Jaguar XJ-S. Manual - part 129

 
 

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ratchet and extensions; you reverse it by pulling it off the extension, turning it over, and inserting the extension the 
other way.  Not only is this the cheapest version (about $10), but it also takes up the least space in your tool box. 

An example of that last type is made by Superior Tool Company of Cleveland, Ohio (1-800-533-3244, 

http://www.superiortool.com

), Model No. 03820, “Ratchet Jaw Basin Wrench Attachment” and is sold at Lowe’s.  It 

claims to fit 1/2” to 1-1/4” hex and square nuts. 

While you’re shopping, also note that basin wrenches come in at least three sizes.  The one described is the smallest and 
perhaps the most useful for Jaguar work, but it probably wouldn’t hurt to have the larger ones around. 

A basin wrench works perfectly on that hose connection.  Note, however, that you still need a 13/16” crowfoot on a 
separate ratchet and extension to hold the fitting still while breaking this big nut loose; if you don’t, you may break the 
fitting off the evaporator, and if that happens -- trust me on this --you will end up deciding that air conditioning is just a 
frivolous luxury you don’t really need.  Fortunately, a 13/16” crowfoot is not hard to find. 

 

SERVICE PORTS:  The service ports on the 70’s and 80’s XJ-S are a standard 1/4” flare fitting.  Well, actually, no, 
they’re not.  They are a standard 1/4” flare fitting except there’s a Schrader valve screwed down into the center.  Hence 
you cannot slap together some hoses with standard flare fittings on the ends to service this system.  You need the kind 
of fittings that have a pin in the middle to depress that center pin to open the Schrader valve. 

 

SCHRADER VALVES:  ...aren’t particularly durable.  Each Schrader valve has two items in it that are made of rubber: 
the seat of the poppet valve itself, and the seal around the body that contacts the fitting when the valve is installed.  
Both pieces of rubber are subject to the same sort of age and deterioration as an O-ring.  Whenever the system is 
discharged for whatever reason, it is recommended that the Schrader valves in both service ports be replaced as a matter 
of course. 

If you’ve waited too long the old Schrader valve may come out without the seal that was around its body, leaving this 
tiny ring of rubber stuck down in the hole.  Just screwing the new Schrader valve down on top of it is not a good idea.  
Before installing the new valve, take a good hard look at the old one and make sure it’s all there. 

The guy who works on my A/C for me has a tool for replacing the Schrader valve in an R-12 service port without 
discharging the system!  It screws onto the port, then allows you to unscrew the valve from the fitting within a glass 
housing so nothing leaks out.  Put a new valve in, problem solved.  Only a little squirt of freon is lost when you remove 
the tool, the amount that filled the volume of the tool itself. 

 

EVACUATION:  Before charging a freon circuit that has been broken into, it must be evacuated.  You can buy a 
vacuum pump that operates on compressed air from Harbor Freight Tools (

http://harborfreight.com

) for $10 plus S&H, 

but if you don’t have an air compressor (and this thing requires a pretty large one, 4.5 cfm at 90 psi or so), a suitable 
electric vacuum pump will probably cost you more than it’s worth; you’ll opt to just take the car to a shop for this work. 

Some suggest ripping the compressor out of a refrigerator and using it as a vacuum pump.  This might work, but then 
again, it might not; that’s a compressor, not a vacuum pump, there’s no telling how much vacuum it can pull.  Squirting 
a little oil into the suction line on the refrigerator compressor before connecting it up reportedly helps it pull vacuum 
better. 

Before you start you might want to connect your vacuum pump directly to your vacuum gauge and see what its 
capability really is as indicated on that gauge.  This can save confusion when trying to pull a vacuum on the system; if it 
doesn’t seem to want to pull as far as you expected, you’ll know whether it’s the vacuum pump or something else 
causing the holdup. 

There are two distinct purposes of evacuation: to remove gaseous contaminants (air), and to remove moisture. 

To remove moisture, it is only necessary to obtain an adequate level of vacuum and to hold it for an adequate length of 
time.  This is because the vacuum is causing moisture within the system -- including within the dryer -- to boil off and 

 
 

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get pulled out as vapor.  Since some of the water vaporizing removes heat from the water remaining and causes it to get 
very cold, it doesn’t want to vaporize.  The vacuum must be held long enough for the remaining moisture to warm back 
up to surrounding temperature and boil away.  Using a hair dryer on components of the system will help, especially if 
you’re trying to do this job on a cold day. 

The fastest way to remove the moisture, of course, is to fire up the vacuum pump and let it run continuously for at least 
an hour, preferably several hours.  You can accomplish the same thing by letting the pump run for a few minutes to pull 
a good vacuum, then valving the system shut and turning off the pump, and coming back every hour or so and running 
the pump a few more minutes.  In fact, if you leave the gauge in place, that’s one way to tell when you’re done: when 
you come back after an hour and the vacuum is still as low as you left it.  Moisture vaporizing will diminish the 
vacuum, so if the vacuum holds there’s no moisture remaining in the system. 

Another way of knowing there’s no moisture remaining in the system is to pull a vacuum that would be unattainable 
with moisture in the system.  When pulling the moisture out of a system, you will gradually pull the vacuum down until 
you reach the point where the moisture starts boiling off.  The vacuum will then remain at that level despite additional 
pumping until all the moisture is gone.  Once dry inside, then the vacuum will start to move again.  Taking advantage of 
this fact of physics, the directions typically given are to pull a vacuum of such-and-such, with the specified vacuum 
being lower than the boiling point of water at any reasonable ambient temperature.  The only way you’ll ever attain that 
level of vacuum is after boiling off all the water in the system and then continuing to suck.  This is a simple and 
excellent way to assure the system is fully dry.  Remember, though, that for moisture removal, you only have to get the 
vacuum to the point where the moisture has all boiled off; additional vacuum does nothing for moisture removal. 

Just exactly how much vacuum is needed to remove the moisture?  It depends on temperature of the system, and since 
vacuum is normally expressed relative to outdoor conditions, it will also depend on the barometric pressure where you 
are.  As an example, if the surroundings are at 70°F, you will need to attain an absolute pressure lower than 0.74 in. Hg 
within the system to boil the moisture off.  If the barometric pressure outside happens to be 29.92 in. Hg (standard 
atmospheric pressure), you’ll need to attain a vacuum of at least 29.92 - 0.74 = 29.18 in. Hg.  Heating things up helps, 
though; if you do this when the surroundings are at 80°F instead, you only need to attain an absolute pressure below 
1.03 in. Hg, so at the same barometric pressure you only need a vacuum of 29.92 - 1.03 = 28.89 in. Hg.  If you operate 
your 12-cylinder heater for a while and get everything in the engine compartment up to, say, 140°F, water will boil off 
at an absolute pressure of 5.89 in. Hg, so you only need a vacuum of 29.92 - 5.89 = 24.04 in. Hg -- although you 
probably won’t get the evaporator behind the dash that warm, so you probably won’t get all the moisture out of the 
system unless you pull to 29 anyway.  Getting everything you can as warm as you can while evacuating is always a 
good idea, though. 

To remove gaseous contaminants requires pulling as much vacuum as you possibly can.  There is no time function; pull 
it, valve it off and you’re done.  If you have a vacuum pump capable of pulling to micron levels, the procedure is then 
to turn it on and let it run until it reaches such a level.  The vacuum level will pause along the way as the moisture boils 
out of the system, and then once dry it will resume dropping until it reaches the limit of your pump.  Valve it off, shut 
off the pump, and begin charging. 

If your vacuum pump can’t pull all the way down to the micron level, there is another way to help purge impurities 
from the system.  Pull the best vacuum that you can from one service port, making sure that it’s low enough to have 
removed all the moisture.  Then connect up a hose to the other service port and allow some non-contaminant gas to fill 
the system and dilute the remaining gaseous contaminants.  Nitrogen is ideal; refrigerant also works and is convenient, 
although using R-12 this way would be irresponsible and expensive and the EPA would frown on you using any 
refrigerant this way.  Once the system has some gas in it, fire up the vacuum pump and suck it out again.  This way, 
whatever the vacuum pump fails to pull out is mostly innocuous gas anyway rather than contaminants.  Of course, you 
could repeat this two or three times to really get things pure in there, but that’s a matter of diminishing returns. 

Many conscientious mechanics will do a nitrogen purge anyway regardless of how good their vacuum pump is.  It can’t 
hurt. 

 

FREON CHARGING:  It would normally not be within the scope of this book to describe how to charge a freon 
circuit, since it should be common knowledge; almost every automotive A/C system is charged basically the same way. 

 
 

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 However, here in the Big Bend region of Florida, it is somewhere between difficult and impossible to find an auto A/C 
shop that will actually charge a freon system fully. 

Sometimes the shops install a specified weight of freon and declare it charged, apparently unaware that the sight glass is 
the final say on the matter; if the spec says one amount and the sight glass says that ain’t enough, the sight glass is 
correct.  Other times they charge until they reach a specified pressure on their gauges and declare it charged, apparently 
blissfully unaware that pressures are only an indication of temperature in a freon circuit, not of the amount of charge.  If 
the pressures are too high, it’s because something is too hot -- usually, in the case of the XJ-S, there isn’t enough air 
flowing through the condenser in front of the radiator, so the condenser is getting hot and not cooling the freon enough. 
 But this is no reason to undercharge the system; rather, it’s a reason to investigate and correct the airflow problems at 
the condenser; see page 514. 

There are mechanics that advocate undercharging the freon system because it helps keep the engine from overheating.  
An undercharged freon circuit will not operate nearly as effectively, so it won’t reject as much heat from the condenser, 
so the air entering the radiator is not as hot to begin with.  Makes perfect sense.  And if your mechanic thinks this way, I 
recommend you leave his shop posthaste and never return. 

After a shop does work on your A/C system, you should check the sight glass to confirm that it is fully charged.  If not, 
you can go back and bitch about it -- but that’s not likely to work since they know it all and you are a lowly customer 
who doesn’t know anything.  If all else fails, obtain a license to purchase freon and “top up” the system yourself.  
Michael Minglin found a page on the WWW “for those of you who would like to be able to purchase R-12, R-134a, or 
just about any other kind of coolant, to do your own A/C repairs: 

 

http://www.epatest.com

 

“At this website you can download the EPA section 609 manual free, take the test online for under $10, and print out an 
instant temporary EPA certification certificate, if you pass the test.  The actual certification card is mailed in about 30 
days.”  This author now has his card; the online test works just fine -- but the price had gone up to $19.95.  The author 
has also used his card to buy 12-oz. cans of R-12 at AutoZone, which also works just fine. 

Note also that it is an open-book test -- which, when working online, means that while taking the test you can click a 
button to review the manual.  Since computers are very helpful, note that you can cut-and-paste a string of words from 
the question and use it to “find in page” the exact same string of words in the manual in a matter of seconds.  It is really 
easy to pass this test. 

R-12 is not cheap, but is available locally.  The hose, valve, and fitting you’ll need to connect a freon can to the car and 
charge it costs perhaps five bucks.  All that fancy equipment you saw at the shop -- the set of two gauges on the long 
hose, the vacuum pump, etc. -- is unnecessary for topping up. 

So, allow me to describe the procedure for topping up a freon circuit:  Connect a can of freon to the low pressure fitting 
on the car.  On the XJ-S with the A-6 compressor, the low pressure fitting is the one on the line from the fuel cooler to 
the compressor.  It is suggested that the valve on the filler hose be cracked open just enough to hiss a little while 
connecting to the car, as this will help blow air out of the fittings themselves while connecting and therefore keep it out 
of the circuit.  With the system running at full cool (fans on high, doors open), hold the can upright and open the valve 
and allow the system to draw the freon gas into the system.  This will cause the can to get cold as the liquid freon in the 
can boils off and is sucked into the car.  The rate at which freon enters the system is a function of how warm that can is, 
so if you heat it a little it’ll fill faster.  Don’t use a torch or anything, but holding it in your hands (if you can stand it) 
will work, as will holding it near warm parts of the engine.  Keep filling until there are no bubbles visible in the sight 
glass, then close the valve, disconnect the hose, and cap the fitting.  If you wish, it won’t hurt anything to add a little 
freon beyond the point where the bubbles disappear -- say, less than half of a 12-oz. can -- but don’t stop before the 
bubbles disappear. 

Don’t turn the can over.  This allows liquid freon into the system instead of pulling gas in.  It is a trick used by some 
A/C mechanics to fill faster, but it risks getting liquid into the compressor -- especially with the connection as close to 
the compressor as it is on the XJ-S.  Just take your time and leave the can upright. 

 

 
 

514

TEMPERATURE VS. PRESSURE:  Within a sealed container containing refrigerant in both liquid and gaseous form, 
temperature and pressure are interrelated.  If you heat the container some of the liquid refrigerant will vaporize, 
increasing the pressure.  If you cool it some of the gaseous refrigerant will condense, decreasing the pressure.  If you 
increase the pressure some of the gaseous refrigerant will condense, releasing its heat of vaporization, causing the 
temperature to rise.  If you decrease the pressure, some of the liquid refrigerant will vaporize, absorbing heat and 
causing the temperature to drop.  The net result of all this is that temperature is positively linked to pressure, and this 
relationship is called the “vapor pressure” of that refrigerant.  There are charts available for all of the common 
refrigerants. 

This situation exists in two places within a running freon circuit: in the condenser where vapor is condensing into 
liquid, and in the evaporator where liquid is boiling off to become vapor.  Within either coil, the pressure and 
temperature are related according to the vapor pressure chart of the refrigerant used. 

At any place in a circuit that contains pure liquid or pure vapor, it’s possible for the temperatures and pressures to be off 
the vapor pressure chart.  Specifically, pure gas can be warmer or lower pressure than the vapor pressure chart would 
indicate (superheated), and pure liquid can be cooler or higher pressure than the vapor pressure chart would indicate 
(supercooled).  Being off in the opposite directions is not possible because it would cause boiling or condensing. 

In general, noplace in a running A/C freon circuit gets too far from the vapor pressure chart.  The pressure within the 
condenser determines the pressure all the way upstream to the compressor outlet, and presuming there are no blockages 
neither the pressure nor the temperature will change much along that route.  After the refrigerant condenses into liquid 
in the condenser, it’s possible to “supercool” it further so that the liquid in the dryer and liquid line to the expansion 
valve is a bit cooler than the vapor pressure chart would indicate, but there’s not much point to doing that so systems 
typically aren’t designed to supercool much.  One might note, though, that any heating of the line along this route might 
cause some of the liquid inside to vaporize.  Specifically, any heating of the dryer -- say, by heat rising off the radiator 
or condenser -- might cause bubbles in the sight glass if the sight glass is downstream of the dryer.  This is a good 
reason to make sure there is a box fan in front of the car when checking the sight glass, making sure heat from the 
radiator is blown the other way and the dryer only gets cool air around it.  It’s also a good reason to ask for a dryer with 
the sight glass on the inlet end. 

One place where a deliberate attempt is made to get away from the vapor pressure chart, though, is in the suction line.  
After the refrigerant boils to become vapor in the evaporator, the system will usually be designed to try to squeeze yet 
more cooling from this vapor, thereby superheating it.  This is to ensure it remains pure vapor as it enters the 
compressor.  If something cooled the vapor a bit along the way, there could be some condensation, and you’d have 
some liquid freon sucked into the compressor -- and that would be bad.  

An evaporator is usually designed to provide some superheat to the vapor.  In the case of the XJ-S, there is an 
additional superheater: the fuel cooler.  Plus, of course, all the lines from evaporator to compressor pass through the 
engine compartment (and the upper part of it, too!) so it’s not likely anything will be cooling that line enough to cause 
condensation inside. 

So, the temperature in the suction line is usually a bit warmer than the vapor pressure chart would indicate.  Don’t let 
the term “superheat” fool you into thinking it’s hot, though; typically, the vapor pressure chart would indicate that it 
should be around 30°F, so if it’s actually around 40°F it is superheated! 

Also note that the line that comes out of the evaporator turns directly upward.  This isn’t only to get over the engine; it’s 
also to help insure that only gas comes out this way, any liquid remains in the evaporator until it boils. 

 

HIGH FREON TEMPERATURES AND PRESSURES:  When charging or checking a system, the A/C mechanics 
keep an eye on the high side and low side pressures.  From the relationship between the two pressures they can tell if 
the system is working properly or if there are blockages in the circuit, the compressor is defective, etc.  They can also 
tell if the system is properly charged. 

Unfortunately, an XJ-S sitting and idling is likely to develop the following indications:  the low side pressures are about 
right and the high side pressures are really high.  These are symptoms of an overheating condenser.  The pressures often 
get high enough to concern the A/C mechanics and cause them to stop filling the system even though it’s not really