Coverting R12 to R134, who's happy?

[bp9129]

Well I found the resistor and tested it and found it was burned up. So a new resistor and my aux fan's working and the A/C blows colder. I'm still up the air about converting it, as I already bought a drier and adapter fittings.

Where's the resistor? Mine only blows on the highest level....

[91 318is]

Where's the resistor? Mine only blows on the highest level....

No the resistor I replaced was for the AUX fan for the condensor. I also found out my blowe resistor is on the way out because everything other then 4 blows VERY slow. That resistor is under the hood, under the cover on the firewall.

[Stanley Rockefeller]

I had an 86 325es that only required a new dryer and a flush of the old R12. After that I had them charge it with R134a and the system was colder than anything I was expecting. Worth the $ and time to jsut get it done right the first time

['89e30]

how many can of freeze 12 do you need to fill in the ac system?

[Hey_You]

If I'm not mistaken, an e30 system holds about 2 lbs of refrigerant. There should be a sticker under your hood that tells you the capacity.
Note: If you have more than 2% air in your a/c system, your system will not perform to it's potential. If your refrigerant contains ANY stop-leak shit, shops WILL NOT service your a/c as it can/will damage/destroy their machines. If you don't have a vacuum pump, don't complain that your a/c doesn't work correctly. It is NOT the same amount of charge of R12 as it takes for 134a, use an online calculator for the conversion. When you add refrigerant after your system has been on vacuum, ADD PAG OIL (or ester oil if keeping the r12) or you can burn up your compressor!

[redsubdivisions]

I did a conversion last summer and I replaced everything (compressor, drier, expansion valve, o-rings, condenser, and flushed out all the lines). I am running R-134 with aux fan set on high every time the A/C is on. I am pretty content with it, however if you have access to R-12 and all your components are suited for it, I say stick with that. The only reason why I converted everything was because the previous owner did a halfass job on trying to retro fit it.

To this day, my E30's A/C system does a great job at keeping me cool in the cabin.

[Eddie Haskell]

The correct R134a amount is .9 by weight of R12. Here is a synopsis in which a A/C tech describes and teaches a short course in auto A/C mx from e30tech. BTW you can measure refrigerant by weight with a cheapy postal scale and 12 oz cans if inclined. It's a little touchy but will get you within 1 oz of desired total weight.


Quote:
If the E30 A/C system was working perfectly and using the stock condensor, what should the R134a high and low pressures be? If everything was working normally, is there even a need for a new style pressure switch that allows for higher head pressure?

I also want to know what the normal operating pressures should be so i know what to expect when filling the system with R134a.

thanks
Never charge a system by just looking at high side pressures. There is a rule of thumb that says the high side psig should be something like 2.2 to 2.5 times the ambient temperature in 'F but this is for troubleshooting a previously working system... not for charging. If it's 100'F outside then with good air flow over the condenser the ROT says you should have around 220 to 250 psig head pressure. You can still have these pressures even if the system is undercharged somewhat. That's why you never charge a system based on pressures.

There's isn't a clear answer of what the pressures should be. The high side (condenser) pressure is not regulated and will be all over the place depending on ambient air temperature, if the condenser fins are dirty, air flow through the condenser and how much heat load there is on the evaporator. One thing that is typically constant (for a given air flow across the condenser and heat load) is the condenser's temperature split. This is the temperature difference between the outside air temperature and the condenser's saturated liquid temperature. The condenser's sat liquid T is the temperature read from a P-vs-T R134a table or it can be read directly off the manifold gauge set on the same dial you read the pressure off. If you know the high side pressure then you can directly find the condenser temperature from that.

On both of my cars that have R134a systems they have a typical condenser split of 40'F. That means if it's 80'F outside then condenser is at 80+40=120'F saturation temperature. On the chart (or gauges) that corresponds to 172psig. Now, if the evaporator soaks up more heat the condenser has to dump more heat in the same amount of time. To speed up the rate of condenser heat dissipation the condenser's temperature has to go up compared to outside... so the split will likely go to say 50'F -> 80+50=130'F which is 200psig. Anything that makes the condenser less efficient will require the temperature split to get greater. If you have less air flow or say dirty condenser fins then condenser heat removal slows down... so the split has to increase to compensate. Anything that gives the condenser less surface area for internal refrigerant condensation to occur over makes the condenser "look smaller" and therefore less efficient and therefore the temperature split again has to increase. If say you overcharge the system and then there's a lot of liquid refrigerant taking up half of the condenser tubing... that only leaves half of the condenser area for condensation to occur. Efficiency is drastically reduced and therefore condenser split has to increase in order to dump the heat fast enough.

Now consider the impact of outside temperature alone. Say you got the system on recirc so you're only cooling down cabin air that's already cool. If outside air T goes up there's no additional heat load on the evap because the cabin air is already cool. Heat load on the evaporator is constant. The rate at which the condenser has to dissipate heat to its surroundings is constant. That means the condenser split is fixed at something like 40'F. Now, if outside air T rises the condenser temperature will have to rise to keep dissipating heat at the same rate... but the split stays the same. So if outside air T was 80'F and condenser temperature was 120'F but outside air T goes 80'F -> 100'F then the condenser temperature goes from 120'F to 140'F. That means the head pressure will go from the previous 172psig to 231psig.

Always charge the system by weight of refrigerant. You want around the same volume of R134a in the system as the R12 volume that you had. You want there to be a certain amount of liquid level backed up from the expansion valve to fill the last couple tube rows in the condenser.. no more because that will take up too much condenser area and drop its efficiency.. no less because there will be no reserve of refrigerant in times when there's big heat load and the system has to circulate more refrigerant. The sure way to add the correct amount is by weight, because it doesn't change if it's hot or cold outside. The volume changes with outside temperature due to refrigerant expansions. The density of R134a is less than R12 so for a given weight of refrigerant the R134a will take up more volume. It just so happens that the density of R12 at 25'C is 1310kg/m^3 and that of R134a is 1206kg/m^3 which means that, taking the ratio of both 1206/1310=0.92 tells you that if you want to charge the system with the same volume of R134a as the previous R12 then you need to charge the system to around 0.9 times the original specified weight of R12.

There is another way you can verify that you have charged the system to the correct amount is by measuring the condenser's subcooling. This is the liquid line temperature coming out of the condenser compared to the condenser's saturation temperature. 10'F to 20'F is a typical amount of subcooling that tells you the refrigerant has fully dissipated its heat and has pooled in the bottom of the condenser and waiting to be used. If the condenser pressure was 172psig then you know the condensing saturation temperature is 120'F and that means a fully charged system's liquid line should have a temperature somewhere around 110'F to 100'F. Do a Google search if you want to know more about this. There's two methods to charge a refrigeration system. One is by subcooling (just mentioned) and another is for systems that use an orifice or capiliary tube expansion device which is charging by superheat measurement.

As for the pressure switches... The compressor pumps volume, not mass. The cooling capacity is determined by the amount of liquid mass available in the evaporator. Under high evaporator heat load the mass flow through the evaporator must increase to soak up & carry away the heat. Because the R134a is less dense than R12 that means the compressor has to pump more volume to move the same mass flow. That means the compressor has to do more work to push the required amount of R134a through the system. All the work done by the compressor adds an equivalent hp amount of heat to the refrigerant that must also be dissipated by the condenser. Therefore, a harder working compressor means more heat load on the condenser and therefore more condenser temperature split, resulting in higher high-side pressures. With higher high-side pressure that means the compressor has to push the refrigerant into a higher pressure which makes the compressor work harder again... creating more heat... that the condenser must dissipate. Therefore, that causes condenser split to go up some more. Therefore, a system with an R12 condenser will run higher pressures if used with R134a. If the existing high-pressure kill switch has a compressor kick-out pressure setting of 385psig, convert that to T, that means the condenser can operate up to around 177'F. Say your condenser inefficiency is causing you to run with a split of 70'F one day when you first start your car in the parking lot, trying to cool her down. That means if the outside air temperature is more hotter than 107'F then likely that pressure switch will kick off the compressor. If you change to a 435psi switch, convert that P to T, that's 187'F. That means with the same split it can be 117'F outside before the compressor will kick off.

personally I don't think I want my head pressures to get up to 435psi.. that's a lot of strain on the compressor and you know what can happen to hoses if they get too much pressure!!

A parallel flow condenser is more efficient and will appear as bigger unit. As a result, the temperature split will be lower, hopefully in the 40'F range or even lower, and therefore the condenser temperature (and therefore head pressure) won't have to be all that high for the system to operate. It will also reduce the work load required by the compressor and hopefully let it live a longer life. To add to my scenario, where I live in San Diego the temp rarely gets above 90'F so I believe with the more efficient condenser I may get away with using the lower pressure switch. In your case, if your temp's get to 110'F and you plan on using the original condenser then I would suggest moving up to the 435psi switch.