Jaguar XJ-S. Service manual — part 134

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BLOWER FAN RESISTOR PACK -- FUN WITH SCHEMATICS: Some A/C system schematics, including Fig.
10.125 in the Haynes, show the blower fan resistor pack (component 188) incorrectly. They show the GS wire to the
fan motors themselves being connected to one end of the three resistances, as though all three resistances operate in
series. In fact, the GS wire connects between the 0.85 and 0.425 resistances. In M1 speed, power is supplied to both
ends of the resistor pack (Y and U wires) so that the 0.85 resistor operates in parallel with the 0.425-1.275 pair. In M2
speed, power is supplied to the R wire in addition, so the 0.85 resistor is now operating in parallel with the 0.425 alone.

The “Air Conditioning” system schematic in the ROM at the end of section 86 as well as Fig. 10.130 in the Haynes
show the fan resistor pack correctly.

Also of note, the schematic in the ROM and Fig. 10.130 in the Haynes show one additional circuit by which the high
speed position of the control knob will, via diode #D4, operate the M2 relay. Reason unknown; it almost appears to be
a backup in case the high speed relay fails.

DEMIST DUCT VANE: The ROM, Section 82.25.21, step 35, says “Ease demist duct vane securing studs from
screen rail and recover demist duct assembly.” John Nuttall felt this was unclear, so when he finally figured out what it
was talking about he provided the following explanation. Note that this applies to his 1977 XJ-S.

“Removal of this vane uncovers a screw which secures the unit to the car body just below the windshield. The main
difficulty with the manual is that it does not make clear just what is the demist duct vane, and I don’t think the official
name is a very good description. It turns out that this object is a piece of vinyl covered rubber about 34 in (85 cm) long,
3/4 in (2 cm ) thick, and 3in (8 cm) wide in the centre, tapering at the ends. Along its back face is screwed (with 13
screws) a plastic air deflector to direct air to the windshield. This vane fills in the gap between the centre of the fascia
and the windshield.

“To remove the demist duct vane you must undo two screws placed vertically downwards at the ends. With the
windshield in place it is impossible to do this with a conventional screwdriver. I was able to turn these screws with vise
grips. The underside of the vane has two prongs about 17 in (44 cm) apart which engage slots in the unit below.”

CONTROLS: Rob Reilly provides the following system description: “Outside air is drawn in through the grille in the
center of the cowl between the bonnet and the windshield. Vacuum operated flaps on the fan motors close off this
outside air under certain conditions and draw air instead from inside the car (known as the recirculating mode).

“From the fan motors the air goes through rubber branch hoses to the front chamber of the heater unit. Here all the air
passes through the air conditioning evaporator and gets cold (if the compressor is on). Then there are four flaps
controlled by mechanical linkages which can be seen on the right hand side of the unit. These control what percentage
of the air passes through the heater core and becomes warm (if the water valve is open and the water is warm) on its
way out to the vents.

“The upper front (meaning toward front of car) flap (“upper bypass flap”) opens or shuts off cold air to the dashboard
side vents, center vent, and windshield defrost vents. This air has not passed through the heater core. Clockwise is
open, counterclockwise is closed, viewed from the right hand side. The center vent and windshield defrost vents also
have vacuum-operated flaps.

“The upper rear flap (“upper heater flap”) opens or shuts off warm air, which has passed through the heater core, to the
same upper vents as the first flap. Clockwise is closed.

“The lower rear flap (“lower heater flap”) controls warm air to the lower vents on the sides of the transmission hump
and to the duct hoses going to the rear vents under the center console armrest. Clockwise is open.

“The lower front flap (“lower bypass flap”) controls cold air to the lower vents. Clockwise is closed.

“On the left side is a vacuum actuator and linkage which, when the selector is on DEFROST, will be up (vacuum off)
and prevents the bottom heater flap from opening.

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“There is a servo motor on the lower right which operates some more micro-switches, vacuum line valves, and
mechanical linkages, and is in turn controlled by the temperature setting and several temperature sensors through the
amplifier and relay on the lower left.

“The left hand knob (temperature control) turns a variable resistor. The resistor should have a resistance of zero ohms
at the 85 degree mark and 10,000 ohms at the 65 degree setting. There are three solder pins on this thing, one of which
should not be used. If you had a high resistance in the circuit the unit would think you’re asking for cold air and would
turn the stepping motor to give it to you.

“The small device mounted on the lower tube of the heater core is a temperature-controlled on/off switch which
disables the blowers until the water warms up.” Actually, it limits the blowers to low speed.

CONTROLS -- VACUUM LOGIC: The following logic chart was graciously provided by John G. Napoli:

Jaguar Climate Control Vacuum Logic -- Representative of 1982 XJ-S HE

Compiled by John G. Napoli

Control:

Vacuum Switch Cam Switch Vacuum

Solenoid

Item

Controlled:

Screen

Flap

Lower

Heater

Flap

Water

Valve

Center

Dash

Flap

Blower

Flaps

FULL

COOL

Control

Status:

On On

Open

Open Energized

Item

Status:

Closed

Open, and
overridden

by flap

Closed Open

Closed

Vacuum

Status:

Vacuum Vacuum Vacuum Vacuum

Vacuum

FULL

HEAT

Control

Status:

On On

Closed

Closed

De-energized

Item

Status:

Closed Open Open Closed

Open

Vacuum

Status:

Vacuum Vacuum

No

Vacuum

No

Vacuum

No Vacuum

FULL

DEFROST

Control

Status:

Off Off

Closed

Closed

De-energized

Item

Status:

Open Closed Open Closed

Open

Vacuum

Status:

No

Vacuum

No

Vacuum

No

Vacuum

No

Vacuum

No

Vacuum

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Notes:

In FULL COOL, recirculation is enabled because the vacuum solenoid is energized.
A NORMAL A/C mode is therefore implied with the vacuum solenoid de-energized.

In FULL COOL, everything gets vacuum.
In FULL DEFROST, nothing gets vacuum (default system operation if system fails).
In FULL HEAT, only the items fed by the vacuum switch get vacuum.

The vacuum switch is attached to the right hand climate control knob (Positions: Low, Auto, High,
Defrost).

The cam switches are part of the servo. The servo is mounted on the right of the climate control unit
under the dash (next to the right hand occupant’s left shinbone).

The vacuum solenoid is mounted on the left of the climate control unit under the dash (next to the left
hand occupant’s right shinbone).

Water valve is located on center of firewall in engine compartment.

Dashboard may have to be removed to access flaps. Always check operation of and adjustment of servo
linkages when troubleshooting this system. There are two blowers (left and right). Check them both!

Some later XJ-S’s may have a manual override enabled by pulling the right hand climate control knob out
and turning. Reference to this feature was seen in a 1983 XJ-S Owner’s Manual.

CONTROL MICROSWITCHES: The right side control knob has microswitches behind it that are often found to be
the source of trouble. However, Ron Whiston points out that the problem is often not a failure of the microswitches
themselves, but simply that they are incorrectly positioned relative to the cam; turning the knob doesn’t move the
switch enough to make it click. Even though the mount holes are not slotted and are not intended to provide any
position adjustment, merely loosening all the mount screws and holding the switches inward while tightening them
back down will often correct all problems.

TEMPERATURE CONTROLLER: Reportedly, the wirewound pot has problems. See Exotic Auto, page 704.

CONTROL LINKAGE ADJUSTMENT: The ROM describes an adjustment procedure that is unclear and covers only
a couple of the adjustments needed. The illustrations are tiny and poorly labeled. Below is a procedure that should
enable a more complete adjustment of the linkage. See Figure 26 and Figure 27.

While performing these adjustments, keep this concept in mind: The difference between a flap being fully open and
90% open is insignificant; there is a lot of airflow in either case, and the passenger will not be able to detect a
difference. On the other hand, the difference between a flap that is fully closed and one that is slightly open is huge.
Therefore, the objective of the adjustment procedure is to make sure that all flaps close fully, and how far they open
will be of little concern.

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UPPER PULLROD

UPPER HEATER FLAP

ADJUSTABLE LINK

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

UPPER BELLCRANK

MAIN BELLCRANK

Figure 26 - Climate Control System Linkage -- Early MkII