If you are low on 134a you will also see the pressure drop as you rev the engine and eventually the clutch will disengage. Then the pressure will recover and the clutch will re-engage. Is it doing that as well?

 

hoping a resolution here..
IMO..
- I would get the gauges and adapter to read high and low
- Take many pics of your system and piping condenser fans etc..
- verify the thermocouple is on the pipe from orifice to evaporator is contacting the pipe surrounded by goop... pretty sure that is the one that cycles compressor off.
also where are you located? someone local may have gauges etc..

 

When there was only 12 oz refrigerant in the system it cycled on at 60 psi, off at 10 psi (low side reading). Now with all the refrigerant added, if I rev it higher like 3000, it drops lower just not enough to turn off.

I did add in a bit more to the system, but it was taking forever so the system now has a low side of about 43 psi. Will work on putting in the rest later. But vent temp wise not much has changed, slightly cooler like low 60's.

I have the gauges to read high and low. Although there is no high side connector in the system. Only the low side at the compressor suction pipe.

I will better verify the thermocouple, as far as I saw it goes down alongside the pipe into the goop.

 

I'm pretty sure there is no heat from the heater core. There is a valve for the hose pushing coolant to the heater core, I have it receiving a vacuum constantly, being that it's summer. And even with the engine running at operating temp I can set the heater on and a/c off I don't get hot air from the vents.

I would like to clean out the heater box/evaporator core of any debris, but I haven't gotten to doing that yet. Hopefully I can verify the blend door is working properly then.

 

Sometimes when you think the heat bypass valve (especially vac-driven) should be functioning as it should; it ain't. Hence meteor's p#8 about installing 2 valves.

I installed two ball valves from plumbing supply; when Both are closed, it absolutely isolates heater core from hot coolant. I don't guess or think it, I KNOW it.
* and if you ever have a heater core leak, those extra two ball valves are a godsend.

 

It is an orifice system.
So it must have a "cycle switch"
Likely near the accumulator?
Do you have both hoses hooked up.
High psis are important too.
I can not believe it does not have a high side connection, somewhere?

Did you replace the cycle switch as well?
I had major troubles with one of the many AC systems I have done. The cycle switch would trip at 48psi and turn the compressor off. Then it crept back up to 50-60+ and it turned on again.
Once it has the right charge it should pull the low side down until the cycle switch cuts out the compressor.
I had to adjust the screw on my mis-calibrated cycled switch to get it to pull down to 29psi before cutout.
That few psi change really drops the temps in the evaporator and made the car vent temps 10-15* lower. Down to 34*


Cycle switch on the right has an adjustment screw.

My favorite R134 pressure chart:

Although I do not settle for vent temps until they are at least in the mid 40s.

Tips:

You will likely need a large Walmart style box fan on high for enough airflow to the condensor in your driveway. It still will work even better at 30 mph on the road with even better airflow.

If the high pressure is creeping up in your driveway, spray the condensor with some water, even a mist will help a lot, to keep it cool.

High pressure should stay under 250psi. While low drops to 30psi.Cooling the condensor more will help.

Make sure you use a fresh drier/accumulator the dessicant pulls out the moisture.

And pull a long vacuum, like 30min to an hour, and see if it holds for 30min. You want to suck out as much moisture as you can, and let the dessicant do the remainder, and to be sure there are no leaks.

Add what the system calls for, R134 amounts are a little different than R12. It is OK to add just a little more, like 2 oz, to make sure it stays cold on a really hot day. You are way better off with just a little too much than too little. There is room for it. (90% of the Freon in there is as a liquid and sitting in the condensor or it's tanks/pipes, and you have a little extra room. A condensor should run about 50-75% full with liquid.

When I get close, I add a little, then monitor the vent temp, and add a little more. When the vent temp stops dropping, add 1 or 2 oz more and stop. Keep the condensor cool and keep the high side under control.

There is a lot of chemistry in there, but it is not rocket science.

Some cars have cycle charts to monitor the on/off times. But GM does not do that. It should still cycle tho. But Corvettes and converted systems are both condensor challenged.

 

Here is a pic I found online of a 79 vette
He said: "Passenger side, behind the front fender there's a little panel triangle take that off . Your fitting will be in there. You will need an adapter. You'll have to remove the valve stem and put on the new adapter to fill the new freon. You can reach it from the top but it's at the bottom."

Because it is on the bottom of the accumulator, and large lines, that looks like the low side port. And on the right is the low psi cycle switch too that an orifice system must have. The copy says this is the high side. But the accumulator is low psi. But it is likely the fitting you could not find.

The older cars had a POA system, and two fittings on the compressor. In 78 they switched to an orifice system.

Here is the hose for a 1979. It only has one fitting on it. The fitting size will tell you which one is low and which one is high.


The upper hose is large, so that is the low psi side, to the accumulator. The lower hose is small, so that is the high pressure side going to the condensor. The fitting is on top, next to the large low psi hose, but I would guess it has to be the high psi fitting, and just put on top for convenience access. Connect both hoses and you will know for sure.

If that is not the high pressure fitting, then it has to be somewhere on that line. Follow the small line out of the compressor, into the condensor, out of the condensor, all the way to the orifice valve you found. That is the entire high psi side, and there must be at east one fitting in there somewhere.

If that is the high psi fitting, and you only get 50psi there, well you are not even close to being fully charged, or the compressor is shot.

You got this!

 

Leigh, unless my 78 is different to a 79, then the picture you show of a 79 low pressure fitting is not standard as I don't have anything like it. Also my system does not have a low pressure switch on the low side, it's on high side right next to the high side fitting (yellow arrow in the photo). As far as I can see the only purpose this serves is to turn off the compressor if you lose all the refrigerant. The only thing that cycles the compressor on my system is the temperature switch at the orifice tube, which prevents the tube icing up. For my car the low side fitting is the one on the hose that you show.

 

Terrific photo UKJohn!

Yes the yellow arrow does look to be the high pressure fitting, on the small diameter high psi line.
That should help our OP.
A switch on the high psi side is likely just a high psi cutout or safety switch.

That would mean the fitting on the compressor is likely the low side fitting. Which puts the two fittings close to each other, a foot or two apart.
That makes more sense than the photo I pulled off of some guys facebook, but it does look like a cycle switch down there, at the bottom of the accumulator.
There is no other logical explanation for a switch on the accumulator.
The cycle switch will stop system operation when the freon is too low.

Anyone have a picture of the orifice tube location and/or the switch by the fitting?

Personally I have not worked on a 78+ orifice tube system, but these things are just not that hard.
At least once you identify which component is what.

As a team I will bet we could help the OP.

 

Here's a photo from underneath the accumulator, the yellow arrow is the thermal cut out switch, the blue arrow is the orifice tube location. As you can see there is no fitting at the base of the accumulator just a connection to the low pressure hose.
The pressure switch is a low pressure cutout, there is no high pressure switch, just a relief valve on the compressor. I've replaced my pressure switch with a binary switch, to provide both a low pressure and high pressure cutout.

 

nice pics..
79 here.. and a thread if helpful.. i do not think the switch is adjustable , also the 4 speed does NOT have an AC kick up solenoid. high pressure line needs an adapter if original configuration. POST more pics
https://www.corvetteforum.com/forums...s-oriface.html

 

I agree completely, I don't trust the original vacuum valve. I installed two ball valves for each side of the heater core connections this morning. Unfortunately this made no difference, but it is nice to know the heater is for sure off.



The blue cap near the compressor is for the low side on my system, although it is right at the compressor. There is also connections at/near the accumulator where I could probably get an adapter to read the low side there (Picture below) . Which might be more accurate. Not sure where I'd get an adapter for those.

For the high side of the system a rubber line connects to the compressor, straight to the condenser. Then a hard line goes towards the orifice tube. The low pressure cutout switch is in that line, however unlike some of the pictures you guys have sent, there is no high side connection.



Not the greatest picture in the world, but past the orifice tube the thermocouple sensor runs down the pipe.

I have to wonder if my system is actually overcharged. Although the pressure for the low side shows around 43 psi (95F outside, 1500-1800 rpm of engine, fans on etc.). When at idle (750 rpm) the pressures are at 55-60 psi, and the performance is quite terrible. However when I had only around 12 oz of r134a in the system, the compressor would cycle on and off, getting the tube past the orifice nice and frosty real fast. However now at it's best it becomes slightly cold.

I have to mention that there has to be at least 44+ oz of freon in the system now. That's being kind, there's probably a lot more. But was really hoping if the pressures get close to matching the chart a change would be noticeable. The coldest vent temp I have gotten was around 59F, however I have not been able to recreate that and it really hovers around 62F.

 

Start over..its cheap enough. Use a scale, you can get it all in if you heat it without running engine.
your ac to me looks different than mine. Maybe a video or more pics close and overall. Not havng a high side valve is not good. Looks kinda custom.

 

Agreed not having a high side fitting is not good.
I have never seen a system like that, and I have done dozens. But not a 79 C3.

Joeywhite
It does seem like you could be overcharged.
R134 fill capacity should be around 85% of the R12 capacity.
Sneak up on it like I mentioned, and when the dash temps are at their coldest, stop, or add 1 or 2oz more only. But not any more.
Do not overcharge it or the high side pressures will go very high.
And the dash temps will get warmer.
It's a shame no one knows where the high side fitting is.
I find it very strange for the car not to have one.

Without a high sidereading, you must be sure to put only the correct amount in, and not go over.
Buy a $20 electronic kitchen scale, and use the gram scale for best accuracy. 454g = 1 lb.
Scale Link

R134 does put more stress on the condensor, so adding a bigger one or a parallel flow one would help, as well as double-checking all the airflow seals.
A borderline condensor or airflow usually rears it's head at idle, with dash temps creeping up.
The blue ford orifice or the Smart VOV variable orifice valve both help conversion systems.
The VOV is a drop in and especially helps conversion systems at idle.
I use the Smart VOV on all retrofits. It is available many places.
It helped me a lot.
VOV Link

I get my AC parts from Arizona Auto Air, they make many quality parts for older systems.
ACKITS.COM

 

Ya that doesn’t look stock, even accumulator.
i tried to show 79 ac lines and high side valve. Accumulator buried in fender. Made a video if helpful.

 

Hopefully this is an important discovery I made. I managed to get 45F air out the vents, however this is with the blower motor set to low.. so barely moving air. Once I increase the fans speed, the low side pressure goes up quickly, along with the temperature out the vents. Quickly the temp rises back up to 60F. I really hope I can expect more out of the system. Even in 80 degree days that probably wont keep up.

I would think that means the a/c system works. Just not a very strong system.

I have tried spraying water on the condenser, with extra fans. While it does help the vent temp, it's not enough (roughly 5F difference). Besides when driving I still get terrible vent temps.

 

I thought you mentioned there is more than 44oz in the system.
That is way too much.
If the R12 capacity is 40 oz, then for R134 you only need 34 oz.
Too much freon just chokes it all up.
It should get very cold just past the orifice. Near freezing.

 

At this point without a high side reading it's hard to tell if the compressor is functioning correctly, or to fine tune the 134A charge.
It does now sound overcharged.
When you were at 34 oz you added another 10 oz? It probably only needed maybe 3 or 4oz. You gotta sneak up on it slowly when near full. I differ from leigh in that I run it slightly low vs slightly high on 134A. But I can monitor the high side pressure to insure it's not rising too high, indicating an overcharge.

 

At 34 oz I was experiencing the same symptoms, as I am now at 44oz. I did slowly add the freon. Over the course of about 30 minutes yesterday and another 30 minutes today. However once the system reaches a certain capacity of freon it really doesn't seem to get any better/worse.

I'm thinking I should restart again. But I want to get a new high side hard line with the high side quick connect. Because it is entirely possible the compressor is junk, although it appears fine and is new (remanufactured).

 

get some cheap digital thermometers with remote sensors, use pipe insulation to insulate them and install one just downstream of the orifice and one on the outlet of the evaporator.
add enough refrigerant to stop the frosting at the orifice with the compressor running continuous, this gets the coil inlet temps just above freezing, then focus on the coil outlet temp, should be in the area of 40* to 55* depending on the temps in the cab and the airflow across the coil (temps will be on the lower end if airflow is low). if you are seeing inlet and outlet temps in this range and vent temps are still above around 55* then you have other issues outside of the refrigerant system causing the high vent temps like air bypassing the coil or hot air being drawn into the airflow like a recirculation door not closing.
The suction line going back to the compressor should be cool/cold to the touch and sweating with ambient temps in the 90* range
The AC system is capable of coil temps below 32* and will cause condensate to freeze on the coil and block airflow, enough refrigerant has to added to keep the coil temps above freezing in all operating conditions but it needs to be just enough to do so or the coil temps will be to high for sufficient cooling.
If these coil temps cannot be attained then there is something in the refrigerant system that is not correct like inadequate airflow across the condenser, air in the system or a faulty compressor.

This is a generalized troubleshooting suggestion,

Neal