Rear window demister

[Corbo36]

Hi all,

Rear window demister has stopped working. Trying to find the fuse to check it but not sure where to look in the boot for it, and what amp it is. Button is working and goes off at normal time interval. Cant see any breaks in the element either

[Magnet]

Hello Paul,

Fuses blow to protect the wiring if the is a short circuit in the wiring. They just don’t blow without there being a fault. 
Can we take it you have a handbook for the car, and you have checked for fuse locations in there? 
Kind regards,

Gareth. 

[Corbo36]

Yes i have a handbook and that pointed me to the fuse box in the boot. Checked all fuses except for one 40amp square fuse. Couldnt get it to pull out and didnt want to force it incase i damaged it

[Magnet]

Thanks, so the appropriate fuse was in tack then -? 
If so, I would suspect a break in the elements. 
Have you got a multi meter to test it for continuity? 
Kind regards,

Gareth. 

[cliffcoggin]

If that is the relevant fuse, and I can't think of anything else on the car that might need a 40 amp rating, it may have melted or blown itself apart. I don't know how to extract it without damaging the fusebox but it does need to be tested. Can you get to the terminals with a meter?

[Corbo36]

Sorry fellas, just home from work. Had a look in passenger footwell fusebox as well, couple of 30amp and 40amp fuses in there but look to be ok. Dont have a fuse puller so dont want to use pliers to pull out the fuse. Have attached photo to show 40amp fuse in boot

[Corbo36]

Just had someone check all fuses with a meter and all seem ok. Also checked the connections at the window on each side but nothing when heater was turned on. Also didnt hear any click from the relay in the boot when heater was turned on

[cliffcoggin]

While you still have the meter there are two further tests worth doing:

/1 Measure the continuity through the element at its terminals with electrical power turned off. No continuity means the element is broken.

2/ Measure the voltage at the element terminals when it is switched on. No voltage confirms no supply. That means you need to work back to the relay, the fuse, and the switch.

[Corbo36]

Havnt got round the issue of the rear window, but have juat been out in the car and heated side mirrors are not heating either. Are both on same circuit or is it it just a coincidence? Havnt had the need to use side mirror heaters until now, so not sure if they stopped working at same time as rear window

[cliffcoggin]

Mirror heaters are usually on the same circuit as the rear window heater. Have you done any testing with the meter as advised four months ago?

We would appreciate an update on your progress with this problem.

[Corbo36]

At the time i had the fuses tested, didnt know which one for heated windows so just done them all, in front of the car and in the boot. None were blown. Didnt get window elements tested as didnt have time. Will maybe go down the path of changing the relay to see if that fixes the problem. If not then will have to get someone to check everything again. Will reply back when i get relay changed and if it fixes it or not. Thanks

[cliffcoggin]

A new relay will be money wasted if the current one is fine. It will be cheaper to do some testing before throwing money at the problem. After all it may be the switch that is faulty.

[Corbo36]

What is the best tester to use? I havnt got one as i just got others to do it. Maybe time i learned to do it

[cliffcoggin]

Any cheap multi meter (less than £10 from eBay,) will be adequate for testing voltage and continuity on a car, but don't be tempted to use it for serious electrical work at higher voltages despite what the vendor may claim.

However you will need to learn how and where to use it to avoid injury to yourself or damage to the car. That's beyond the scope of what we can do in a forum like this, so you may be better off employing somebody who knows his way around a car.