R-134A...does anyone know how to make this conversion in a R-12 system work decent. 20* below ambent is what I have.

Asked by Jul 25, 2011 at 07:09 PM

Question type: Car Customization

converted my 1985 Chev C-10 to R-134 with the kit. System was evacuated to 38" vac before charging, cooling is terrible only 20* below ambent. R134A does not work well in vehicles built with it but anything over +90* and this refegerant really makes no difference unless you are on a long drive on the interstate.

9 Answers


First of all your gagesare suspect, it is impossible to pull any deeper than 29.9" of vacuum. You also didn't say how long you pulled the vacuum, just because it pulls down to any said number doesn't mean that you have pulled all of the contaminants out of the system. Did you let the system set with the vacuum pulled on it? Normally I let the system set with just the gages on it. If it won't hold the vacuum for at least a 1/2 hour, it won't hold refrigerant very long either. Also, the vacuum pump has to run for at least another 1/2 hour to get all the. remaining R12 and air and moisture out of the system. When all that is done successfully you can .charge the system. The system was designed for R12 so the condenser is a bit small for 134A bu it will work

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Thanks! sorry Type o. 28" of vac is what I had, not 38. Evacuator was on for half an hour, pulled down under 20" of vac in a few minutes.Turned off gage valves and left under vac for an hour (no loss). Pulled vac for another 15-20 min or so after that. compressor is ok (43000 miles on truck). I charged R 134A 2 months ago with no loss. I did replace all O rings and used retrofit oil. System seemed to cool better undercharged, but would drop to 25psi low side and cycle compressor alot, but would condinsate low side lines like this. With correct charge does not condinsate lines at all and 40psi at low side. I don't remember what high side reading was. It cooled better/ quicker with a partial R-12 charge. There is a different low psi switch for R-134, but compressor runs all the time so I did not think this would help. The LP switch it looks like I need for undercharge operation and fair cooling is off@25psi, on at 50psi, but is not offered.Thanks for your help/suggestions!

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Mike, you are correct in saying you will have to undercharge the system by R12 standards.There is no way you want the compressor clutch releasing at 25# with 134A, and 40# of suction pressure is high for 134A. Have you put your gages on the system and brought the RPM up to highway speed?? At highway RPM the pressure should be 25 to 30 PSI, absolutely no higher. You will either have to get the LP switch for 134A or jumper out the R12 switch, (I don't recommend jumpering) Forget what the original sticker says about how much refrigerant should be in the system, charge it by your suction gage. 25# at highway RPM.... no higher than 45# at idle or low speed.

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Thanks! I have checked with gages with engine RPM at 2500. Spec is 3.25lbs and I have 3lbs in system now. undercharged (2 lbs in system) at 80* ambient(psi goes up w/ambient temp) low side was 25 and would cycle compressor out for 15 seconds when psi reached 40 compressor in for 30 sec or so, then out. bypassed switch like you suggested and would freeze lines at 95* about an hour on interstate. spec now is 30psi to 40 low side rpm does not effect it as much as ambient, Thanks again My 66 buick with R-12 turns the black interior into a meat locker in 10 min on a 95* day. I appreciate the help on getting the '85 to work better!

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Mike, If the suction tube is actually icing at highway speed you might want to consider adding a little more 134A. You'll have to experiment with it a little, but if the tube is icing so is the evaporator coil. I also reccommend running the system on outside air not recirculation when your on the interstate, this will possibly stop the icing problem. Good luck with it and keep me posted, happy to help.

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thanks again. When it iced on interstate I had lp switch bypassed with a jumper, max a/c (recirc cab air) low charge (2lbs), 95* amient and it did freeze evaporator restricting airflow thru evaporator. I see what you mean about pulling outside air as this will help keep evaporator temp higher. I don't know the psi when I had this iced condition but undercharged I jumpered to get pressures and it was 16 to 21 psi low side dependent on ambient temp and engine RPM, then I hit interstate to get iced condtion in about an hour. I charged system to 3lbs at which point the expansion valve temp and evaporator outlet temp were close to the same, (How I was taught to tell a system is fully charged R-12 no sight glass) and it reduced cooling, but compressor does not cycle in and out. I checked on LP switches and fact R-12 currectly in truck was off 25PSI on @40. There is a retrofit switch that was off @ 6psi, on @ 50psi, the other 3 switches I checked were not close with off specs at 30psi, on at 50 or 60. This system uses the LP switch to control evaporator temp. I don't think the off 6psi switch would fix as the lowest pressure i was getting was in the teens. Thanks again for your expertise and suggestions!

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If you are still getting suction pressures in the teens at highway speed you must still be slightly undercharged, unless that pressure occured when the evap was freezing. Good luck with it , I'm surprised that the factory was using the LP switch to control the evap. temp. Actually that sounds like kind of a fun project. I'd enjoy hearing the ongoing saga and the final results.

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Think I got it! Engine at 2500 RPM 30psi LP @82* ambeint seems to be the best for R-134A. It is very Temp sensitve to pressures and was dropping to 25psi at idle at 72*, down from 42psi@82*. I bypassed LP switch and installed thermal limit on this circut (from 70's GM application) to prevent evaporator freez-up. Takes a while to cool down but I think that's a R134a trait also. brought engine idle a/c solinoid up to 850RPM up from 600 and now seems to stay cool while in town. Not an artic blast but it works! Think my R-12 charged cars have spoiled me:) Thanks again for your advice and goodwill!

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I'm glad if I helped you Mike, I'm just outside of Cincinnati, Ohio, I know about high temps and high relative humidity. I don't know where you are located, but it sounds like very similar weather. I was in the HVAC business for longer than I want to talk about, but I'm more than happy to share what I know about it. Enjoy the vehicle and use it as it should be used. Zoneman (John)

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