NOTE: The following procedure may not be official, prototypical, or otherwise "correct", but it works for Bill. This is definitely NOT the normally recommended process.
Two things to keep in mind:
1) Merely changing freon will not correct something else wrong in system. If compressor was weak pumping R12, it will be just as weak pumping R134.
2) These are 22 year old systems. Unless you start replacing components (including hoses!), is only realistic to expect some degradation of integrity. You may be lucky and have a tight system (one of my Lincolns hasn't lost an ounce of freon since I converted). Or you may have a system that leaks like a sieve...
Issue is price and convenience. R134 costs $4.50 a can (a complete charge is less than $20). Even if a genie gave you all the components for free, on a DeLorean accessing anything from the compressor forward is a colossal pain. Bottom line: is it worth more than $20, or the labor, to return A/C system to factory condition (or to do a "proper" R134 conversion)?
That said: this is how I convert R12 systems to R134:
- R134 quick connect adapter fitting (low side - blue). People say these are permanent, but I have no trouble using temporarily to attach my gauges on R12 systems. You of course want to leave on a converted system.
- At least one R134 charging hose (I have several to keep from swapping devices. They're only $5)
- Adapter to connect R134 charging hose to a vacuum barb (made mine from two brass fittings: M12 male to NPT female, NPT male to 1/4" barb). Need vacuum hose too (note most nipples on PRV manifold are 3/16" -- can lubricate 3/16" hose to fit above adapter, or use 1/4" vapor canister purge line)
- R134 can tap (different thread than R12 cans)
- Pressure gauge. I have a cheap one and an expensive one -- guess which
gets used most...
- 1/2 quart flushing fluid
- Three or four 11 oz cans R134 ONLY. Stay away from cans with oil -- too much oil in system will puddle in bottom of evaporator and prevent freon from evaporating. Most of your oil never leaves the compressor anyways.
1) Evacuate old R12. Supposedly anyone with a recovery machine will do this free of charge. I've found A/C techs to be leery of unknown cars (don't want to contaminate their tank with whatever's in your system. Was a time before R134 became so common that people experimented with LP. Have also heard of people trying to run R22.) However you do it, any remaining R12 must be bled THROUGH SCHRADER VALVE (don't go disconnecting hoses while system's under pressure!).
2) Flush system. Remove high (output) and low (input) hoses from compressor. Pour 1/2 quart flushing fluid into high side hose and reconnect. Leave low side hose disconnected from compressor so that compressor can draw outside air. Start engine and jump the compressor to force clutch engagement. (To force compressor clutch on, you run a 12v line to the compressor itself. Run a wire from the jump start terminal to compressor quick connect.) You need to do this because low pressure switch open. Compressor will pump fluid through entire system and out low side hose. When all fluid, crap, and old freon is out (looks like bubbling green acid), reconnect low side hose. You don't want to flush through the compressor because it will remove lubricating oil.
3) Attach R134 quick connect to low side. Everything from this point forward can be accomplished through low side (only need high side to diagnose abnormal low side readings).
4) Vacuum system. THIS IS SINGLE MOST IMPORTANT STEP. Moisture trapped in system (remember was flushed with outside air) WILL freeze. Freon evaporates at 22 degrees, well below freezing point of water. Will totally stop up low side. Can actually see moving rings of ice on hose as clogs form and melt. This process uses engine vacuum rather than a separate vacuum pump. Find suitable vacuum nipple on engine (preferred) or use vapor canister purge line as a vacuum source. (Good source is the vacuum connection on the bottom side of the left "ram's horn". Hard to see but easy to reach.) Start engine. Do not jump compressor. Attach R134 charging hose with barb adapter to vacuum source, then to low side quick connect. Let engine vacuum suck on system for 10 or 15 minutes. Remember to disconnect from low side Schrader valve BEFORE turning off engine.
5) Charge system. Start engine and jump compressor to force clutch on. Add cans of R134 through low side. Cans empty much faster if you turn them upside down. If you want to charge with cans right side up, may need to turn radiator fans on to get last can in.
6) Assuming all other components in system OK, should now have cold air.
"Normal" pressure readings on low side:
System at rest: ~100 lbs
Refrigerating: ~30-40 lbs (remember cut off switch opens at 28 lbs, so you may want to aim high)
Thoughts on R134:
Is every bit as cold as R12. Have actually had fins on an evaporator ice over like an old freezer (you know when that happens because air flow stops from vents even though fan spinning -- defrost with outside air).
On cars with engine mounted fans, is sometimes not enough air moving across condenser from fan alone to get good refrigeration cycle going (car needs to be moving). Can lose cycle when stuck in rush hour traffic. This isn't problem on cars with electric fans.
Every person I've helped with unhappy conversion did one of two things wrong:
1) Did not vacuum system before refilling
2) Used the big cans with oil (each one of those has 2 oz of oil. Add four to system and you've got 8 extra oz in there. Compressor itself already has that much -- system is now at twice capacity. If your compressor needs oil, take it off and pour it in the back the old fashioned way).
Above procedure may not be official, prototypical, or otherwise "correct", but it works for me (and several people I've helped). Come summer, my windows are closed. And I haven't spent hundreds of dollars at an A/C shop. Even if your system leaks like a sieve (I've got one of those too), you can vacuum and refill every summer cheaper and easier than replacing components. Think of it as reverse winterization.
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