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Rear Window Demister Problem

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hi all,

i noticed that the rear window demister doesn't seem to be working properly, or at all. usually even on light rain drops, the function will work as you can see the lines clearing it away. the past four days it hasn't worked. i tested it today with my 20min journey home, and i would safely say if faulty. the strange thing is the button on the centre console illuminates. has anyone had this before. can someone help me diagnose and hopefully solve the problem?

my car is gs300 sport mk1

thanks :)

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  • 3 weeks later...

I'm not exactly great with car electrics, but I'd start at the fuse and work down towards the heater. I imagine the switch only throws a relay so it may well illuminate even if the fuse has died.

It might also be worth checking the connection between the body of the car and the window itself. Seems fairly unlikely that it would be the problem but it could be worth a look?

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  • 1 month later...

hi all,

im really suffering with this problem which is still not cured. i've checked the fuses and they are not blown. there is also a fuse box in the inside of the drivers cockpit by the accelerator pedal. fuses are not blown. i can't figure out why the demister is not working. any ideas?

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Looking at the wiring diagrams (albeit for yank versions but hopefully should be pretty close), if the mirrors are working, then the main fuse and the relay should be working. The mirrors have their own 10 amp fuse (MIR-HTR), but the power feed for that fuse is via the 40 amp DEFOG fuse and the defogger relay.

If any of the element wires are broken, then most or part of the rear screen heater should still work, so it shouldn't be that unless somehow every one is broken.

So it would tend to suggest wiring to the heated rear screen, or the noise filter just before the heated rear screen, or the connections onto the rear screen. Without taking panels off in my car, I can't see what this filter looks like, or exactly where it is located, but I would assume it will be almost directly by the positive side of the screen heater, or even on the heater itself. I think all you can do is trace the wiring with a multimeter and see where your 12 volts disappears. Checking either side of the screen and its connectors would be the easiest place to start I would think.

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can you describe exactly how i can diagnose this with a multimeter? if i was for example opening my boot from the outside, so im facing the rear window , which will be the positive side? i have a multimeter which looks exactly like this:


how can i use this to diagnose the wires?


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Find where the wires go to the rear screen, switch it on [ignition on].

Set your meter to 20V DC, connect probes to screen connectors, if no reading, swap probes around

it won't do any harm.

If no reading of 12 volts, check for corrosion, broken wires, bad earth connection,

if there IS 12 volts coming to the screen, look for breaks in the element in the screen.

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