Charging up the AC in my Backhoe

TractorGuy · Apr 18, 2016

Flushed the system out on my backhoe and installed a new compressor, dryer, and expansion valve. The PDF manuals have had so many things in them that do not fit with my tractor I can't believe anything in it unless it is an obvious match but it says the system only holds 1.6 lbs of R134A. It was charging ultra slow but I think that is the plunger in my HF gauges just barely opening the schrader valve. I put three 12 oz cans in it but the gauges still read low and the sight glass is pure foam. I'm pretty sure it isn't leaking since it held a vacuum for 24 hours before I charged it. The vent temperature got down to 34 degrees so it's cooling pretty good. This was this evening so it isn't a true reflection of what it will do on a hot day. I'm kind of reluctant to keep adding freon but according to the gauges and sight glass it is still low. Low side pressure is reading 8 lbs and high side was at 150 lbs. I think it should be 20-30 low and 175-225 high. I guess I'll wait for a hot day to check it again but it's probably just one of those if it looks like a duck and quacks like a duck, it's a duck things.

Tx Jim · Apr 19, 2016

1.6#s of R134A doesn't sound as if it's an adequate amount. I agree your posted pressures seem low but ambient temp has a large bearing on pressures.

Gary Fowler · Apr 19, 2016

I think the key issue here in your case is foam in the sight glass. It should be completely clear when properly charged. OF course the pressures should also be in the right ranges which I believe they will be when charged.

L39matt · Apr 19, 2016

A good way to get close to the correct charge with most systems is to charge until the sight glass is clear. Google a pressure temperature chart for 134A and it will give you a decent idea of were your pressures need to be. If your low side pressure was only 20 psi and there wasn't enough heat load, the unit would freeze up and not work correctly. You will want to have at least 29 psi to keep from freezing. To get a ballpark number for the high side, take the outside air temp and add 30 to it, then convert to pressure using the chart. So at 70 degrees you would have 125 psi. It sounds to me like your high side is too high and the low way to low. I would search for a restriction in the system like a plugged filter drier or expansion valve. Also if the evaporator and condensing coils are plugged up with dirt it can cause issues like this too. My experience with this comes from large industrial units so it may not be perfect advice, but it is where I would start. Hope this helps.

TractorGuy · Apr 19, 2016

L39matt said:
A good way to get close to the correct charge with most systems is to charge until the sight glass is clear. Google a pressure temperature chart for 134A and it will give you a decent idea of were your pressures need to be. If your low side pressure was only 20 psi and there wasn't enough heat load, the unit would freeze up and not work correctly. You will want to have at least 29 psi to keep from freezing. To get a ballpark number for the high side, take the outside air temp and add 30 to it, then convert to pressure using the chart. So at 70 degrees you would have 125 psi. It sounds to me like your high side is too high and the low way to low. I would search for a restriction in the system like a plugged filter drier or expansion valve. Also if the evaporator and condensing coils are plugged up with dirt it can cause issues like this too. My experience with this comes from large industrial units so it may not be perfect advice, but it is where I would start. Hope this helps.

Further research shows this should have more like 3 lbs refrigerant. I will wait for a warmer day and add some till I see the gauges looking right. 134 systems won't necessarily run a clear sight glass but it should not be full foam.

Evaporator and condenser was cleaned and blown out with nitrogen inside and out. All functional components are brand new. I have never seen a system run at 125 psi on the high side and I was a certified master auto tech for 30+ years but then I wouldn't be running AC at a 70 degree outside temperature either. I have seen high side pressures run higher until a full charge is achieved then come down at full charge which I can't explain but I have experienced that on several occasions.

Here are the approximate high pressure readings shown in the service manual. But again, this same manual says 1.6 lb charge which cannot be right.

TractorGuy · Apr 19, 2016

This is pretty much what I was seeing so like I said, it quacks like a duck.

L39matt · Apr 19, 2016

When I did the calculation I forgot that the condensing unit would be in the engine compartment, so you would take the ambient temp in there and add 30 to that. Your high side pressure would be considerably higher. Working on industrial units the condensers are outside so I would use outside air temp for that calculation. I did read that you replaced all functional components but I have on occasion seen new components plugged from moisture or solder junk from the repair work, and it explained what I assumed was a low low/ high high pressure problem. It sounds like you have it figured out, post the results after you charge it again. Good luck, and sorry for the bad info.