Jaguar XJ-S. Service manual - part 137

 
 

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VACUUM SCHEMATIC:  Finding a vacuum schematic seems to be difficult, so I have included two here -- courtesy 
of Victor Naumann. 

 
 

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BLOWER FANS:  According to Randy Wilson: “Jaguar doesn’t sell the blower separately.  They sell the complete 
blower unit: housing, motor, fan, and electronics!  Yes, there are electronics buried inside that blower case.  Plus, just to 
add a bit more, the right hand box carries the ambient temp sensor.  There are also two relays inside the box.”  The 
blower assembly is reportedly incredibly expensive. 

 

BLOWER SPEED CONTROL REPAIR:  Stefan Knappe saved himself a lot of money:  “I noticed that the right fan 
(it’s got two, left and right) did not work except when the fan speed was set to max.  I took the blower motor out, which 
is rather easy on the right side of a LHD car, finding a little solid state circuit located in the intake part of the fan.  This 
circuit gets information from the A/C computer and regulates the fan speed unless the setting is to max, which actuates 
a little relay situated in the blower case too.  I found out that the power transistor on this little circuit was blown.  I went 
to the Jag service asking whether this is available seperately.  But, you might have guessed it, the blower is only 
available as a complete assembly, being very expensive.  So I went to an electronic store and bought a transistor 
equivalent to the type built into the fan-circuit (Texas Instruments #2N6284).  I soldered it in, put the stuff together and 
the fan worked from that time on without problem.” 

Martin Sellars adds, “Air con blowers: these devices contain an electronic circuit which gives speed control for all 
settings but “high”.  This comprises a Darlington transistor 2N6284, a 68 Ohm 2.5Watt resistor, a 1N5401 diode and a 
small glass diode, probably 1N4148 or 1N914.  All these parts are easily obtained cheaply from electronic component 
outlets (example; in the UK Farnell Electronic Components, +113 2636311, but ask for a catalogue to get the right 
order numbers).  Blowers that won’t run at low speeds almost always have a failed transistor, but replacing that alone 
won’t always fix the fault.  The resistor is there to protect the transistor from voltage spikes produced by the motor, so 
check the value and replace if it isn’t right (most hobby multimeters have an Ohms measurement feature).  The small 
diode feeds the blower motor voltage back to the A/C computer, and has steel leads which corrode away, breaking the 
connection and preventing speed control from working.  I replaced mine with 1N4004 types, which are much more 
rugged, more easily handled, and cost just a few pennies more. These diodes are fitted with their cathodes (marked by a 
bar on the body) towards the transistor collector (the steel case).  The 1N5401 diodes are very rugged, and unlikely to 
need replacing.  When fitting new diodes or resistors, don’t try to fit them inside the blower interior, like the originals, 
where they are prone to corrosion.  Fit them on the solder side of the PCB (having snipped out the old parts) and then 
cover them with the original plastic flap using some tape. 

“I also fitted new brushes to the blower motors, using power drill spares filed down to the right size (Kirby’s tip!). 

“Inside the blower assembly is the high speed relay, which is also prone to dirt and corrosion.  Standard car accessory 
shop units can be used here, but check the pinout - I think they are not standard, I had to rewire mine.  Considering the 
cost of a new unit, changing the parts mentioned above gives “as new” performance at a tiny fraction of the cost.” 

 

COMPUTER PROBLEMS WITH BLOWER CONTROL:  Stefan Knappe cheated on this one:  “Another problem I 
had on a friend’s car, on which one fan did not work.  After checking the fan, I found that the A/C computer (located on 
the right hand side of the centre console, take cover underneath glove box out) delivered signal to one fan blower only, 
the other signal line being dead.  Computer gone (at least in this respect).  I asked for the price of a new one and 
decided immediately to go for another solution.  Since I’m not so deep into electronics to repair this difficult thing 
itself, I had another idea.  I cut the signal line which was dead and connected it (the line to the blower) with the good 
one for the other side.  Both fans worked again.  Risk was that the computer will be damaged on the good side due to 
higher current delivering two circuitries, but on the other hand the thing was already gone... This solution proved to be 
okay, since it still works and the repair has been done three years ago. 

“Caution:  Don’t check power output anywhere in the system with a lamp or bulb, use a voltmeter 10 MOhm 
impedance, otherwise you may damage the computer!!”