It will come as no surprise to anyone who's owned an XJ40 to hear that one of my heater blowers is only working on max speed, so I get no air at all at low and medium settings. Thanks to some research on http://www.jag-lovers.org, I've diagnosed it as a (probable) failure of the speed control resistor and have ordered the required parts from Maplin (my local shops didn't have them in stock).
The parts required are:
Darlington transistor 2N6284,
a 68 Ohm 2.5Watt resistor,
a 1N5401 diode
and a small glass diode, probably 1N4148 or 1N914
To play safe, I ordered both types of glass diode and bought two of each resistor and diode (they only cost a few pennies each).
Dismantling the heater showed the circuits to be in fairly poor shape, so I'll need to clean it all up, replace the components and thoroughly re-solder the whole lot. I'll take more pictures as I go.
After removing the under dash trim, unclip the relays from the side of the blower, along with the vacuum line to the recirc actuator and undo the two electrical multiconnectors to the right of the blower - this will give better access for removing the blower. If you follow the wiring from the blower itself, you will find two multiplugs - one round, one flat - which need to be unplugged.
You'll need a 10mm socket and long extensions to undo the two mounting bolts - which are tucked out of the way! You may also need to undo the 13mm nut that supports right hand steering column brace if you find space a bit tight when removing the blower unit.
Right hand side:
Left hand side:
The blower unit may be rotated clockwise and down. This will allow you to remove the unit and separate it from the flexible duct which connects it to the main heater unit (this duct isn't easy to connect - it may be better to remove it and reinstall it already attached to the blower).
The dodgy electronics live under this panel which is held on by 4 rivets - the picture shows the "after" shot, with self tapping screws replacing the rivets:
The speed control PCB can now be rotated out of the blower to be repaired - you don't need to split the blower unit:
If you do want to split the unit, there are two screws and half a dozen clips holding it together:
The Darlington transistor is held on by two bolts (with insulating washers) as well as being soldered to the PCB. At this point, I have to confess that as the new small diodes looked different from the originals, I chickened out when it came to replacing them - I just de-soldered them, cleaned the contacts and soldered them back! I did, though, replace the big diode and the Darlington resistor:
Reassembly is the reverse of the above - I chose to check the function before bolting it all back together.
Hope this helps.