Answers to A/C Service Questions

Q: I can’t find dye anywhere even after injecting it in the system, but I don’t want to condemn the evaporator too quickly. What should I do?

A: This is a real tough one, and there is no perfect method. Most of us rely on the process of elimination (i.e.: “It’s not leaking where I can see it, so it must be leaking where I can’t see it!”). See Fig. 1.

However, there are a couple strategies you can try. Try removing the cabin air filter or blower motor in order to get a black light and visually inspect the evaporator. An A/C sniffer might also help. Another method is removing the core from the Schrader valve and putting CO2 gas into the A/C system with a smoke machine. Then, stick an emissions analyzer in the vicinity of the evaporator. If CO2 levels escalate, you nailed an evaporator leak. However, CO2 might not be detected if you need real-world system pressures to force a leak. It’s an imperfect method, but it’s another bullet in the gun belt so to say.


Q: What special equipment do I need to deal with sealant?

A: First, you do not want to get your A/C machine contaminated with sealant. It will cost you too much to have your machine down in the middle of A/C season. Every single vehicle before you service it needs to be checked for sealant.

Think about it: Your parts store sells the stuff. Do-It-Yourselfers use the stuff. Shops use the stuff! There are too many people out there adding sealant to vehicles to avoid expensive A/C repairs. Because of this, you need to take a few extra minutes before every A/C job to test for sealant. (See Fig. 2 .)

When you find sealant, you need to charge your customer to remove it from the vehicle. An A/C sealant filter can separate all the harmful sealant from the R134a, but this equipment needs to have its filters changed every 50 hours of use or so (See Fig. 3). Then, after removing the sealant, recharge the system with dye. Chances are that sealant was added to that system because there was a leak somewhere in the past.

As a side note, it should be said that there are new sealants out there that meet the SAE 2670 standard that are not supposed to harm the A/C system or any service equipment. These sealants can get pricey (about $15 an ounce) and may pose their own benefits and issues. If using such sealant, be sure to explain to your customer the benefits and also risks involved in using a sealant.


Q: I believe that the compressor is bad, but I cannot rule out a bad expansion valve. I can command the A/C compressor clutch on and off with my Power Probe. Is there anything I can do other than changing both?

A: Infrared thermal imaging technology shows some real promise in finding leaks and parasitic draws, because it can view differences of temperature and essentially “see through walls” so to say. With an understanding of A/C system pressures, accurately seeing temperature differences can really hone in our diagnosis. (See Figs. 4, 5 and 6.)

Let’s get this straight: The refrigerant entering the compressor should be cool, and it should be hot leaving the unit. Refrigerant then is cooled going through the condenser, but still hot. This temperature should remain about the same until it hits the receiver dryer. A restriction should lead to a large temperature difference. The expansion valve should have a large temperature difference in normal operation, because cool refrigerant should be leavening it. When using a pressure gauge a restriction in the receiver dryer or expansion valve should cause very high pressures on the high side, and low pressures on the low side. Sealant clogging up the condenser can also cause a restriction, with the same results as a clogged receiver dryer or expansion valve as reflected in system pressures.

Thermal imaging helps because it helps us see where the restriction is. If you have a normal operating receiver dryer, but the expansion valve shows little heat difference, pressures will tell you if the compressor is still pumping pressure into the system and all you need is the exansion valve.

You can use a temperature gun to employ this strategy on an A/C system, but you have to be careful with your aim. For example, the temperature of the cooling fins on a condenser are much different than the hot pipes beneath.

Thermal imaging or having very good aim with a temperature gun is more useful in finding partial blockages than complete blockages. A complete blockage can make it very hard to figure out whether the compressor is dead in the water or something else is rendering it useless, simply because if refrigerant flow is stopped, pressures are affected universally. That’s why no job should be under warranty unless the expansion valve and receiver dryer are replaced. A shop should not take liability over things that are not testable to a degree of 100 percent accuracy. There will always be a gray area with A/C, and a customer should understand that.