Ac low pressure reading

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Ac low pressure reading

Postby Dinosoar » Wed Jul 15, 2020 9:25 pm

My truck has a new compressor condenser dryer and expansion valves (tractor trailer w/sleeper) and new binary switch and high pressure switch. Also all new hoses.

When I charged it with the specified amount I lost some when I changed cans and I decided to put a little extra in and I certainly put in too much (I ended up putting probably about a can in) because the high pressure went way up on the gauges. So I drained some out and the high pressure went down to where it would cycle.

The fan switch comes on at 275 and goes off at 225. The switches are new and the switch actually is supposed to be 250 and 300 but works at 225 and 275 for whatever reason.

The binary switch is 28 and 40.

On the advice of a friend I let more refrigerant out. The low pressure side reads about 40 as the truck idles and the high pressure side cycles. I don't think that the low side really went down at all, but I may not have let enough out.

It was about 85゚here today. I took the truck out and it blew pretty cold. I was comfortable in the truck. It could blow colder but I was comfortable.

I am wondering if I should let more refrigerant out. Since the binary switch will shut off at 28 I was wondering if I should let more come out until I get down to say 32 or something like that. My friend thought that it should run at about 25 but I think that will shut the compressor down.

Just wondering what kind of advice you could give me.
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Re: Ac low pressure reading

Postby Dougflas » Thu Jul 16, 2020 4:14 am

So you just vented the extra refrigerant? You're supposed to recover it; not let it get into the atmosphere. Now you do not know how much is in the system.
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Re: Ac low pressure reading

Postby JohnHere » Thu Jul 16, 2020 7:01 am

As said previously, you don't know how much refrigerant is in it now because of the adjustments, and you can't charge or determine the charge level by pressures alone.

Regret to say, but it looks like you'll have to start from the beginning by recovering the refrigerant while noting the amount of oil that also comes out so that you can replace it. Then, evacuate and recharge to spec by weighing in the exact amount of refrigerant using a precise refrigerant scale or an RRR machine (or have a pro shop do all of this for you).
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Re: Ac low pressure reading

Postby Sorefoot » Thu Jul 16, 2020 11:32 am

My wife lost A/C in the Kenworth she was driving for work (water truck at a big construction project) and I replaced a couple of bad O-rings, pulled a vacuum and re-charged it for her with the little cans to get her back working in A/C. It can be a pain with those little cans but it is doable.

I did notice the fan in that was a few psi off of spec as well.

Did you initially pull a vacuum on the system?

Really, the correct answer is to take it to a shop, have what's in there recovered, pull a vacuum, and re-charge it with the correct amount. This could be done in about an hour and it sounds like all that is needed so you shouldn't have a huge shop rate.

Having said that, by looking at a classic R134 pressure chart for an 85 degree day, your pressures seem reasonable. . .

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