A/C too cold...

[OilBurner]

something irritating about my 86 GM...is that when using the A/C, it works fantastic...but often gets TOO cold...It seems to NOT shut the compressor off when it gets down to adequate temperature, and i have to cycle it manually on and off, or compensate by letting the heat run which seems inefficient...It also ices up if its humid...20* air with FOG coming out of the vents, receiver/pipes under hood coated with ice...and the compressor just keeps on running. I drove cross country this summer...manually modulating the air on & off got very old driving 700+ miles a day..as well as getting frozen out even driving thru the Utah desert...

I just dont understand if this is normal or if its malfunctioning...iv been into the guts of the A/C in my Toyotas and even my Yugo and they all shut the compressor off when the cabin gets down to temp and then cycle it as needed...

My mercury does NOT have Automatic A/C...however after reading the write up on HVAC on here it appears that even cars with automatic AC do run the compressor all the time and use the heater/blend door to compensate..?


It DOES seem to cycle automatically however when on defrost...or atleast pulse the compressor on & off at intervals determined by ..(what exactly? is there a temp sensor or is it just a timer?)..or something.

[gadget73]

If the low pressure switch is bad, it will cause the system to ice up. Check/replace that first. It screws into the side of the black or silver can on the passenger side of the engine compartment. 2 wire plug, then it spins off. There is a valve in there to seal things up when you remove it, though it will hiss a bit as you unscrew it. Make sure the O-ring is in the new switch when you put it on, otherwise it will leak.


It should be very cold coming out of the dash if its working right, but large amounts of ice doesn't sound quite normal. If its just too cold, move the temperature control upwards a bit. That will mix in hot air from the other side of the air plenum. It works just like the knob in your shower really, one side is cold, one side is hot, and the door gives you the full range between them. The door will not move on a non-automatic model, if its too cold you have to adjust it yourself.


The compressor cycles in defrost, though be damned if I could tell you exactly what causes it to do that. It may be a timer, its definitely not a temperature sensor. Its to remove humidity from the air.

[OilBurner]

so it is actually normal for the compressor to stay running all the time whenever A/C is being used? thats the key thing that was confusing me. I thought it was supposed to cycle on & off when it gets to the correct temp...like a household AC...or a fridge or every other refrigeration device iv seen.

[gadget73]

There is not anything to sense temperature on these. The auto systems aren ot any different in that respect, it just moves the air door towards the warm setting for you. Thats the only thing automatic about it. The only thing that shuts the compressor down is that low pressure switch and thats only to save the system really. If it gets too low, it ices up. If you want it warmer, you have to move the temperature slider. Even the late models with the electronic auto temperature control work the same way. The AC is just on unless the pressure switch says otherwise, and it uses electric motors to move the blend door for temperature control.


Frankly, your AC must really kick ass if this is even a concern. Very few people with these cars now have the issue that the AC works too well. If its still R12, that would explain it. That works a lot better than a 134a conversion does.



I presume this is a summer issue. Its cold outside, turning the AC on now would make it cold as hell inside the car. Opening a window would do the same thing.

[85crownHPP]

Doesn't the compressor run until it reaches its set operating pressure, then the high pressure switch shuts it off? Then as it 'cools' (or just dries the air) and looses pressure, then kick back on?
I definitely don't know much about A/C but that's how it be in my limited memory banks.

[OilBurner]

Mine is actually R290 based now...i realize this is controversial...but it does kick ass, is extremely cheap to maintain, and is standard in Europe...and I have used it in 5 R-12 vehicles now with good results.

I may add a simple electronic or mechanical thermostat that interrupts the compressor clutch to get it to shut off when it gets down to say...60* in the cabin the same way the Toyotas operate..

[85crownHPP]

aaahhhh, I think I am thinking about regular air compressors, for using air tools and such lol

[gadget73]

oh, R290 propane. Yeah, propane works fine as a refrigerant. Its probably better than R134a at any rate. I've given some thought to doing my Mark VII with that. The 134a in a fox car sucks a bit due to the smaller condensor area vs the panthers. The underdrive pulley on the Towncar makes it's AC a bit feeble too. propane runs with mineral oil too, right ? Thats what the R12 worked with, so no harm there. SHould just be a matter of a flush to get clear of the ester oil, evacuate the system, and charge. What do you use for charge amount with that, same as the original R12 charge ?


Also, whats your low side pressure at? if the pressure to temperature curve is significantly different than R12, you may need to adjust the cycle switch. I you unplug the connector, there is a phillips screw under there. I forget which way it works but turning one direction increases cutout pressure, the other decreases.

[tjc78]

Both of my boxes almost never cycled the compressor. The 89 MGM with R12 on a humid day would blow frost. It never iced up, though. The 89 LTC that was converted never cycled either but it also never performed nearly as well as the R12 system. My 04 cycles all the time whether screaming hot or in defrost on a 50 degree day.
If it's icing up it could be improperly charged or the low pressure switch as suggested

[OilBurner]

Yes, Propane. It does mix with mineral oil and any leftover R12 in the system very well, and actually it more efficient because it functions at a lower pressure, so the compressor & engine don't work as hard. It is not compatable with 134a however..so a 134a system would need to be completely flushed and redone with mineral or an r-12 safe synth oil...it is perfectly legal to convert a 134a system to 290...but not r12 to 290...unless you "already converted it to 134a first"...

I never got real scientific when i do mine, I just add til its cold. You do need less Propane by weight than R12, so it would be less than what the sticker says...on systems with sight glasses, (not these apparently) you can just add until MOST of the bubbles are gone...from what iv read, technically you want it slightly under charged going by the original R-12 standards. I use the small 16oz Coleman bottles...they are supposedly more pure than larger BBQ type cans...to get it chemically equivellant to R12, you need to mix it like 80% propane to 20% isobutane...and isobutane is apparently different than regular Butane...Iv always just used straight propane and it seems to work fine...even tho straight propane is technically closer to R22. You'll need to verify any percentages or info i just said as im going from memory...like i said i just evacuate and add it in til its cold. I also use a manifold gauge set til it looks good enuff.

As for the legality issue...who cares...and as for the flammability issue...that is a risk i suppose...however i dont consider it much more of a risk than the fuel line rupturing and spraying gas all over the exhaust manifold in a good accident...R134a and R12 also are flammable when violently escaping and mixed with mineral oil...so its very negligible. Once again, R290 is widely used in Europe in factory equipped A/C systems.

[OilBurner]

R134a also causes testicular cancer...FYI. My truck i believe was converted to 134a by the previous owner...or it at least has 134a fittings...and it seems to function well. The F250 probably has a larger condenser than the GM/TC and i dont know for sure whats in it as i havent had a need to find out yet...it could be any of the 3 discussed refrigerants or who knows what...i suppose i could ask the guy as i still have his number.

[gadget73]

The fire thing doesn't bother me that much, mostly because of the 18 gallon gas tank in the trunk. Theres what, 2 lbs of propane in the system?

[OilBurner]

it might be 3lbs...dont remember...i thought it was absurdly high compared to other systems...but that would also explain why it works so well and takes forever to leak out even with bad seals...i redid all my O rings so mine only needs topped off every other year or so...might still have a slight leak at the comp shaft seal and if theres any O rings inside under the dash..i replaced everything under the hood.

another benefit is that propane is totally safe for the environment...you can vent it all out and it does virtually no harm at all compared to even 134a.

[lincolnlarry]

I would need to do some research but if you can find a cycle switch that cuts off at a higher pressure. See what the running prressure is and you will need a sensor that cuts out slightly above that. Might solve the icing problem and increase effic. at the same time?

[DerekTheGreat]

I dunno about 134a sucking in a panther. I had my 85 LTD converted and damn, that thing would freeze your nuts solid, coldest A/C I've ever experienced. It would ice the evaporator up though and I'd have to switch to vent from max as using the temp slider only delayed ice up. Car eventually ate two compressors. Never found out why.