A/C pressures off -what's up?

[MikeInCtown]

Haven't had A/C in my truck for two years. Today, I pulled the old hoses and replaced them with boneyard hoses 5 years newer. (and cleaner inside) Also replaced accumulator and all the rubber o-rings I could find. Used A/C flush on entire system except compressor and then sealed everything back up.

Vacuumed out the system down to 28" which is about all we can get at this altitude. Added a 2oz can of oil to the system to replace what was lost in the lines when I flushed. Then added two cans freon (24oz)

The GM manual I have says that I need 1.875 IIRC pounds of freon. The manual also says that at 70* and 1,500 RPMs and high humidity I should have about 21.5 or 22 lbs on the low side and about 250 on the high side.

When I checked with just the two cans put in I was getting about 18 on the low side and about 210 on the high side.

I added a small portion of the third can and it didn't raise the low side pressure any but did seem to raise the high side just a touch.

I am wondering if I just need to wait for some real heat to check again or if it's possible that my compressor is just weak and needs to be replaced? (is it possible that I am just reading the guages way wrong?)

Any help is appreciated.
Mike

[droptdime]

what is your temp inside the car and what is the temp. of air coming out of vents?

[johnmyster]

Sounds good enough to me. However, both numbers are a bit low compared to your target. You're tempted to blame the compressor (for not getting the high to be high enough) but that's not the case, because your low is still below target.

Either your system isn't as hot as they expected, or you have lower humidity than those specs consider, or your fan is moving more air then they consider, or you've got an underdrive pulley.

You've got enough gas in the system. You're fine. Roll with it. Factory specs are hard to duplicate. If you had the right amount of gas in the system and your low was too high and the high was too low, then I'd be concerned. Or if the high was too high and the low too low, then I'd be concerned with the orifice.

Too many variables to get too concerned.

[droptdime]

nicely put,

[droptdime]

make sure your coils are clean, will help with getting cooler air inside

[MikeInCtown]

Quote: Originally Posted by johnmyster

Sounds good enough to me. However, both numbers are a bit low compared to your target. You're tempted to blame the compressor (for not getting the high to be high enough) but that's not the case, because your low is still below target.

Either your system isn't as hot as they expected, or you have lower humidity than those specs consider, or your fan is moving more air then they consider, or you've got an underdrive pulley.

You've got enough gas in the system. You're fine. Roll with it. Factory specs are hard to duplicate. If you had the right amount of gas in the system and your low was too high and the high was too low, then I'd be concerned. Or if the high was too high and the low too low, then I'd be concerned with the orifice.

Too many variables to get too concerned.

Excellent info. I'll have to try and get some interior temp readings as well as actual outside temperatures when it gets hotter this weekend. The news guy said it was 66 out but I was working in the garage and it was a lot warmer in there. I was using the specs out of my service manual and estimating the humidity since it was drizzling outside. (all charts showed above 20 on the low side though.)

I'm going to have to look at the guages again to make sure I was reading them right. There was a line for the R-134A on th eguage as well as lines for the other new type freons, but at the center was also just basic pressure. I'm assuming that I should have been just looking at basic pressure at the center of the guages.

if for some reason the low pressure is too high and the high is too low, does this indicate a compressor problem?

[Oo-v-oO]

Are we talking R-12 or R-134?

I'm relatively new at A/C repair, but the Mastercool service manual I have lists only one condition that can cause high low-side and low high-side readings - bad compressor.

What altitude are you at? Must be a little less than 2000'.

I'd be curious to see what your GM manual calls out for specs on a '96 pickup. I found specs for mine that called out 32 oz. for a full charge, but I goofed and put in 36. Still seems to cool OK. I'm at about 950' and I was seeing low side numbers around 20-25 and high side around 150-175. The Mastercool book lists 25-55 on the low side and 150-350 on the high for '93-'98 GM, other than a couple of models. These specs are for sea level and an ambient temp of 80°F.

[johnmyster]

Quote: Originally Posted by MikeInCtown

if for some reason the low pressure is too high and the high is too low, does this indicate a compressor problem?

Or that the orifice is flowing faster than expected. Or that the evaporator isn't picking up as much heat as expected (cool or low humidity cabin). Or that...

Seriously. You're pretty close. This isn't an exact science. If you've got the right amount of gas, just roll with it. Double check gas requirement and go.

Were your old lines dirty? Orifice clogged and dirty? When compressors begin to wear, compressor bits show up in the system, usually on the orifice.

Sounds like your pressures are fine. More importantly, is it cooling well? If your compressor is shot, it'll let you know eventually, but it doesn't sound like that's the case. You've already got the system charged, so may as well give it a shot.

As long as the outlet (towards the front) of the dryer is good and cold, it indicates you've got droplets coming from the evaporator. If so, it indicates you're getting as much cooling as you can possibly get at the given ambient/cabin/airflow conditions.

[johnmyster]

Quote: Originally Posted by Oo-v-oO

I'm relatively new at A/C repair, but the Mastercool service manual I have lists only one condition that can cause high low-side and low high-side readings - bad compressor.

But he's low on both sides - indicating he's low on gas, but he's not. Thus, indicates that the system is too cold, which still doesn't seem to be the case.

250 or so has always been my goal, but at 1500 RPM you're still turning pretty slow. I forget when the high side switch cycles it out, but it's not much higher than 250 I don't believe. I think you're fine, but test it out and see.

Look at it this way. You're about 20% low on each side. If you're sure you've got enough gas in it, no biggie. There just wasn't enough heat in the system for you to get the readings you wanted.

[MikeInCtown]

Quote: Originally Posted by Oo-v-oO

Are we talking R-12 or R-134?

I'm relatively new at A/C repair, but the Mastercool service manual I have lists only one condition that can cause high low-side and low high-side readings - bad compressor.

What altitude are you at? Must be a little less than 2000'.

I'd be curious to see what your GM manual calls out for specs on a '96 pickup. I found specs for mine that called out 32 oz. for a full charge, but I goofed and put in 36. Still seems to cool OK. I'm at about 950' and I was seeing low side numbers around 20-25 and high side around 150-175. The Mastercool book lists 25-55 on the low side and 150-350 on the high for '93-'98 GM, other than a couple of models. These specs are for sea level and an ambient temp of 80°F.

Book for the 96 says something like 1.87 pounds, which is about 30-31 oz. The book also says there are two types of A/C ststems on that truck and I really don't know which is which. For pressure purposes it doesn't seem to matter. I did find out reading the book a bit more that the pressures they show are meant for garage type testing (no sun load) I also found a chart about 15 pages after the pressure charts that gave a graph with low pressures on the left and high pressures on the bottom. In the middle was a white box and if the pressures intersected anywhere in that white box then the system is supposedly fine.

If anything, I think I may have put in about 2 or 3 oz too much, but I did lose a small amount because my lines kept loosening up when I was moving the guages around.

Johnmyster, yes the accumulator is cold and water drops were forming up really good. It's supposed to be 80+ here over the weekend so I'll get a chance to test it out better. Hopefully the new o-rings and seals did their job and I don't have to put another 30oz of freon in there come spring.

[johnmyster]

Quote: Originally Posted by MikeInCtown

Johnmyster, yes the accumulator is cold and water drops were forming up really good.

What I meant was that a cold accumulator/dryer outlet indicates that some Freeon liquid is getting passed from the evap. I've had them ice up before (R-12 systems), but typically, cold condensate is about as far as I get on R-134.

Let us know how she does come weekend. Doubt 80 deg will be too difficult. The stock fans on these things have great airflow (even unlocked) which is great for A/C systems.