A/C Charging - Which measurement to follow?
 

I had to evacuate the A/C system on my '94 Accord recently and when I charged the system with freon, the measurements wouldn't line up. The manual states that my car should use 600-650g of freon. However, by the time I unloaded one can containing 396g of R134a, the High pressure reading on my manifold showed that it was already at the upper bound given ambient temperature. Moreover, the Low pressure reading was about 5psi below the lower bound.

I'm really confused about this. Which measurement should I follow in such a situation?

There are no hard measurements. It is best to charge by using the required weight of R134a. Most Accords require about 24 oz which is conveniently about 2 cans.

A rough charge measure by pressure is as follows:

Park car in shade and run at fast idle 1500-2000rpm, max fan, recirculate, AC on. Let system stabliized for about 5 mins or so.

HP pressure in psi will be about 2.5*ambient air temp in degF measured at condensor face. So 90F = 225psi. If HP measurement is low continue to add charge. Pressure measurements are always when compressor is running not when compressor cycles off.

Low pressure goes down to about 25-30 psi before compressor cycles off.

Temperature pull down (air temp on floorboard - air temp from duct) should be 35-45F, so 80 on floor boards should produce 35-45F vent temps.

When I've "topped off" I tend to err below 2.5*amb air temp.

good luck

I charged up the system to weight spec and the high pressure reading was almost 400psi at ~85F. Low pressure reading was also above upper bound. These readings are in stark contrast to my 2000 Civic Si (recharged both cars back-to-back) which had both pressure readings below their lower bounds.

Another thing I noticed was that the High pressure hose was very hot on the Accord and when I tried to pull off the connector, there was a burst of high pressure air.

Any clue what's going on? Radiator fans work fine as far as I'm aware (car has never overheated even on really hot days).

Why did you evacuate the system? What parts did you change out?

If you charged spec amount into an empty system, you have a blockage somewhere. Perhaps a stuck-closed expansion valve, debris blockage, or debris stuck in expansion valve.


If system was repaired for a compressor seizure, you need to replace receiver/drier and flush the system.


good luck

To my knowledge, everything in the A/C system is stock. When I first got the car, there was no freon in the system so I topped it up and the compressor clutch kicked on as it should. However, the freon leaked out so I used one of those "A/C leak fixes" and it seemed to fix the leak (held freon for a few months without issue). However, at the time, I didn't use a manifold gauge set or measure the weight of freon I put in.

I chose to evacuate and recharge the system because the A/C wasn't particularly cold (has never been). I figured I'd try putting in the correct amount of freon and hopefully the A/C would blow cold. Instead, I got 400 psi on the High pressure reading.

At such a high pressure, I'm genuinely concerned that something might blow up. Would it help to partially evacuate the system to bring the pressure down to a reasonable level?

You have a leak(s) in the system. I would add dye, then find/repair the leaks in the system. I think the leak additives are marginal at best.

When you turn on the a/c, do both fans turn on?

With system operating at idle to reduce pressure, try checking for cold spots in the condenser and HP line from condenser to firewall. There may be a blockage indicated by cold spot. If nothing, the blockage is likely at the expansion valve which would be replaced in an overhaul, which is what is necessary for a lasting repair.


good luck

I'm pretty certain that the leaks have been fixed. The freon I used contained dye and I couldn't find any leak spots in spite of searching high and low with a UV light.

I read that excessive oil in the system can reduce its capacity, and each can of freon has some oil in it. From the time that I've owned this car, there's been a total of four cans of freon that I've used (first one leaked out, second one after fixing the leak, and last two from most recent evacuating/recharge). Could it be that there is excessive oil and so 600-ish grams of freon is in effect overfilling the system?



UPDATE 1:

I hooked up my manifold gauge set and turned on the A/C to max setting. It looks like the Low pressure reading reaches a steady state pretty quickly and then slowly rises by about 5psi and stays there (about in the middle of the acceptable range). However, the High pressure seems to rise consistently and stabilizes around 80psi above the upper bound of the acceptable range. The Low pressure stabilizes a couple of minutes before the High pressure reading does.


UPDATE 2:

I tried evacuating some of the freon. In order to lower the High side pressure to the upper bound, the Low pressure reading is about 5-8psi below its lower bound. I then charged it up slightly to increase the Low pressure reading to its lower bound, and now the High pressure is about 25psi above the upper bound. This process was done out in the sun (no shade).

Pressures in the system will vary due to outside temperature, humidity, ...

I attached a bulletin that will give you an idea of the proper pressures. The 94 shop manual that you can download from a link in the common diy thread in the general tech forum has a chart you can use as well.