Jaguar XJ-S. Service manual — part 135

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UPPER PULLROD

LOWER BYPASS FLAP

ADJUSTABLE LINK

SERVO LEVERS

MAIN PULLROD

MAIN

TENSION

SPRING

CLOSE

UPPER

BYPASS

FLAP

CLOSE

CLOSE

UPPER

HEATER

FLAP

LOWER

HEATER

FLAP

CLOSE

LOWER

BYPASS

FLAP

FACIA

CONTROL

CABLE

UPPER BELLCRANK

MAIN BELLCRANK

Figure 27 - Climate Control System Linkage -- Later MkII

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1.

Remove the glove compartment, the “underscuttle casing” (panel above the footwell) on the right side, and the
grille and padded panel on the right side of the transmission tunnel.

2.

Remove the short steel duct that serves the footwell grille. Be careful not to drop any screws into the works.

3.

Loosen the locking screw on the lower bypass flap adjustable link. Loosen the locking screw on the upper
heater flap adjustable link if there is one; if the car has a facia temperature control (a slider under the stereo), it
will not have an upper heater flap adjustable link as the facia control takes its place. Loosen the locking
screws on both pullrods where they connect to the servo control levers.

4.

Move the servo to the full cool position, which is where the levers on the servo are held at their most
downward position. Note that the pullrods are normally held upward by the linkage springs; with the linkages
disconnected, the servo levers will fall down regardless of servo motor operation. You must pull upward on
them with your fingers to determine what position the servo is actually in. Note also that operation of the
servo doesn’t move the levers continuously from one extreme to the other, but rather goes back and forth
somewhat; be sure the motor has moved all the way to the extreme position that holds the levers in the
downward position before proceeding.

To move the servo, disconnect the main electrical connector to the servo unit, which is a 13-wire connector in
the same general area you’re already working in. Connect 12V power across the solid purple and solid red
wires; if the battery is connected, 12V power is available at the nearby fusebox, at fuses 2 or 3 (RHD cars) or
fuses 13, 14, 15, or 16 (LHD cars) so it’s a simple matter to run a jumper from the fusebox -- or simply from
the cigar lighter, and connect the other wire to ground. If the battery is disconnected, a 9V transistor radio
battery will also work well, running the servo a little slow but it gets there. To run the opposite direction,
reverse the power.

5.

Turn the main bellcrank clockwise until the lower heater flap is held firmly shut; it may be easier if the main
tension spring is disconnected. Hold the main servo control lever up. Tighten the locking screw on the servo
lever.

6.

If the car has no facia control, hold the upper heater flap in the fully closed position (clockwise) and tighten
the locking screw on the adjustable link.

If the car has the facia control, move the slider to the full right position. Loosen the clamp holding the cable
housing, and move the cable housing until the upper heater flap link forms a straight line. Lock the cable
housing into this position.

To check the operation of the facia control, move the slider to the left. The upper heater flap should fully
close, and further movement should be taken up by the spring in the linkage.

7.

Move the servo to the full heat position (levers in their highest position).

8.

Turn the lower bypass flap to the fully closed position (clockwise). While holding this position, slide the
adjustable link to its longest possible length and tighten the locking screw.

The lower bypass adjustable link has a slotted opening to allow the flap to be farther closed than the linkage
calls for. To check that this slider works properly, move the servo off the full heat position. Turn the lower
bypass flap towards closed, against the force of its spring. The lever should move smoothly in the slotted
hole. If it doesn’t move smoothly the linkage is probably misaligned, causing it to jam. Determine which way
the parts are misaligned. Take the lower bypass adjustable link apart by removing the locking screw entirely
and removing the screw holding the lever to the flap. Once out of the car, carefully bend the parts to correct
the misalignment. Reinstall, readjust, and check for proper motion again.

9.

With the servo in full heat position, allow the spring to turn the upper bellcrank clockwise until the upper
bypass flap is closed. Hold the upper servo lever up and tighten the locking screw.

10.

Reassemble and test drive.

Note that on later cars there is a vacuum actuator above the left side footwell that holds the lower heater flap closed
when the defrost is called for. Vacuum permits lower heater flap operation, lack of vacuum prevents it. To observe the

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operation of this actuator, it will be necessary to remove the underscuttle casing on the left side. There is no real
adjustment necessary, however, since it is either on or off. Make sure there are no wires or anything obstructing its
motion.

Note that the connection of the pullrods to the bellcranks is a nut-bushing that is fitted into a slotted hole on the
bellcrank. The procedure above does not address location of the nut-bushing within the slotted hole. This position
determines the sensitivity of the system; if the motion of the servo causes the flaps to move too much, the nut-bushing
should be relocated farther away from the pivot so that the same amount of pullrod motion won’t turn the bellcrank so
much, and vice versa if the servo doesn’t move the flaps enough. It is recommended that these nut-bushings not be
tampered with. If their position is altered, the entire linkage adjustment procedure above should be repeated.

CONTROLS - RANGE PROBLEMS: Olov Carlsson sends this comment: “I had a problem with the temperature
control knob being out of range. Even when I put it in the coldest position, the system wouldn’t cool properly. It did
work, however, which I determined by using the cigarette lighter and holding it under the interior temperature sensor.
This sits in a hole that can be felt under the dashboard centre shelf. When doing this the system adjusted itself and
cranked out wonderfully cool air.

“There is an adjustment for this on the amplifier unit. I turned it to the cold end position, but this wasn’t sufficient to
bring the temperature control knob properly on scale.

“My final solution to this was to connect a 10 kohm resistor in parallel with the external temperature sensor, which is
mounted inside the right side external air intake. Careful analysis of the circuit diagram and the wiring allowed me to
put the resistor alongside the internal temperature sensor. I connected a wire to the appropriate lead on the amplifier
unit, the result being that the new resistor effectively was in parallel with the external temperature sensor. This one, by
the way, did measure the correct resistance (the same as the internal sensor, when they both had the same temperature).
This brought the adjustment on the amplifier unit into range making it possible to calibrate the temperature control
knob.”

TOO MUCH COOLING: Mike Morrin says, “If the system works properly for a while (10 minutes) and then drifts
slowly towards full cooling, then the fault is likely to be in the plumbing to the in-car air temperature sensor. This is a
reference to a couple of cases I have heard of where the rubber connector inside the dash had come adrift so that the air
temperature inside the dash was being measured. There is no cold air supply inside the dash, just heat soak from the
engine and heater unit, causing the measured temperature to creep higher and higher, and the A/C to pump full cold into
the car.”

TOO MUCH HEATING: Tom Bernett says, “Note that the function switch removes the supply from the temperature
control circuit when in the DEF position, and this causes the system to move to full heating, so a faulty function switch
can cause the system to stay on full heating. But, I was getting considerable air flow from the side and center vents
when it was in full heat mode.”

Mike Morrin explains: “Yes, because the function switch also has a vacuum valve which directly controls the demist
flaps, and is independent of the electric circuit.”

SERVO REPAIR: Mike Morrin says, “After cruising for an hour at about 70 mph. with the A/C on (of course), the
system all of a sudden went to full heat. I fooled around with the temp selector and after several minutes it went back to
normal operation.

“It turned out to be a faulty feedback potentiometer in the servo, so that the amplifier thought the system was on full
cold, so was sending the system to full warm. To check this you need to leave a voltmeter connected to the feedback
circuit (as per instructions) and see if the voltage goes the right way or the wrong way when the fault occurs.

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“The actual fault on mine was that the feedback potentiometer is a wirewound type, and the wiper arm had worn
through the wire right at the end of its travel. I did a quick fix by painting some silver loaded paint onto the end of the
winding, and it has survived for 3 months so far, I expect another 10 years out of it.”

RELAY CHATTER: Mike Morrin, who owns a 1975 XJ-S, reports on “a tendency for the amplifier relays to chatter
after moving the servo. An oscilloscope across the supply lines showed noticable spikes when the amplifier relays
operated. I have managed to significantly improve this by fitting a 1000 uF 50V capacitor across the 12V supply close
to the amplifier. The amount of relay chatter is reduced by about 80%. I can confirm that driving the car today, the
operation of the servo system was smoother than previously.”

CONTROL AMPLIFIER REPLACEMENT: Aftermarket A/C control system amplifiers are available for those who
either don’t wanna pay Jaguar prices or don’t expect that a new one of the same type will last any longer than the
original did. See H. D. Rogers & Sons on page 709 or Welsh Jaguar on page 694.

CONTROL AMPLIFIER REPLACEMENT -- CHEAP VERSION: Mike Morrin suggests, “...a double pole switch
(like a window lifter switch) can be arranged to drive the servo manually.” Simply connect 12V power through the
switch to the red and purple wires on the servo connector; the two positions of the switch should provide power in
opposite polarities. Label the button “cooler” and “warmer” or some such.

NAPOLI MOD: The idea of this mod is simple enough: provide a switch to enable operating the climate control
system without the compressor when cooling is not needed. John Napoli says, “You are basically adding a switch
in series to the inline fuse for the compressor located on the transmission tunnel near the air flap linkages.” The
wire in question is a GN wire.

Note that operating with the compressor off will not only result in no cooling, but will eliminate the ability to
dehumidify as well. In high humidity situations, you will probably need to turn the compressor back on to prevent
window fogging and other humidity problems. You will definitely need it on if you use “defrost” mode to clear
condensation on the inside of the windshield.

Also note that operation of the system in hot weather with the compressor off might cause the “Auto” mode to
increase fan speeds, since it’s trying to cool and the servo is moving to ever more powerful cooling modes to try to
lower the temperature. However, normally the occupants will want to switch the compressor on long before this
happens. Even if it did happen, it’s simple enough to switch to “Low” mode.

There is a fuel cooler in the freon circuit, and shutting off the compressor will obviously result in a loss of fuel
cooling. According to Roger Bywater, “the fuel cooler was added as an emission device, not because of fuel
handling difficulties like vapour lock, although this could happen on early cars before the fuel rail temperature
sensor was introduced. The high underbonnet temperatures of the V12 always resulted in a lot of heat being
transferred to the tank with the recirculating fuel and I recall measuring over 60°C at the tank which means a lot of
vapour can be given off. To pass a full EPA emission test for certification there are strict limits on the evaporative
emissions of the whole vehicle (including adhesives, plastic mouldings, paint, etc.) and the fuel cooler eased this
situation by reducing fuel vapour loss. This is why all cars for the USA market had air conditioning - they were not
certified for sale without it. Also the convertible would have to have the compressor running continually because
the air con would probably not be used with the top down.” Hence, the Napoli mod might technically be considered
detrimental to the emissions control systems of the car. Note, however, that you would normally only switch the
compressor off on days where the weather was cool anyway; presumably, the fuel needs cooling the most on the
same days that you do, when you will have the compressor on.

The guys who have computers in their cars report that shutting off the compressor results in an instant fuel economy
increase along the order of 2 mpg. As always, your mileage may vary, and in this case some of that benefit may
deteriorate as the fuel in the tank gets warmer; warm fuel hurts engine efficiency. And if you roll the windows