531
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.