The HVAC system on amodern car is a techno-logically diverse andfascinating operation. Itcombines the principlesof refrigeration, elec-

tricity, vacuum controls, computerprogramming and the old-fashionednuts & bolts of a whirling compressor.In the space of this article, we can’texamine the fundamentals of all thesesubsystems, so we’ll concentrate onheating and refrigeration.

Heating and refrigeration are two

sides of the same coin, which is heattransfer. In our rush to troubleshoottoday’s sophisticated climate controlsystems, we often jump past basicheat transfer principles to the high-tech realm of electronic controllersand computer programs. Stop: Let’sreview the basics.

Heat always moves from a hotterobject to a colder one. That’s somekind of law of thermodynamics, butyou don’t have to be a scientist to un-derstand the applications of the idea.The primary cause of common HVAC

complaints is that the heat transferprocess has broken down somewherealong the line. Most often, trou-bleshooting the HVAC system is sim-ply a matter of knowing what a partic-ular symptom means in terms of heattransfer, then recognizing the causeof that symptom.

The Refrigeration CycleThe a/c system cools the inside of thecar by transferring heat from warm airto the cooler tubes and fins of theevaporator and to the refrigerant in-

BY KEN LAYNE

HVAC SYSTEMSHVAC SYSTEMS

37April 1999

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side the evaporator. It’s this refrigerantthat makes the operation interesting.

Whether it’s R-12 or R-134a, refrig-erant is a unique compound thatchanges its state from a liquid to a gas as

it’s pressurized and depressurized. Thisrefrigerant circulates in a continuous cy-cle inside a closed system. Before it en-ters the evaporator for the cooling oper-ation, it’s a warm high-pressure liquid.The refrigerant flows through an expan-sion valve or a metering orifice, whichquickly depressurizes the liquid, causingit to vaporize. Another of those thermo-dynamic laws says that when a liquidquickly vaporizes to a gas, its tempera-ture drops just as fast, causing it to ab-sorb heat from anything around it.

The heat is now in the vapor, and youhave to get rid of it. The way you dothat is to compress the vapor; make itsomewhat hotter and a lot denser. To dothis, the low-pressure, low-temperature

gas gets drawn from the evaporator bythe compressor, where it’s repressurizedinto a high-pressure, high-temperaturegas. The refrigerant keeps moving andflows to the condenser, where the heat

transfer processgets reversed. Thecondenser is an-other radiator, andas the refrigerantflows through it,heat is transferredfrom the refriger-ant to the tubesand fins of thecondenser and tothe cooler airflowover the front ofthe car. Anotherthermodynamiclaw says that as ahigh-pressure,high-temperaturegas gets cooler, itcondenses to aliquid.

Now the re-frigerant is acooler (but stillwarm) high-pres-sure liquid, andit flows back tothe evaporator tostart the coolingcycle all overagain. It doesn’tmatter whereyou pick up the

cycle as a starting point, as long asyou recognize the heat transferprocess and the relationships of tem-perature, pressure and heat.

Read the Pressure,Know the TemperatureGood old R-12 has a characteristicthat really helps system maintenanceand troubleshooting—it correlatespretty closely the relationship be-tween the pressures and tempera-tures on the high and low sides of thesystem. With a pressure gauge con-nected to the low side, normal pres-sures should read 20 to 30 psi for anexpansion-valve system or 15 to 40psi for an orifice-tube system. Those

pressures correlate numerically 1:1with the refrigerant temperatures in-side the evaporator.

Of course, temperature rises fromthe refrigerant to the evaporator finsand tubes and again to the air blow-ing through the evaporator. Never-theless, R-12 at 30 psi and 30°F inthe evaporator will give you outlettemperatures of 45° to 50°F—notbad on a summer day.

For the low side of the system, theold saying was: If you know the pres-sure, you know the temperature. That1:1 correlation continues up to about80°F for R-12, which is just belowthe bottom of the pressure range forthe high side of the system. On thehigh side of an R-12 system, you canrelate pressure to the ambient airtemperature, rather than to the re-frigerant temperature.

With an R-12 system working nor-mally, high-side pressure should beroughly twice the ambient air temper-ature at the condenser. For example,if you put a thermometer in the air-flow in front of the condenser and itreads 90°F, high-side pressure shouldbe roughly 175 to 195 psi. At 100°F,pressure should be 200 to 230 psi.This relationship is approximate, butit’s still a useful diagnostic yardstick toindicate generally what the pressureshould be for a given air temperature.

This comparison is based on aproperly charged system and a fancreating normal airflow through thecondenser. If relative humidity ishigh, pressures will increase, and asthe air temperature rises above100°F, the pressures start to increasefaster and rise above the approximate2:1 ratio. Nevertheless, these temper-ature-pressure relationships will helpget your diagnosis off to a good start.

What About R-134a?Refrigerant is refrigerant—basical-ly—and R-134a has to do the samejob as R-12. R-134a vaporizes from aliquid to a gas at a slightly highertemperature than R-12, so its heat-transfer capabilities are not quite asgood. The vaporization (boil-off) tem-peratures are 221.7°F for R-12 and

38 April 1999

A mainstay of technicians for years, a good set of pressuregauges is indispensable for monitoring a/c system operation.

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215°F for R-134a. Therefore, R-134a systems need slightly largerevaporators, condensers and com-pressors for equal cooling. In termsof human comfort, however, R-12and R-134a systems are pretty close.

Low-side pressures are virtuallythe same for an R-134a system as foran R-12 system, and the relationshipsto evaporator temperature are veryclose. The high-side pressures for anR-134a system are just slightly higherthan those for an R-12 system. Thetable on page 40 lists the high-sidepressures at corresponding ambientair temperatures for both R-12 andR-134a systems.

Put the Gauges to WorkWhen you read pressures on the highand low sides of the system, remem-ber what the pressure tells you aboutthe temperature. Most importantly,think about heat transfer and whatshould be taking place at any point inthe system. Let’s start with an exam-ple of abnormally low pressures onboth sides of the system. Lower thannormal pressure on the low sidemeans that the refrigerant is vaporiz-ing in the evaporator all right, butthere’s not enough of it present topick up as much heat as it shouldfrom the evaporator airflow.

Remember, heat causes pressure torise, and less heat transfer results inlower pressures. Now the high side ofthe system has less heat to get rid of,so the high-side pressures also staylower. The conclusion at this stage ofpressure gauge troubleshooting isthat not enough refrigerant is gettinginto the evaporator. But why?

Obviously, the system could simplybe undercharged, so take a look atthe refrigerant lines, particularly onthe high side. You may get lucky andspot a leak. If not, a little dye addedto the system or an electronic leakdetector will help you along.

Suppose you recycle and rechargethe refrigerant to verify the proper ca-pacity or just “add a can,” but the pres-sures remain low. The next step is tolook for some other physical clues. Forinstance, an evaporator pipe that’s

warmer than normal to the touch ac-companied by low pressure on the lowside confirms that the evaporator is be-ing starved. It’s possible for a system tohave low pressures but no leakage.Even with a normal refrigerant charge,pressures can stay low if heat transferis not occurring properly. If you can’tfind a leak, look at the liquid lines fromthe condenser to the orifice tube orthe expansion valve on the high side. Ifyou find a section of line that’s cold to

the touch and perhaps has some con-densation or frost on it, you’ve found arestriction in refrigerant flow.

Dirt in the lines can restrict flowand cause a pressure drop where oneshouldn’t occur. And as the refriger-ant pressure drops, it picks up heatunder the hood. Then there’s less re-frigerant to vaporize in the evapora-tor, less heat transfer for a/c coolingand we’ve got a customer complainton our hands.

It’s not unusual to get confused byhigh-side pressure readings. Supposeyou connect your gauges, look at thepressures and recognize that they’rehigher or lower than spec. Nowwhat? To help keep pressure trou-

bleshooting on a logical path, again,think about heat transfer.

Let’s consider high-side pressuresthat are higher than normal. Remem-ber that the high side has to transferheat out of the refrigerant. As heatleaves, pressure should drop. If pres-sure doesn’t come down to where itshould be, not all the heat is leaving.

The condenser is the outbound heat-transfer machine. To do its job, it musthave airflow across its fins and tubes. If

airflow is restricted or reduced, heatwon’t transfer out of the refrigerant andhigh-side pressures stay high. Look fordirt and debris that might block con-denser airflow. Also look for vehiclebody damage, which can cause air toflow to the wrong places—that is, awayfrom the radiator and a/c condenser.

Besides a blocked condenser, air-flow can be reduced by cooling fansthat don’t work properly. Electric fancircuits are among the most challeng-ing troubleshooting exercises. Here,you get into the world of electricity tosolve a heat transfer problem, andyou should have the applicable fancircuit description and wiring dia-gram to do the job right.

39April 1999

Using optional pressure transducers in tandem with a good graphing multi-meter like this one turns traditional pressure readings into graphs that helpyou monitor both sides of the system over long periods of time.

Pressure Testing SummaryThe two previous examples of abnor-mal pressure readings aren’t the onlyinformation your gauge set can giveyou. We’ll wrap up this review with afew summary descriptions of abnor-mal pressure readings and their mostlikely heat transfer causes.

Low Pressures:•Both high and low sides slightly

below normal. Low refrigerantcharge; possible leakage. If the sys-tem has a sight glass, you’ll probablysee bubbles in the refrigerant flow.

•Low side well below normal, highside only moderately or slightly belownormal. Here’s where you try to pin-point a restriction by looking for frostor condensation on the high-side liq-uid lines and the receiver-dryer or forcold spots near the orifice tube or onthe expansion valve or its sensing bulb.

•Both high and low sides well belownormal. Two prime possibilities existhere: First, the refrigerant charge maybe almost completely lost from seriousleakage. In this case, the system obvi-ously will provide little or no cooling.Second, but equally possible, is a frozen

and blocked evaporator. The compres-sor will be pumping against a blockedsystem, and low-side pressures will bevery low because of it. Heat transfer al-so will be sharply reduced, so the high-side pressures will stay low. If the evap-orator is frozen, look for a compressorthat’s not cycling off when it should.

High Pressures:•Low side above normal, high side

normal or a bit low. If these gaugereadings are accompanied by a com-pressor that cycles off at pressureshigher than usual, look for a bad ther-mostatic or pressure switch that con-trols the compressor. The low sidecan’t pick up enough heat if the com-pressor turns off before it should.

•Both high and low sides wellabove normal. A restriction in airflowat the condenser will reduce heattransfer and keep the high-side pres-sure high. Also, an overcharged sys-tem can contain too much refrigerantto allow complete vaporization andheat absorption.

•Both high and low sides steadilyabove normal. The overall heat-trans-fer capability of the system is reduced

by the presence of air and moisturethat don’t pick up and give off heatbut do displace refrigerant. Air com-presses on the high side, but doesn’tcondense and release heat at the con-denser. Consequently, high-side pres-sure stays up.

•Low side steadily above normal,high side normal or slightly abovenormal. If these pressures are accom-panied by condensation on the suc-tion lines and warm discharge air, youhave a classic evaporator floodingcondition. Remember, if too muchrefrigerant gets into the evaporator, itremains partially liquid and doesn’tabsorb as much heat as it should fromthe evaporator airflow. It then contin-ues to vaporize in the suction linesand picks up heat in the engine com-partment, where it shouldn’t. All thismisplaced heat transfer plus excessliquid on the low side keep low-sidepressures high.

When a customer says, “My air con-ditioner blows hot air,” ask if the prob-lem developed all of a sudden or if thesystem has been going downhill for awhile. Refrigeration problems usuallydevelop gradually over time. Electricaland vacuum control problems, on theother hand, usually occur more sud-denly. This little bit of extra insight canhelp you get off on the right diagnosticpath. If it looks like an electrical orvacuum problem, then you have otherbasic principles to consider...and that’sa story for another day.

For a free copy of this article, write to: Fulfillment Dept.,

MOTOR Magazine, 5600 CrooksRd., Troy, MI 48098. Additional

copies are $2 each. Send check or money order.

40 April 1999

Following is a comparison of normal high-side pressures for both R-12and R-134a systems. Pressures for R-134a are a little bit higher at any giv-en temperature. Pressure will rise in any system as humidity increases.

Ambient Air High-Side Pressure (psi)Temperature (°F) Humidity R-12 R-134a

70 Low 115-135 140-190High 130-150 165-220

80 Low 150-170 150-200High 165-185 190-260

90 Low 175-195 170-220High 185-205 210-290

100 Low 200-230 195-245High 230-250 230-320

110 Low 250-270 235-285High 265-285 260-350

High-Side Pressures for R-12 and R-134a Systems

Answers to PreviousAssessment Quiz

(Charging and StartingSystems, Jan. 1999)

1-B 2-D 3-A 4-B 5-D6-B 7-B 8-B 9-A 10-D