[xj-s] XJS 87, 3.6 Air Con

[Mark Barker <mark@far-out.demon.co.uk>]

Finally able to post after many tries!!


>  The Centre Vent is not getting very much air


I have a 86 3.6 XJS and had exactly same problem.

Mine has the Denaliar mkIII aircon system, I presume you have the same,
being an 87. These don't have the motorised cam-switches the older ones
have. 

I had a few problems too, all contributing to causing lack of air from
centre vent.

Try this check list:

1. Make sure the 2 blower fans are both going. Test by pulling out fuse
and listen the other one. On mine the blower on the RHS was dead. (Fix
info later) Airflow is poor unless both fans going.

2. remove both fuses for the blowers and listen carefully to sounds from
dashboard: RHS switch to NORM, left switch pull out (MAN) turn from
fully left to fully right and listen the sound. You should hear an upper
and lower servo-motor running for a few seconds each time you move the
control. On mine, one of them was sluggish/slow/sticking. (My fix later)
Unless the upper servo can travel the full distance to 'cold', the
computer WILL NOT activate the soleniod valve fro the centre vent!. If
you set the left control to out(MAN) full cold, and switch between NORM
and DEF the upper motor goes from one full end of travel to other,
should be about 10 seconds.  

If you remove the glove-box and peer into the side of the air-con unit
you can see the shaft the servo motor drives. On my RHD car my glove box
is on the other side and I can see the computer's position feedabck
potentiometer turning by this shaft.

3. There is a soleniod vacuum valve that operates the centre vent
mechansim. Access by removing side panels either side of the centre
unit. 4 screws. (Don't over-tighten) There's 4 soleniods, you can use a
circuit tester to see if any are burnt out. Mine had a broken circuit on
the one that.. you guessed it controlled the centre vent! (My fix for it
later)

*** Interesting point for all those with non-functioning (the freon
part) air con. The Delanair system (correctly) re-circulates the air
rather than drawing in fresh air when it's a hot day. You may consider
unplugging one of the wires to the Recirc. soleniod valve so that fresh
air is drawn in always. ***


These fixes I've done to repair:

1. Blower fans. 
Totally dead motor: Fitted new brushes (cut down from bigger ones). Test
before refitting.  

Speed control ineffective: corroded circuit board inside. Diode
dissoveled away. Linked tracks, fitted 1N4148 diode (soldering), the
diode is part of the speed control. Spray generously with clear varnish
after tests ok.

2. There's 2 servo-motor units controlling the upper and lower parts of
the air-con separately. They balance the mix of hot and cold air. In
fact the slider control under the radio 'biases' the upper part air
temperature slightly. The entire system is a closed-loop temp.
proportional control system.  

If these motors are sticking etc. the system goes crazy: fans run fast
all the time, hot air on a hot day and vice-versa. Also
defrost/demist/defog can take ages.

If you suspect the motor, try swapping the connectors over between the
upper and lower, the wires just about reach. NEVER apply voltage to the
motor directly! It'll jam at one end of travel. 

Mine had a motor that was slow/sticking. Was the motor itself. Had to
remove the armature and clean the out the commutator. OK after
that. Be warned, these motors are difficult to remove.  

3. Centre vent's soleniod vacuum valve not operating. Was a broken
circuit in coil. Use a circuit tester/ ohmmeter.  This device feeds
vacuum to the centre vent opener mechanism. All 4 of them are located
behind the radio. Take off the side panels (4 screws).  I re-wound mine
with wire after much hacksawing and cutting later. Test before
refitting!.  


Note on the computer unit: You shouldnt have many problems with it,
though it tends to be blamed for causing trouble. I did have trouble
once, but caused by short circuiting soleniod. Had to replace a
transistor (BC548).




Mark Barker 

mark@far-out.demon.co.uk
http://www.far-out.demon.co.uk

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