Automotive Air Conditioning Information Forum

I have installed all new "stock" style components into my 1979 f250 with brand new everything including a sanden 508 compressor. I am lucky to get a 20 degree differential when the ac is on. Even after driving for 30 minutes the lowest temp I have seen from the vents is 50 degrees. When gauges are on when parked and rpm at 1200 the low side pressure is 10-15max and high side about 200-215 with ambient temp of 83 degrees and extras fan blowing across the condesnsor.

When watching the gauges I can see the txv opening and closing and controlling the pressures. Is there a recommend placement of the txv bulb as far as clocked around the suction side? Any other things I should be checking for to get the air a little cooler?

with the low side this low isnt it freezing the evaporator?

The HS (condensing) pressures look good, but the LS pressures are low. The TXV bulb is usually located at about 3 o'clock or 9 o'clock, but I'm unsure whether clocking even matters.

Is the bulb tightly attached to the tube, and is it well insulated from engine compartment heat?

At those LS pressures, the evaporator will be well below ice-up temperatures, which could be part of the problem.

Suggest testing the system at ~1,800 RPM.

Thanks that’s the weird thing the evap is not freezing
Up or getting anywhere near cold enough. I moved the “anti freeze” switch probe around to different sections of the evap. And checked with a thermometer the lowest temp
I can find is around 40 degrees. The compressor never cycles it runs all the time.

I don’t know how the low side can be so low and not freezing

Some additional potential causes:
● Insufficient refrigerant flow through the TXV.
● Something blocking the TXV and refrigerant flow, such
as beads from a ruptured desiccant bag.
● Non-condensable air in the system.
● Sealer in the system.
● Too much oil.
● Saturated R/D desiccant.

Did you convert this vehicle from R-12 to R-134a?

With numbers like this we need to start from scratch.
Are you saying ALL components were changed out?
That would be comp, condenser, F/D, txv, evap, and all hoses.
Had the old comp grenaded?
Did you use the electric powered vacuum pump or powered by car engine vacuum?
How long did you vac for? Did you use a vac gauge?
Where did you install the system oil?

The txv sensing bulb must installed at 3 or 9 pm as John stated. Is it installed in the correct direction?

Expansion Valve system? Those numbers could be closer than you think. A little more charge is in order.

all new system, every component new and replaced. I replaced the expansion valve and drier again on friday just to be sure. pulled vacuum with pump for 45 mins down to -25psi and let set for an hour no leaks. It still is only getting about 20 degrees Delta across the evap coil. Which is ok i guess but not as cold as i remember the original R12 systems being. I am running a new stock size 16x20 parallel condenser. Would a larger one give me lower temps? Even while cruising the vent temps dont really drop for a long time until the average cab temp comes down.

sedlacek269 wrote: ↑Mon Jun 17, 2024 1:33 pm pulled vacuum with pump for 45 mins down to -25psi and let set for an hour no leaks.

I think what you meant is 25 inHg (inches of mercury), which is a measure of vacuum. If so, that's not nearly enough for a thorough and complete evacuation at sea level or slightly above, especially for only 45 minutes.

In this instance, if the above elevation holds true, you should be evacuating the system to ~29.92 inHg, or more precisely, ~500 microns as measured on a good quality micron gauge, to remove all the air and moisture, which are probably still present in the system.

Moisture is a contaminant that forms acids in conjunction with refrigerant and oil, and air is a non-condensable gas, both of which can cause problems with higher center vent temperatures, like you're experiencing.

sedlacek269 wrote: ↑Mon Jun 17, 2024 1:33 pm all new system, every component new and replaced. I replaced the expansion valve and drier again on friday just to be sure. pulled vacuum with pump for 45 mins down to -25psi and let set for an hour no leaks. It still is only getting about 20 degrees Delta across the evap coil. Which is ok i guess but not as cold as i remember the original R12 systems being. I am running a new stock size 16x20 parallel condenser. Would a larger one give me lower temps? Even while cruising the vent temps dont really drop for a long time until the average cab temp comes down.

The biggest condenser you can physically fit in the core support opening the better performance you will get. The core support opening on my 1987 G20 is 28 x 17". I have a microtube parallel flow replacement for Peterbilt 3S010633 that is 31.25 x 15.25 in the opening. Because of the gap between the radiator and condenser the extra width is still getting air drawn across it. Mine has the factory auxiliary transmission cooler on one side and I added a Spal 30101504 straight blade 12" pusher on a relay controlled by the same wire that turns on the high blower relay for the high blower motor speed. I have a reverse rotation 11-bladed plastic fan and clutch to match my serpentine belt water pump setup but I have used the 3.0L Ford Ranger 12-bladed plastic fan (Ford F57Z8600B equivalent) matched to a Hayden 2799 severe duty fan clutch on my 1983 G20 with a standard rotation V-belt pump and it worked equally well. Big condenser and as much airflow as you can provide it will always help. It was 102F and 46% relative humidity yesterday, finally hot enough to optimize the charge on my custom system and I had the gauges on it. I adjusted the charge by adding 6-8 oz until I had 6-9*F of superheat at the accumulator outlet on that system. At 850 rpm idle was getting 45F center vent temp. At 1,250 rpm dropped to 40F. At 2,000 rpm it dropped to 35F. At 1,250 rpm I was observing 30 psi low side and 265ish high side. I am running a Sanden 4261 SD7H15 replacement for the GM HT6 with 155cc displacement, which struggles a bit at idle compared to the Four Seasons 10 cylinder 210cc unit I have in my 1997 Express with the same engine accessory bracket setup. I am not a fan of the Sandens at all now that I have run them. Sanden claims the smaller displacement works as well as larger displacement compressors but my experience has shown that simply not to be true especially at or near idle. The 508 has even less capacity. I had a new genuine Sanden SD7H15 shipped to me from Sanden in Wylie, TX on my 1997 Express that I used for a brief period of time on it prior to it being used on this 1987 G20. The Sanden was completely inadequate on the 97s dual evaporator system over ~90F ambient temps and marignal with the large GM single evaporator system in the 1987. The SD7H15 is more capable but I have retrofitted a TM21 in place of a couple of them on larger vehicles. The TM21 is more inline with the GM A6 or York 2 cylinder BTU capacity wise especially at lower rpm. All 3 are in the 40K BTU range at 3,000 rpm compressor speed.

I am assuming this Ford truck like the 1978 Lincoln Versailles I worked on a few years ago had a GM A6 compressor. Someone had previously Sanden 508 swapped it, never cooled right and failed from improper service. I went back to the A6 which has more than 50% more displacement, correctly made new barrier hoses, new parallel flow condenser, 12 bladed Ranger fan on a severe duty clutch and it would cool mid-low 30s center vent in 110F at highway speeds and low 40s at idle with R134a. The 18" Ford Ranger blade and Hayden 2799 are my go to setup here in Texas heat. NOTHING else that fits in place of an 18" fan will touch one cooling and airflow wise when you are actually driving the vehicle including dual electric fans.

I am an absolute believer in the newer style aggressively pitched 9-12 blade plastic fans, matching severe duty fan clutch, a proper shroud and the largest newer style microtube parallel flow condenser I can fit in the core support. They will drastically outperform older style tube and fin, pico or serpentine style condensers 2-3x as thick while blocking much less airflow through the cooling stack. It is a bad joke that manufacturers are even still making tube and fin, serpentine or pico style units as they are junk AC performance wise.