AC Overhaul (yes another one...)

Before anybody says it, yes I did search on this before making this topic.


My 89 F250 is still rocking an R12 system, but it is struggling to keep the cab cool and I can see black death buildup on the outside of the compressor. I don't see it surviving another summer so I've decided to get it fixed now.

After reading through the various topics on AC over the years here on OB, I've decided to just replace everything in the AC system with new components and convert it over to use R134a with a variable orifice tube. I'm not keen on using R12 alternatives and want to make it to where this is serviceable by a shop going forward in the future. From what I've gathered, there are two different ways of going about this:

Option 1: Get all replacement parts for the 1989 year, charge with R134a and call it a day.
Option 2: Use Evaporator, Condenser and line between Evap/Condensor from 1995 truck since it should be more efficient with R134a and the rest of the components needed for a 1989 truck.

Both options will receive a new low pressure switch calibrated for R134a. Option 1 is probably going to be the easiest/safest route to implement. I'd like to consider doing Option 2 if it'll be worth it, but am not sure if the 95 Evaporator will connect to the line that runs to the AC compressor or if the fitting is different. Since I'm using a V-Belt setup on my compressor, that limits me to using the older HVAC line configuration (I'm not ready to convert to serpentine at this time). If anybody can provide verification that the line will connect to the EVAP without issue, I'll go with Option #2.

Other miscellaneous questions I still have are:

1. Regardless of which option I go with, should I still drain all oil from the new compressor and add PAG oil (4oz to compressor and the remaining 4oz throughout the rest of the system)?
2. Do I need a high pressure cut-off switch? If so, where do I add it and how?
3. Is the rule of thumb to charge 80-85% of R134a to what an R12 system would normally have? What pressures should I be looking for for R134a in my case?

I'll be getting gauges and a vacuum pump, so I'm hoping I can do this on my own without having to pay a shop a crap ton in labor to do this. I'm open to suggestions and feedback. Thanks.

The new compressor comes with oil? I usually have to put the oil in myself. You have to pour it in, turn the compressor a little bit, and pour in some more till you get enough in there, sometimes you can't get it all in and just have to pour it in the suction like before you hook the lines up.

If you are going to do a complete new system, new coils and new lines, then you can use PAG oil. That is what most of the factories use when using 134a. If you are going to use the old evap coil in the firewall, if you can get it really clean with solvent and get all the old oil out, you can still use PAG.

If you are worried about having any old oil from the r12 in there, then use the ester oil that is commonly used for conversions. It's compatible with any old r12 oil laying around in there, which is why it is used for quick conversions when the system is not totally cleaned out.

I see no need for a high pressure switch. It already has a high pressure safety valve on it, believe me I tested mine accidentally when I put electric fans on my truck and forgot to interlock the fans with the compressor.

On the fill, I just fill mine till it is cool enough coming out of the vents. It does usually work out to being just slightly less than the factory fill for the r12. But you will find that the system will work ok if you are in the ballpark on the amount of refrigerant in the system.

I know some compressors say they already come with oil and not to add more while others say to add oil. Since the original compressor is showing signs of black death already, I'm not even going to try and flush/re-use old components and just buy everything new and be done with it. I've seen other people make reference to the high pressure cutoff switch in the past on forums which is why I asked here.

I put a '95 condenser and variable orifice tube in my '91 15+ years ago when I converted to R134 but I kept the evap core as that was more than I wanted to get into at the time. I did end up replacing the evap a few years later as it was clogged with dirt and leaves so in for a penny, in for a pound; I would recommend doing the evap core at the same time. Being that the later condenser core is larger, you have to modify or get the later hood latch support rod as it's shaped to go around the larger core. The fittings on the later condenser are different so I had napa make custom hoses for me. I can't remember if they just cut the fittings off and clamped new ones on or if I had them make all new lines but it wasn't expensive whatever I did and those hoses are still on the truck and not leaking. If you are using a '95 condenser and evap, the stock line will obviously work so that would eliminate the issue from that line but the one from the compressor to the condenser would still have to be custom. Make sure you clean the evap housing out when it's all apart and make sure your fan has good speed. I don't think there's a need for a high pressure switch as long as the over pressure valve on the high side of the compressor is working. When I fill these converted systems, I put enough in for the compressor to stay on while the engine is running about 1K (make sure a fan is running over the grill). Then I go drive it for 15-20 min to get some of the heat load out of the interior. Then put the gauges back on and, again with high idle and a fan over the condenser, I try to get the low side to refrigerant temp as close to freezing as possible while making sure high side pressures don't go over 350-375 (the converted systems can suffer high pressures at low vehicle speeds) depending on the ambient and how much air I can move across the condenser. The variable orifice adds a bit of complication to what you will see on the gauges but it sure made it comfortable inside.

Just did this with my 93. 94 was the first year for R134.

I had black death so I replaced everything (and I mean everything!). Old r12 system was running r134 and struggled to get vent temps under 60 degrees.

The new evaporator, condenser, liquid line and compressor are for a 1994 R134 system (however I think the compressor is the same for both years), everything else were 1993 parts. A note came with my compressor saying the oil in it was for shipping only and to drain it and fill with pag, also said to put half the oil in the compressor and half in the accumulator.

The evaporator was physically a different size (mostly a little shorter) so I had to modify the box/cover it goes in and create a new set of seals around it out of weather stripping to keep air from bypassing the core. I also started with a variable orifice tube (supposedly a high quality expensive one), it failed within 10 minutes, my low side pressure dropped to near zero and high side went sky high causing the compressor to constantly cycle off with very short on times. Didn't want to deal with that again so I went with the red office tube the late 90 R134 systems use.

After assembly I kept the system under vacuum for two hours. Book says you only need 40 minutes to an hour but I live in humid Houston, TX so I wanted to be sure that all the moisture was out.

My AC will now put out 40 degree air at idle, and 46ish running down the road. Much better then the old system that never worked well. I suspect that the charge could be tweaked to get a little colder but I have a nasty habit of over charging so I'm leery of messing with it any more.



PS: Using a 1994 condenser, evaporator and liquid line, along with a 1993 accumulator, I was able to use a 1993 manifold hose set and didn't have to have any custom hoses made. You will have to bend the 1994 evaporator outlet tube a little to fit the 1993 accumulator. There is a risk of causing a crack/leak, thankfully mine seemed to do OK.

Strangely enough the 1994 manifold hose set WOULD NOT fit my truck, even with a 1994 accumulator, the low pressure hose was nearly a foot to short, wouldn't even get all the way across the engine. Seems strange since the 1994 truck also had the 7.3 IDI with the compressor in the same spot.