Rear window demister problems

[worms]

12 year old Defender (200Tdi 110 CSW). Rear window demister blows fuses instantly, but after fuse blows, switch light stays on and if left on for a long journey, battery drains. To me this suggests that it leaks to earth somewhere but I am surprised that the switch is lit when the circuit should have closed with the fuse blowing or alternatively if the earth leak is before the fuse, why should the fuse blow?

At the same time, the clock is very erratic – minute-hand suddenly runs on for an hour as if it was a second-hand but must then stop for a while since it is never more than about 40 minutes out. Don’t know if the two faults are related.

Winter is on its way so I can't ignore the demister problem for much longer!

[130CC]

The standard for wiring in a car is that a switch such as the demister one is only used for switching low current of a relay and the like, the high current the actual demister circuit is switched by a relay.
High an low current is a reference to Amps not to volts.

So the high current fuse may blow but this will not affect the light in the switch as it is fed from the low current side which will most likely be on a different fuse.

If you remove both spades lugs from the demister circuit one should short to ground and the other should mate it's way all the way back to the switched side of the relay. to test this use a multimeter or a circuit test pen to follow the wires. if you need to find the relay leave the blowen fuse in as this should not affect the operation of the relay only the load (demister) on that relay, turn the demister switch from the on to the off position and listen to the relays this will give you the one you are after. the other way of doing this is to check at the fuse and put one of multimeter propes on the demister side of the fuse and the other one on the ground if this makes a circuit then you have a short on that wire (make sure you remove spade lugs from demister circuit) it's easy now replace the affected wire and this should fix the problem.
As to the problem with the clock it sounds like you drive to close to the Bermuda Triangle, I think that it is just a fault with the clock, but I might be wrong.
Hope this makes some sense
Adam

[worms]

Thanks! That's a really useful reply. I did not know about the high and low amp circuits - learn something new every day! I was also using the multimeter with the demister spades still attached which may have added to my confusion.

As to the clock, I have long suspected that I live in my own personal version of the Bermuda Triangle! And what's an hour here or there anyway?!