Ac issues

[87GrandMarq]

This one is a little strange. My buddy does ac and has taught me a thing or two but it’s I think a combination of issues maybe. When I turn the car on and let it idle with the ac going it’ll get down into the 40s with ambient temps in the 80s. It can do this indefinitely it seems but the compressor stays on. If I drive it while in motion it’s around 40* and stays there until I stop. While in motion I can feel the compressor cycle. Now after I drive it and park it the compressor stays running like before but the ac blows around 60* and will get warmer if I turn up the fan speed. Why does driving it make a difference? It can be idling at full operating temp and still be in the 40s and when I get back it’s still at full operating temp. Could it be a faulty pressure switch or overcharged? With said ambient temps the low side is around 30 psi which is on the low side. Any info would be great.


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[sly]

If it's a little low on refrigerant, the increased pumping of higher RPMs will allow the compressor to draw down the low side below the cut-off pressure. This can cause the evaporator to freeze up (get too cold and create ice on the surface). This is much more common with R12 systems. R134a systems won't cool that low and don't do this that easily.

Another possibility is if the liquid line has drooped and is close to the exhaust manifold as this will help warm up the liquid before it gets past the expansion valve. This usually induces damage to the liquid line and it doesn't last long in this condition, so I doubt that's the issue.

The last think I can think of is the blend door on an automatic system... if the temperature control valve hose is not attached to the barb on the air plenum at the firewall (behind the glove box) it won't pull air over the coil that controls the valve and it may soak the cooler air from behind the dash and not cool as well.

Of course, if the blend door itself has no seal left or the door is broken and not holding full closed for cool, it'll do this as the heater core warms up as it's mixing hot air in as well.

[87GrandMarq]

I know it’s not low as I’ve put in more than enough at this point. Originally I put in what would be the equivalent to the original spec converted to r134a. 70% comes to mind but I forget. Anyway since then I’ve added some with the same result and it seems to not make a difference and I hate to keep adding refrigerant.

All the lines are new and in stock locations. Also it has manual controls.

Could it be overfilled and the condensor gets heat soaked?


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[gadget73]

too much refrigerant makes for high pressure, and that increases temperature. Clue there is the compressor will not cycle, low charge will have it cycle.

Charge is somewhere in the 70-80% of the R12 amount but you can also judge it by evaporator outlet temperature. When the evap is fully flooded, the outlet pipe temperature will start to drop considerably. Since I don't have an AC machine and have no way to figure out whats in the system to do a top-up by weight thats how I do it. Basically when the drier gets cold with the engine at ~1800 rpm, its pretty close to right. High and low side pressure will help confirm its not over or under-charged. I usually aim for low side pressure in the 20-25 range, basically as low as you can get it without the compressor kicking out. The lower the pressure, the colder the refrigerant but unless the evaporator is full of cold refrigerant the vent temperature won't be very cold.



The other thing that makes the AC start to suck as the engine heats up is a bad fan clutch. Heat from the radiator cooks into the condenser and that makes it less effective. Poor airflow makes this much worse.

[87GrandMarq]

The fan clutch is new but I guess I shouldn’t rule it out. Do you think it’s overcharged? Maybe having the compressor run continuously while driving for 20-30 mins at a time is causing it to heat soak. I have everything to wire in a pusher fan but I’m afraid if I do I’ll need to do a 3g swap which will just add more complexity to things lol.


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[bgreywolf]

Did you replace the AC cycle switch with one set for r134a? I've had some cars (like my CV) where it made a big difference, and some where it didn't make much difference at all. My CV was one of the ones that made a difference. The Low and high pressures are a few psi different; on the CV it made the difference from cycling all the time at idle to just cycling every few minutes (although the temps were pretty close inside the car).

[87GrandMarq]

To be honest I don’t remember. I believe I originally had a 134a switch but it fit poorly and leaked so I got an r12 switch. There are no markings on it but I believe that’s what’s on there now.


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[gadget73]

unplug the pressure switch and back the screw out about 1/4 turn. You have now converted your R12 switch to R134a. Cutout with 134a should be right around 20 psi, its like 28 with R12.

hard to tell if its overcharged without high and low side gauge pressures.

[87GrandMarq]

I’ll try that out tomorrow. I have an old set of gauges that the high side is broken and the new ones I have are ebay specials. I doubt they’re accurate. I’ll see if I can get a reading tomorrow though.


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[87GrandMarq]

I adjusted the screw as advised and tried putting the gauges on. The high side wouldn’t go on so I took it off and was greeted by a nice leaking shrader valve... luckily I have the tool with the ball valve built in to swap valves with the system under pressure so I did just that. It lost a little refrigerant during the process but it’s probably not a bad thing. So I started it up and still no cycling. But I put the fan speed up to max and it was about 50* then I took it for a spin and it dropped a little then sat outside idling and it was steady around 48-50*.

One thing I forgot to mention before was that in my first post I noticed all the issues with the fan speed on low. When turning it up the temps climbed. So it only cools right when the fan is on low.


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[87GrandMarq]

I have a laser thermometer and when playing with it a little when the fan speed is turned up the higher the temps are and the lower the fan speed the lower the temp is. My guess is the evaporator isn’t getting as cold as it should.


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[bgreywolf]

Did you replace the orifice tube when you switched from r12 to r134a? Did you flush out the evaporator with the orifice tube out?
Gadget, I didn't know that the switches were adjustable. Handy info!

[87GrandMarq]

Yes on both. When I bought the car it had nothing so everything is new except for the condensor but like I said it was cleaned. I need to get a new set of gauges and get an actual reading for you guys.


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[87GrandMarq]

So I took a quick pic of the pressures at idle after it warmed up.



Ambient temps are in the low 90s with high humidity. Low side seems low and the high side is a little high. And the high side was fluttering 20 or so psi the whole time. My guess is air in the system.

Also the compressor would cycle like normal a few times when first started then stay on.


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[gadget73]

looks like 30 on the low and 215 on the high? Honestly looks pretty good for 90F. What does it do with some throttle? I usually wedge a dime or a penny under the throttle stop screw. That puts the rpm around where cruise speed is. Expect the low to go lower and the high to go higher.

Flutter on the high side could be internal valve problems with the compressor. Its a 6 cylinder pump, and if one of those reeds is bad it will dance pretty rapidly. Unless its just the hood vibrating and making the needle dance.