. I know high side pressures are needed to get the full picture. I read around and some say 35 PSI is where Accord's should lie on a 90F day with high heat load.

The gauge has an adjustable dial and it says 50 PSI is what the pressure should be on an 85F day. This seems too high to me and is probably why it's so easy to overcharge using these things.

Any A/C pros in the house care to comment?

 

[Rand]

if its working dont touch it.

 

[DemoFly]

A check and charge at a local indy shop is about $70 for 20oz...

 

[Chris142]

Low side varies. Must use a high side gauge

 

[Oro_O]

Not a pro, but I went through this last weekend. 90F, 35psi on our 2002 Lexus was ideal. Any higher would overcharge it and cause it to cut out. I had been trying to set it to the mid-range -45psi- of what my chart said (40 to 50). No go.

I had a manifold gauge set, but it had been around since my dad bought it in 1974 to charge his 1973 Vega Wagon, first car with a/c he had. The high side line sprung a leak while we were checking it out. The low side gauge got a little wonky sometime around the late Clinton years, so instead of replacing the hoses, he bought a new kit at O'Reillys. Which, get this, did not include an adapter for tapping the can into the charging line. That requires an extra-cost adapter - that O'Reilly's does not stock and is special order. What a product failure (this is the Interdynamics kit - avoid). So I ended up having to use an el-cheap gauge on the low side to charge, though I had a dual manifold gauge set to check afterwards.

It was 90F and moderate humidity, peak performance was at 35PSI on the low and it at ~215psi high side. At a steady 40mph on "Max Cold" it would put out 40F.

I was happy with that and did not worry it was out of the "normal" low range.

 

[DoubleWasp]

Don't forget that you are supposed to charge with the engine at 1500 rpms, with a blower forcing air at the condenser.

The high-side pressure is important in diagnosing the system. If you have a restriction in the system, you are going to have your compressor cutting out (that switch is on the high side) without ever getting to the appropriate low-side pressure, and have absolutely no idea why.

Where the former is concerned, you can easily end up with a "false charge" because low compressor rpms and inadequate flow over the condenser are causing elevated pressures through the system. This is another situation where it is important to have a high-side reading as well, because if you read the high-side pressure is unusually high, you can diagnose a condenser issue by misting the condenser with water to see if the pressures go down.

Having low-side readings only is fine, so long as your system and conditions are absolutely perfect, and you know for certain that they are.........which is impossible to know without a high-side reading.

 

[Gokhan]

Low- and high-pressure specs vary from A/C system to A/C system. You need to look it up in the OEM repair manual or somewhere else.

Temperature of the refrigerant increases with increasing pressure, which is known as the Gay-Lussac's law. Therefore, higher pressure results in higher refrigerant temperature and less cooling. If you lower the pressure too much, you have too little refrigerant circulating in the system, which also results in lower cooling efficiency. Therefore, you need to find the optimum pressure to maximize the cooling.

Pressure too high = refrigerant runs too hot = poor cooling
Pressure too low = too little refrigerant circulation = poor cooling
Pressure optimal = maximum cooling

 

[mclasser]

So I ended up topping off the system a little. I estimate adding about 4 oz of pure R134 since the can felt about 1/3rd empty in the end (very scientific, I know). Low side was about 35-36 PSI with it being 90F in my garage. The air's definitely colder now. Vent temp at idle with fan on low and recirc off was 42F. Not bad for a 14 yr old system I'd say.