Possible Weak Compressor, Low High Side Pressure

[GriffinB]

I've got a 1987 Bronco with a 460 from a 1992 F250, I bought all new AC parts for said F250 with a 7.5l. AC system works pretty well now that I added an electric pusher fan in front of the condenser. But I am having an issue with my low side being around 45psi running and my high side only 150psi when it is 85 degrees out. The AC worked well on that day and it started to feel cool within one minute of turning the AC on, after 15 minutes the center vent temp was 56 degrees not great but not bad. When driving it will get down even colder as well. Another time I tested it was 80 with the sun setting so not that sunny after a 30-minute drive and sitting in the driveway idling low side was 45psi, the high side was 100psi, the vent temps was 42 degrees, and once again high side pressures were very low.

I filled it up with a manifold gauge set and 12oz cans, I am aiming for 38oz of R134a and there are 27oz currently as the compressor wasn't really sucking it down as fast. There are 11oz of PAG oil in the system, 2 in the condenser, 2 in the accumulator, and 7oz in the compressor which it came with. Before filling I vacuumed it for 2 hours and it held the vacuum for 30 minutes. It works very well at getting the humidity out of the air even in middle Georgia only 60 miles from the coast. Biggest mistake I made was not purging the fill line when switching cans, it just flew over me that I needed to do that.

So I don't really have a complaint on how the AC performs but the pressures scare me, notably the lack there of pressure on the high side. I know this is a key sign of the compressor being bad, but it cools quickly on first startup in a hot truck and cools well in general, making me think it's not weak. However, these are just my thoughts, which is why I want to hear them from the pros on this forum.

Should I recover the refrigerant, vacuum down again, refill properly to 38oz, and see what the pressures are? If they remain the same I would assume the compressor is bad. It is a UAC from RockAuto, if I replace it I'll get a Four Seasons instead of cheapening it out.

Thanks for reading hopefully I can figure this out from here.

Here is the link I have started on the FullSizeBronco Forum: (Link removed)

[JohnHere]

I've got a 1987 Bronco with a 460 from a 1992 F250, I bought all new AC parts for said F250 with a 7.5l. AC system works pretty well now that I added an electric pusher fan in front of the condenser. But I am having an issue with my low side being around 45psi running and my high side only 150psi when it is 85 degrees out. The AC worked well on that day and it started to feel cool within one minute of turning the AC on, after 15 minutes the center vent temp was 56 degrees not great but not bad. When driving it will get down even colder as well. Another time I tested it was 80 with the sun setting so not that sunny after a 30-minute drive and sitting in the driveway idling low side was 45psi, the high side was 100psi, the vent temps was 42 degrees, and once again high side pressures were very low.

The vent temperatures appear erratic, the low side is too high, and the high side is too low, all of which lead me to believe that the system is (A), low on refrigerant; and (B), that the system contains non-condensable gases (air and moisture).

I filled it up with a manifold gauge set and 12oz cans, I am aiming for 38oz of R134a and there are 27oz currently as the compressor wasn't really sucking it down as fast. There are 11oz of PAG oil in the system, 2 in the condenser, 2 in the accumulator, and 7oz in the compressor which it came with. Before filling I vacuumed it for 2 hours and it held the vacuum for 30 minutes. It works very well at getting the humidity out of the air even in middle Georgia only 60 miles from the coast. Biggest mistake I made was not purging the fill line when switching cans, it just flew over me that I needed to do that.

I don't have the A/C specifications for an '87 Bronco, but I do have them for a '92 F-250, which are 44 ounces net weight of R-12, and 10.0 fluid ounces of Mineral Oil. So your specs of 38 ounces net weight of R-134a and 11 fluid ounces of PAG oil (you didn't mention the viscosity) seem about right. Where did you get these specs, from an under-hood decal on the '87?

Bear in mind that a system might not leak under vacuum, but it can readily leak under pressure, especially on the high side.

If you didn't purge the yellow hose each time you changed-out the 12-ounce cans, you probably do have air and moisture in the system. The remedy is to recover the refrigerant (or have it done), replace the accumulator, evacuate the system for as long as it takes to get down to ~500 microns, and re-charge it to the adjusted specification for R-134a.

On this Forum, we follow US-EPA regulations for approved refrigerants, which also means no venting of refrigerants—ever. For the DIYer, a professional MVAC shop can recover the R-134a for you at a reasonable price—usually $50 or less. That's for recovery only.

[GriffinB]

My system is low on Freon and there is air and moisture in the system, I will get it recovered, vacuum it down for an hour, and refill it properly. I am using PAG46 viscosity oil, if you need to know anything else about it please let me know.

I got the specs for 38oz R134a and 11oz PAG46 from this (Link removed). He used to be a very knowledgeable member of the FSB forums before he got banned. He gets the specs from a TSB from Ford.
Oil TSB: TSB 97-10-05
Freon Amount TSB: TSB 94-19-20
He was doing these repairs on a 1993 Bronco which still came with R12 I am fairly certain, so should be accurate.

Interesting and good to know a system can leak when under pressure but not when under vacuum.

Sucks I have to replace the accumulator but oh well the systems gonna be empty, they're cheap, and they're easy to replace. I'll order that and get some more freon and see what happens.

[GriffinB]

I got the refrigerant recovered and filled it back up with nearly 36oz, the reason I stopped was the pressures were quite high on both the low and high side. After filling the system and properly purging the fill line each time I changed the can, the low side was at 65psi and the high was at 230psi. It was nice to see the high side looking proper but then the low side was too high, combine that with the vent temp blowing 70 degrees I wasn't sure what was going on. It performed better when I started driving and got better as the drive lasted, around ~20 minutes. Eventually, I got cold and had to turn the blower to half speed and at that point, it got down to 42 degrees at the vent while driving, after coming back into my driveway and idling for a couple of minutes while I hooked the gauges up the temp held at 52 degrees while idling. Then while idling after that drive it was a big shock to see the low side at 45psi and the high side at 110psi, but its still working well, but not optimal in my opinion. The ambient temp during all testing was 80 and partly cloudy, also replaced the accumulator and added the same amount of PAG46 oil that came out back into the accumulator.

I also tested my 2001 Crown Vic which has a new AC system that is a couple of years old that I put in. Vent temps go down to 44 after a couple minutes of idling and low side was 30psi and the high side was 120psi, so either my high side gauge is messed up or something else I'm not sure. The outside temp on the electronic HVAC controls said 84, my weather app said it was around 77. The low side correlates more with the high side here but still seems the high side is low, the clutch was cycling once the cab got cool on my Vic. It will not really cycle on my Bronco, at least that I've seen.

Hopefully, all the info here helps you and is enough, I'm not really sure what's going on but I will test it again today, says there will be a high of 90 so I will really be able to test the system on a hot day to see how it performs.

[tbirdtbird]

Be sure to use the temp in front of your grill, not the weather channel. I am gonna let the other responders keep their hand in here, it is not clear what steps have been taken.

At one time we had a format as follows; (this is an example only)

2011 Camry
Rpm. 1800
Fan max cool
Max fan speed
Windows open
Lo pressure. 60
Hi pressure. 120
Ambient 82
Humidity 52%
Stem thermometer reads 55 (in center vent)

Wish we still had this format, it is easier to follow

EDIT!!!
These numbers are totally made up, I am merely giving an example of the kinds of numbers a poster may bring to the board, NOT the ideal numbers that a system should be .

[GriffinB]

Be sure to use the temp in front of your grill, not the weather channel. I am gonna let the other responders keep their hand in here, it is not clear what steps have been taken.

At one time we had a format as follows; (example)

2011 Camry
Rpm. 1800
Fan max cool
Max fan speed
Windows open
Lo pressure. 60
Hi pressure. 120
Ambient 82
Humidity 52%
Stem thermometer reads 55 (in center vent)

Wish we still had this format, it is easier to follow

Copy I have some digital thermometers I keep in each room of the house so I'll put that in front of the grill to test.

Is that format what you would use as a standard when bench marking, if so those pressures are kinda close to what I have been seeing.

Any input @JohnHere?

[tbirdtbird]

please note that the numbers I put into the chart above are NOT the numbers that are desirable; they are way off.
They are an example of a system that is jacked up
Sorry for any confusion

[JohnHere]

No confusion on my end, tbirdtbird. It's essentially the same procedure used currently but with different pressures and vent temperatures in an OEM system.

At 82°F ambient and a full charge of R-134a for a given system, look for a high side of ~190 PSI and a low side of ~30 PSI, the latter of which would yield an evaporator temperature of ~34°F and vent temperatures in the high 30°F and/or low 40°F range, depending on the system and the length of the ducts.

[Carguychris85]

One thing I will throw in here. In Texas heat and humidity I have not seen a R134a retrofit perform well without the corresponding orifice tube size reduction. On Fords the R134a version of the same truck had a smaller orifice and perfomed very closely to the R12 version of the same system. On GM they kept the same orifice tube size with only a few exceptions and performed horribly unless they were at highway speed for several miles. With R134a you need to get the evaporator core to a lower pressure than R12 to get similar cooling. The R134a per unit of refrigerant boiled removes more heat. It has a better latent heat of evaporation than R12. That is a double edged sword because it also means it is harder to condense in high ambient temperatures. I upgrade the condenser to a modern microtube parallel flow, using the largest condenser that will fit the radiator core support opening and occasionally even 2-3" wider than the actual opening. I reduce the orifice size, I have found the Red Ford 0.062" or GM Yellow 0.062" for condenser mounted tubes to work extremely well in high heat and high humidity for most large 70-80s designed fullsize car, truck, SUV and van system regardless if they were R12 or R134a to start with. I have used the orifice in GM, some Fords (unless the factory R134a system in the same model called for the smaller 0.057") and some Mopar trucks, SUVs and vans. I also use a severe duty fan clutch that spins more rpm in freewheel mode as well as pairbthat with a fan with as many blades, and as much pitch as I can fit in the shroud.

Tested my 1987 G20 today in 102F and 46% relative humidity. It has a new evaporator core, resealed evaporator box, Peterbilt microtube parallel flow condenser replacement. The L31 350 swapped into it has a GM 154672 Z82 cooling package severe duty clutch with a Motorcraft YA-249 fan blade that was just small enough to fit in the factory fan shroud but a big improvement over the GM 5-bladed steel fan. I added a thin single 12" pusher in front of the condenser, opposite of the factory transmission cooler that is hooked to a relay controlled off the high blower fan relay input. When the blower fan speed is on high the auxiliary condenser fan fires up. I used a Sanden 4261 compressor and crimped my own hoses. I optimized the charge after the system customization by monitoring superheat and subcooling. With 6-9°F of superheat out of the accumulator outlet at 1,350 rpm it was running 30 psi and 260 psi with 40F out of the center dash vents. At 2,000 rpm it pulled into the mid 30s. At 850 rpm idle it holds 45°F. Adjusting the low pressure cutout to bring the refrigerant as cold as possible before freezing the evaporator core into a solid block of ice is also a critical change. The R12 switch is typically calibrated around 28-30 psi and the R134a switch needs to get down to about 18-20 psi for the best cooling. You will not see a big difference at idle/low speeds with the blower on the higher speeds but it makes a big difference when the vehicle is actually going down the road. Once the interior is cooled down and the blower speed is reduced, it will also blow colder air with the compressor no longer cuting out too early. I have seen modern replacement switches available for many applications calibrated at 21 psi for R134a with the R12 thread size. Often those can still be adjusted 1/4 to 1/2 turn CCW without freezing the evaporator core.

On my 1997 Express with dual ac evaporators I run a similarly setup system. That R134a system has always performed better with the Red 0.062" orifice. It will get in the mid-low 30s at highway speeds out of the center vent. I run a 10 cylinder 210cc displacement HT6 upgrade compressor and a modern microtube parallel flow condenser upgrade for the OE serpentine condenser in it. I have an early duramax fan blade with 1/2" cutoff each blade to fit the oe shroud on it. In 110F that one runs about 30/275 psi at 2,000 rpm. I retrofited the smaller fixed pressure GM cycling switch to an adjustable replacement on that 97 van and my 99 Tahoe as well. On the R134a GMs I use a switch for a 1994 G20. I have found some similar fixed switches on Chrysler products as well that the GM switch fits on as well.