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Hagerty Employee

Finding leaks in an A/C system by pressure-testing

In my book Just Needs a Recharge: The Hack Mechanic™ Guide to Vintage Air Conditioning, I make the case that those “A/C recharge $79.95” signs you see at service stations are perilously close to fraud. If a system needs to be recharged, it’s because the refrigerant has leaked out, so the leak first needs to be found, then needs to be fixed. The time for all that, plus the recharge itself, is always going to make the bill more than $79.95.


The best-case situation of the leak being caused by a bad o-ring—quickly found and easily replaced—is about as likely as a loose gas cap being the cause of your check engine light. Occasionally, however, you get lucky.


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New Driver

You could also use an ultrasonic leak detector to find the tiniest leaks.  It looks for the ultrasonic noise from any gas under pressure or vacuum escaping from a small hole.

It also has many other uses in the garage. Air compressor leaks, tire leaks, vacuum leaks, fuel injector pulsing, worn bearings.  If you have the accessory ultrasonic transmitter, then you can check window and trunk seals.

Intermediate Driver

Big thumbs up to the a/c article.I just did the a/c on my 92 Jaguar XJS. Could never find the leaks. Sprayed it down with soap solution then pressurized the system with argon/co2. Whoa!! The hoses and compressor looked like Lawrence Welks' bubble machine! Problems found. Full rebuild on the compressor along with new high and low pressure hoses and a new receiver dryer plus a proper vacuum down and freon install has turned the interior of the car into a meat locker. I had to get my 609 a/c tech certificate ($25.00 course on line) because I was dealing with R-12 and the necessary tools and equipment cost a few bucks. But if you do a couple of friends cars plus your own you can easily recoup the cost and save yourself and your pals a good piece of change. It is really pretty neat when you solve a problem in a complex "Black Arts" type of system. Heres to hot days and working a/c.

Intermediate Driver

Great tool for the professional A/C tech but I’m a bit scared when I think about Bubba hooking up his cars A/C to his oxy/acetalyne outfit.

Adds a whole new dimension to A/C suicide kit.

Last thing he said was “hey ya’ll what this!”

Intermediate Driver

I meant watch this!


I am looking for a 2002tii to buy. Any sites or dealers I can  go to?

Advanced Driver

Great article! AC can be a complete mystery to most people. Compressed gas is an excellent method to find leaks. Once upon a time when I worked in the HVAC business, that was the go to method to find leaks in a system or to check newly brazed joints for leaks.


One thing I would add is that if you have a low system, never ever, ever, ever, ever use stop leak to "fix" the problem. When the vehicle eventually has to go to a mechanic to be properly fixed, they will not be happy with you and will gladly charge you extra to flush the junk out of the system to get it working properly. Save yourself the money in the long run by just fixing it properly the first time.

Advanced Driver

Excellent point about the stop leak.  Stay away from ALL of the "do it yourself" kits, too. Avoid the cans of refrigerant that claim to "fix leaks, add oil/ dye, and blow 34% colder". And, I just found out last week, Auto Zone R134a cans are NOT the same diameter as a normal can. I like to use can side-taps... and the AZ can was the same diameter as an R12 can, so most of the R134a went spraying out everywhere. If you dont know what you are doing, find help from someone who does!  Those DYI kits arent called suicide kits for nothing!


I just have to ask anyone who's done any of this work: how often is the leak inside the dashboard?

Advanced Driver

I haven't done much in the way of automotive AC work, but I have spent a lot of time in the automotive parts sales business. I would say that more often than not a leak is from an O-ring or at some sort of connection. In my experience and observation, the evaporator is not indestructable but it is fairly uncommon to have go bad.