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Long story short…. Rebuilt one of the P38s from ground up. Replaced many, many parts and she works as good as new…. Except…
This was a tyre up restoration. Everything was removed. When it got the the AC system I put as new compressor, condensor, drier, expansion valve and evaporator in. Made sure all was secure. Went about the rest of the work.
Today, I decided to tackle the AC and vacuum test it. Low side will not vacuum to less than -1 and wont even hold that. High side will vacuum down a bit. I’ve obviously got a large leak on low pressure side.
Knowing where the evaporator and the expansion valve is located I’m dreading the replies I am sure to get here BUT…
If I am unable to get any vacuum at all on the vacuum test will my only solution to finding the leak be my removing and rebuilding the entire system again?
OR would it be worth my while to jury rig the pressure switch, forcing the compressor to accept R134 with dye added, so that I can try and source the leak with a black light instead of ripping the whole thing apart? My understanding of the system is that the compressor will not engage unless the system indicates pressure so byp[assing the pressure switch will allow the compressor to run and thereby pulling the R134 plus dry into the lines etc….
Ideally I should have pressure tested the system before assembling the interior around it….

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I wd go down the dye route. If you put a 9v battery across the compressor terninals the clutch shd pull in, you dont need to worry about the pressure switch.12v also works, but not necessary.

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I install domestic AC systems these days (I know, I retired over 2 years ago but sort of fell into it) and a mate does mobile car AC. When we both did our respective training courses we were told that before vaccing a system, you pressure test it with Oxygen free Nitrogen. On an automotive system it is tested to 10 bar (43 bar on a domestic system) and left for at least 15 minutes during which time you can squirt your leak detecting fluid (or washing up liquid and water) over the system to look for any leaks. As long as you vac it out immediately you've done the test, you won't do any harm using compressed air as long as it is dry. Much easier to find something squirting out than sucking in......

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Romanrob, will the compressor pull in the Freon/dye regardless of there being no negative vacuum in the system? I assumed that a negative pressure was required to “suck” the Freon in?

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It'll draw some in but probably not enough to do anything useful.

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Hmm, it's definitely time to get the Duchess re-gassed. She was last done in May18 according to my notes. Can it really be that long? I guess 4 years isn't too bad.

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JMCLuimni wrote:

Romanrob, will the compressor pull in the Freon/dye regardless of there being no negative vacuum in the system? I assumed that a negative pressure was required to “suck” the Freon in?

Nope, wd need to be vac'd filled, I usually end up at kwik fit/ats at that point. You can also pull off the Ali manifold and pour the dye directly into the compressor, if you then have a way to hook the system up to an air compressor at home ( I defer to Richard on that bit)

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So this is a Catch 22 situation I’m in then….
I can’t add the dye because the system won’t hold vacuum. It will hold maybe -1 while I have the vacuum pump running but as soon as I turn it off it goes to zero…
Any ideas as to how am I going to get the dye into the system with no vacuum being able to be held?
I can’t imagine that stripping the entire system apart is my only option to finding a leak…. Surely

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Paradox intervention … blast the dye with compressed air into the system. Will bring it everywhere and will show the leak, as with the coolant under pressure?

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Well KCR that partly depends on just where the leak is of course, but with such a big leak this could well just 're-decorate' the inside of the car/engine compartment - with bright fluorescent dye spraying everywhere ... ?

Better to use compressed air and spray soapy water on the system as we do with EAS leak-detection ?

Back with the vac. method how about a smoke A/C leak detection approach...

Finally yes forcing the compressor/clutch to run in order to add R134/Dye can work but again with a big leak might consume very large amounts of R134 ?

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As I said previously, no need for dye at all, just pressure with a gas, preferably Nitrogen but as long as you are going to be vaccing it all out immediately afterwards, even compressed air. That will leak out from wherever the leak is and you should be able to see it with soapy water.

The correct process when re-gassing a system is to recover any remaining refrigerant (which you won't have), pressurise with Nitrogen and leave for at least 15 minutes to check for any slow leaks (which you find by squirting the leak detecting spray or soapy water on all potential leak points), let the Nitrogen out then vac down to around 500 microns (760,000 microns or 760 Torr is atmospheric pressure at sea level), leave that for 15 minutes again to ensure it maintains it, before putting the R134a in.

You can skip step 1 as there's nothing in it, so go straight to step 2 rather than jumping to step 3.