AC Troubleshooting 1976 GMC C15

[carfanatic63]

So quick introduction, I'm pretty sure I've made an intro here before but if not, I have a 76 c10 SWB with a 454 and 85 exterior/interior swap. Been on the forum for years and use it regularly to see what is going on as these old trucks rock! (On my 3rd)

Loop this back around to the problem I'm having. After swapping to a serpentine belt setup years ago (failed water pump, figured while you're in there), I haven't had AC since I needed new lines. Living in Houston, its HOT but I've toughed it out since then. Truck is just used for fun and errands so no big deal.

I've got 2 little kids now who LOVE riding in the truck so when I was tackling a radiator upgrade, I figured I would do the AC system while I was at it.

I have replaced the below.

Condenser (2 oz pag oil)
Compressor (New AC Delco) (3oz pag oil)
Accuulator/Drier (2oz pag oil)
Expansion Valve (Ford Blue style)
Custom made AC lines
All new O-rings

After pulling a vacuum for an hour and seeing it hold, I decided to charge the AC system. Everything went fine until the compressor stopped cycling and the high side was at 300 and low side was at 55 with minimal cooling happening (maybe 65 or 70 degree air out vents). I only put about 12oz of R1234a in it which is well below what I expected. After some googling, I figured it was a clogged expansion valve. Figured that was weird but went and had the system evac'd and popped the high side line on the bottom of the evap core and swapped the expansion valve. A ton of PAG oil poured out the line. I estimated about 3oz worth. I figured I must've been overfilled with oil even though I only put 7-8oz in originally but didn't think too much of it. Decided to not add any additional oil to be safe and resealed everything up.

Pulled a vacuum again for an hour, waited to make sure it held, then charged the AC system again.

Things went better this time at first. Compressor started hyper cycling then stayed on and cycled every few seconds then stopped. I maanged to get maybe 20oz of refridgerent in this time but same thing. High side went up to about 250 and low side at 50psi. About the same temp out of the vents in the truck. I'm guessing the expansion valve is plugged again.

Does anyone know if I did something wrong somehow? I've done AC systems before but this was my first time making lines. Never had any issues with AC's like this before and this is the first time I've had PAG oil clog the orifice tube like this.

Anything I can do to fix short of going to a shop? I'm both cheap and also take pride in doing my own work (even when I have to do it 2-3 times) so I would like to avoid doing that if possible.

Also, pictures of the truck for your enjoyment and of my AC lines as well I made. I'm not much of a picture takes so please excuse my poor pictures.

If anyone can help, that would be awesome. Been working on this over the past couple weeks off and on and at this point, I'm out of my knowledge realm.

 

[75gmck25]

Did your truck originally have the large A6 compressor mounted on the driver's side? I had that on my '75 and they were infamous for internal failure and "Black Death" (actually granules of black material) that contaminated the entire system. IIRC, the R-12 charge for my original system with the A6 was 3 lbs, 12 oz, so about 60 oz. I assume that means the correct R134 charge would be around 45 oz.

I made a similar conversion (complete flush, Ford blue orifice, new accumulator, pag oil, etc.) for my '75, except I switched to a Vintage Air Sanden compressor and brackets. I flushed the evaporator and replaced the condenser with a new parallel flow. I had new lines made with the right threads and seals for the new condenser and Sanden compressor.

I charged with R134 at a level of about 75% (that was about 40-42 oz) of what was recommended for R-12, and then tried tweaking it by adding a little more from there. During my later troubleshooting I also found that the high pressure cutoff switch I had added was leaking. I didn't find that leak until I added a little UV dye. Keeping track of the pag oil amount was difficult because the new compressor was pre-lubed, and then I added a few oz for the evaporator, compressor and lines. My goal was 7-8 oz, but since the leaking cutoff switch also had oil residue around it I don't know how much I lost there.

I have the system working reasonably well right now, but it doesn't get ice cold. I'd like to try to make it colder, but I think I'd want to remove all the R134, check the orifice and hoses for contamination, and then vacuum and charge from scratch again. Since it's working good enough for now I haven't wanted to start taking it all apart. Last time I checked, the pressures were about 32/175 when the outside temp was in the low 80's. This seems to be too low for good cooling, and it may get better with more R134.

 

[carfanatic63]

I did have the large A6 on the drivers side. When I removed it, it was still perfectly functional.

I replaced the low side switch as well on this change. For some reason, PAG oil seems to be blocking the orifice tube even after 3ish Oz was removed even though I didn't put more than 8 in.

So far, I was only able to get in maybe 20oz of r134a before pressures were high and compressor stopped cycling and stayed on

 

[Finkaire]

Low pressure is on the high side of normal range. 50psi should mean your outlet temperature should be about 50 degrees. Anywhere between 40 and 50 degrees is within normal range. In my opinion if you could get more air going across the condenser the performance would improve.

 

[carfanatic63]

Maybe? I only put in 20oz though which is around half what the system should take and it was already at those pressures.

Evaporator wasn't getting cold either other than right after the expansion valve. Rest of it and the drier were cool at best so I don't think the refridgerent is circulating as much as it should for some reason

 

[carfanatic63]

Well, I went ahead amd evacuated the system and pulled a vacuum. Going to flush the system and refill with oil again and freon at a later date. Maybe change the evap core as well even though it doesn't leak.

 

[85K304SPD]

Make sure to use 8 ounces of PAG 150 oil for that compressor. Not the lighter oils. Check the orifice tube for debris.

 

[carfanatic63]

Orifice tube was fine. No debris. Just oil blocking it.

I had pag 46 in it. Maybe that caused it

 

[Hunter79764]

Oil shouldn't "plug" an orifice. If get a slug of oil, it will restrict it as it is much thicker than refrigerant, but shouldn't stop anything. Did you drain the oil out of the compressor before you began? They usually ship with some amount in them already, that might be an overcharge of oil if so. Too much oil will act like an overcharge, and 55/300 to me is more of an overcharge pressure vs a blocked orifice.

When you said it "stopped cycling" at 55/300, was the compressor continuing to run, just constant instead of cycling? Or it quit, but the pressure stayed where it was? What engine RPM was it? I might have missed it, what was the ambient temp, and what kind of condenser airflow do you have (looks like stock mechanical fan)? Blower on high? Is your condenser a tube/fin type, or parallel modern style?

Have you tried letting the system equalize when if gets that way and start it up again? If you have a slug to the orifice, it will restrict but should equalize after a while, 55/300 isn't terrible or harmful. When the system is all charged and running with a proper amount of oil, the oil is generally not sent as liquid anywhere, but rather sent as mist through the system, mixed with refrigerant. It will balance out with accumulated liquid in the various components when it has run enough, but it's possible you are getting too much unmixed oil hitting the valve for whatever reason. After it balances out, you can adjust things as needed.

Stupid question, but just checking. Are all of your hoses routed correctly? Compressor discharge to the top inlet of condenser, liquid out from condenser to expansion valve/orifice, to the evaporator, to the drier, back to the compressor?
Last thing, assuming it is running but at some off pressures, you can spray some water on the condenser (probably don't drench it immediately, spray a little on a portion and work your way up). You should see the high and low pressures go down, and vent temps should get colder. Removing water should bring both pressures back up.

 

[carfanatic63]

Thanks for your insight Hunter!

Hoses are all routed correctly to the best of my knowledge. It's a brand new condenser off LMC so I believe it should be the newer style.

I let it sit for a day and next day it blew COLDA for about 5 minutes then pressures got high again

Correct. Compressor stayed engaged and would cycle. Temp was about 90 degrees and fan was on max inside.

Mechanical fan as well.

Regarding oil, the brand new compressor I had said shipped with no oil specifically on box (weird) but I drained it and refilled it anyways. 3oz in compressor. 3oz in drier, 1 Oz in condenser. I was using PAG 46 so I'm wondering if maybe that was the pr9boem possibly too.

The evaporator core pipe right after the expansion valve was ICE cold and frosting up when those pressures were recorded which leads me to believe the expansion valve was plugged and not letting enough refridgerent flow through it.

Also, my high pressure port is right before the expansion valve and it visibly had oil coming out of that port when the garages were hooked up as visible in the site glass which is why I thought it may be oil clogging it.

I could easily be wrong though and probably am.