I'm hoping there are some experienced AC techs in here that can help me.

I built my own AC system. I found an old style dealer add-on under dash evaporator unit and coupled with a modern compressor (I used one of the cool brackets you can find on this forum to mount the compressor under the alternator location on my 390 FE). I built my own hoses using the Mastercool tool.

I've been tinkering with this system for a couple years now. AC isn't a high priority and I lost steam a couple times. Mostly I've had to address slow leaks. One from a flare coupling and another from a hose fitting that did not seal well after crimping it with the Mastercool tool. The system, when it was running, cooled decently well.

I think I've finally got the system sealed up with no slow leaks.

Here's my issue though: I'm having trouble determining the correct amount of R134a charge to put into the system.

I read that the vintage air systems for bumpsides runwith about 24 oz of refrigerant (two cans). But I also read that the system pressures, when the correct amount of refrigerant is flowing should fluctuate around 200 psi on the high side and 50 psi on the low side (give or take depending on ambient air temperature). I started with 24 oz of refrigerant, and my pressures were running around 100 psi on the high side and around 15 psi on the low side (ambient air around 68*). These readings seemed low based on the charts I've seen, so I added 6 oz of refrigerant, bringing the total amount to 30 oz. The high side increased a little bit to a high of around 125 psi and the low side to around 20 psi. The temperature at the evaporator is pretty good when the pump is running (~37*-40*).

But I'm concerned about the seemingly low pressure readings. Do I need to add more refrigerant? I don't want to overcharge the system.

Can anyone who knows more about AC help me out?

Thanks!!

 

Your home-made system will hold more than 2 cans
More like 3 pounds
Go by pressure
I'd have it somewhere near 28-32 pounds on the low side with the compressor running constantly
Do you have a accumulator with a cycling pressure switch controlling the compressor on time?

 

With a unknown internal volume you will need to set the charge by pressure. Add Freon till you get to ~30 psi. with a rpm around 1500. Most of these old trucks had sight glass on the dryer, or in the discharge line. In the old days you charged (R12) till they were clear and pressures on the low side was around 35psi. This would give you a discharge temp around 45F. Expansion valve system give you a good amount of range to play in.

Use nitrogen gas to pressure test with. If you flow it thorough it will quickly dry out a dryer unit making evacuation quicker. If it's not holding a hard vacuum you still have a leak.

What safety switches do you have on your system? The main issue when using R134 as a replacement is the mineral oil used on R12 systems turned to acid and ate the system up over time. Forcing you to replace every thing down the road. Flushing out the old oil is critical to making it last.

 

My system uses an expansion valve, so I have a drier, not an accumulator. The drier is pretty large, so maybe that is why the pressures seem low and it seems like more refrigerant is necessary.

Except for the evaporator, my system was built with all new hoses and components, so I'm hoping there's no concern regarding mixing R12 oil with R134a oil.

It sounds like I should continue to charge since the pressures are reading low. I just wanted to make sure before I added more than 30 oz.

Thank you for your help!

 

Yes, there is a concern
Mineral oil for R12 is NOT compatible with PAG for R134a
You will need some ester oil out or a retrofit kit
Once the ester is in there you are good to add pag in the future for recharges
You might want to switch to an accumulator
Your receiver dryer stores liquid refrigerant
The accumulator on the low side prevents liquid freon from hitting the compressor

 

Take the time and expense and flush the whole system, remove what components you can to clean. If you don't you will be replacing every thing in the system since acid ate every thing up. Cheaper to do it right the first time. Doing so you will be required to add a little extra oil since some components retain oil.

This is the main reason I never did conversion. I would use R12 as long as I had it so I would not have too. Beside R-12 cools better any way since these old systems were not designed for R134. Now there are some R-12 replacements that are a very close match with R12, so nothing needs to be changed or cleaned.

 

I convert and build my own AC systems. There is no known capacity, have to figure it out

Pressures can be all over depending on ambient temp, airflow through the condenser, airflow through the evap, and engine RPM. So there is no magic number. It depends. There are charts as a guide, but maybe only one factor not all of them.

This is my view on the subject; superheat.

You want enough refrigerant in the system to achieve max BTU heat transfer. More won't help. To determine this, under worst case conditions (hot ambient, engine bay/condenser hot, interior of vehicle hot) check the temperature of the outlet of the evap, suction line. If it is warm all the refrigerant has evaporated, and the system needs more. If it is cold there is still liquid in the line, you have enough or too much. So you need to sneak up on the right amount under the right conditions.

If you check the system cold, the suction line will be cold even underfilled. The system isn't transferring much heat, doesn't have to evap much refrigerant

The temp split from air intake to vent out is another determining factor. Again, the whole system needs to be heat saturated, working as hard as it can to get a real number. Absolute vent temp is useless.

Same applies to home or any other type of A/C system. expansion valve, orifice, whatever.

Very simple, but AC industry doesn't want you to know this. They are thieves.

Again, this is my perspective, conflicts with some info above. Not saying others are wrong. there is more than one way. Use as you wish.

 

If helpful, my 1972 and 1976 factory based 134 A/C systems use about 24 oz. More won't help. I live in hottest, worst case climate.

Your results may vary.