Comments on: Car AC Diagnostics

By: Kirk Fletcher

Kirk Fletcher — Mon, 17 Oct 2011 17:49:11 +0000

Hi Mark, Just read this light bit of info that you have posted here on A/C Diagnostics. I just wanted to share some of the things that we are currently teaching in AC Clinics the last couple of years. Myself, I am trying to get more guys looking at temps rather than mainly looking at pressures. Too many of the OEM’s are putting the high and low side fittings in the wrong area of the system to correctly diagnosis by pressures. Many of the high side fittings are located in the liquid line so the high side readings can be much lower than what the compressor is really creating.

We are now on our fifth generation of AC Condensers for R134a systems and the flow passages are now as small as .015 of an inch. When you have a condenser inlet side of just less than 8 1/2 square inches of combined flow path (28 tubes x 20 flow paths per tube x .015 per flow path) the inlet manifold tube of the condenser becomes the primary filter for the A/C system. These small passages become partially blocked and become tiny OT’s. We get substantial pressure drops at these partial blockages and we get corresponding temperature drops. We are teaching tech’s to look at inlet and outlet temperatures of the condenser. We only want to see delta’s of 25 to 35 degrees.

Too little temperature drop and we have an air flow problem through the heat exchanger. Too much of a temperature drop and we have massive amounts of partially obstructed refrigerant flow paths. This industry continues to try to back-flush these new style condensers with Sure-Shot cans and Flush-Guns with zero results. This is one of the major reasons replacement Compressors have very short life spans. The Compressor was not Defective; the compressor was Damaged because of incorrect service procedures. I know there is a need to educate the DIY on proper A/C System service and repair but most DIY’s don’t have the proper equipment to be successful even if they have the proper knowledge.

It is hard enough to get the professional repair shop to upgrade their A/C Service equipment to the current generation and to purchase the dedicated flushing machines that are really needed to flush and clean these new systems. The goal isn’t to go through the motions; the goal is to get the system perfectly clean. How can we expect the DIY to get these systems perfectly clean when only the very best professional repair shops currently do? To me, selling an A/C Compressor to a DIY at the parts counter is like handing a loaded handgun to a five year old and expecting positive results.

By: Mark

Mark — Tue, 12 Jul 2011 15:44:34 +0000

Bo: While reading through your AC symptoms you described I was thinking this sounds like a classic expansion valve problem. Then you stated you replaced it. 2 things come to mind. 1) Is the new expansion valve working properly. 2) Yes the dryer should be replaced if exposed to humid air (in my opinion). Although I do not know if it will solve all your problems. The drier removes moisture and partially frozen moisture in the lines can act like blockages that can cause the pressure problems you indicated.

By: Bo

Bo — Mon, 11 Jul 2011 19:28:10 +0000

I have a 1992 Acura Legend that had a leaking evaporator core. It has already been retrofitted to R134A. I replaced the drier, expansion valve and the evaporator. I vacuumed it down for 20 minutes and put 1.5lbs of R134A in the system. My pressures are to low. Low side is 15psi and high side is 150psi at idle. And its 95 degrees out. It cools fine until thirty minutes of highway driving. During that 30 minutes it cools at 54 degrees but afterwards it hits the upper 70′s through the vents. I notice a tremendous amount of water pouring like a spicket out of drain tube so I pulled the blower resistor out to look at the evaporator and the expansion valve is an ice block. Thinking I might have a defective expansion valve, I pulled the dash back out and this time got a new valve from the dealer. Put it all back together and no change. Same results. What else could this be? Im now wondering if my drier is bad since the lines were open for a weeks’ time. Any help would be greatly appreciated. Thanks

By: Jason

Jason — Tue, 05 Jul 2011 22:05:42 +0000

I have an 03 honda crv. Compressor went out. So vacuumed flushed whole 9 yards put new compressor in, pressure building to high like 400 psi at 2k. So went ahead new expansion valve, new condensor, charged system and still doing the same thing. Any suggestions?

By: randy

randy — Wed, 27 Apr 2011 22:07:01 +0000

Mark, Thanks for your diagnostics tip,very helpfull.

By: syed

syed — Wed, 29 Sep 2010 01:57:49 +0000

a/c condenser was clogged, replaced it and problem solved. thanks

By: syed

syed — Fri, 24 Sep 2010 17:04:15 +0000

Mark : changed both the expansion valves..still the same issue. I got the van from an auction with the suction line broken in half.(probably due to excess pressure. Replaced the line and since then freon has been coming out the relief valve on compressor……come to think of it the compressor does not sound as smooth as it should.

I also noticed the condensor fan comes on when the low side reaches 45psi….as soon as it comes down this pressure starts to come down to about 25psi……fan turns off…..psi start to rise to 45 psi and the fan kicks back in.
Should this fan be on regardless? Already changed fan relay but still functions the same way.

By: Mark

Mark — Tue, 14 Sep 2010 18:11:47 +0000

Syed: A few things can cause this type of condition so in the end a diagnostic test should be performed. Also note that either an orifice tube or an expansion valve is used (not both). I’m not sure which one is on a 2000 Caravan. The first thing I would do is confirm the high side problem and get a pressure reading. The high side relief valve should not operate until it hits about 350psi. It could be a bad compressor relief valve BUT your right the orifice or expansion valve is more common. But remember it could be several other things. A damaged compressor is another possibility.

By: Syed

Syed — Tue, 14 Sep 2010 04:15:58 +0000

I have a 2000 Dodge Caravan. I am having a lot of headache trying to diagnose why my AC system won’t hold freon. Once I put the freon in the van it runs fine for 10 minutes. And then all of a sudden the freon starts spewing off from the compressor relief valve. I have vacuumed the ac system and checked the fans to make sure they are coming on as supposed to. I am not sure why the ac system is building up so much pressure that it has to come out through the relief valve on the compressor. I am suspecting the orifice tube is plugged or a faulty expansion valve but wanted your expert opinion before I proceed. Thanks