Rear Demister not working

[SDIclassic]

Hi all. My Rear Demister is not working on car mk1 Octavia. In the early stages of trying to find the issue have started with the fuse which is fine moving onto the relay next. Can anyone tell me the location of the relay for the heated rear screen your help would be greatly appreciated cheers Colin.

[KenONeill]

@SDIclassic - Do the number plate lights (and rear wiper if applicable) work?

[SDIclassic]

11 minutes ago, KenONeill said:

@SDIclassic - Do the number plate lights (and rear wiper if applicable) work?

Hi yes both of these work 

[KenONeill]

25 minutes ago, SDIclassic said:

Hi yes both of these work 

OK, well, I don't think it's the earth then.

[mrgf]

Silly question but is it not working with the engine running? On my Fabia, the rear demister only works if the engine is running. Try to operate it with just the ignition turned to accessory and it will not come on.

I would then try testing the electrical connectors for power, using one of those probes. Often, the electrical wires on heated rear windows decay and/or break. You can buy a repair kit from motor factors that you kind of "Paint" on to bridge any small breakage in the element.

[SDIclassic]

Yes not working with or without the engine running. When I press the button the microswitch clicks but cannot hear any relay sound around the dashboard. I would really like to find where the relay for the heated window is located so I can eliminate it.

[KenONeill]

25 minutes ago, SDIclassic said:

Yes not working with or without the engine running. When I press the button the microswitch clicks but cannot hear any relay sound around the dashboard. I would really like to find where the relay for the heated window is located so I can eliminate it.

Just been and checked mine. The HRW works, but there is no audible click.

[nige8021]

There are no relays involved, the power comes from fuse 26 direct to the switch pin 1 (White/Yellow wire) then out of the switch on pin 2 (White wire) to a 5 pin connector by the "C" pillar and then out to the rear screen as a Black wire 

[SDIclassic]

Ok thanks at least it eliminates that. I will have a look at the switch and the tailgate area for any signs of breakage. Many thanks Colin.

[MikeTheThinker]

@SDIclassic: If you go to this link you can download the owners manual for your car as a pdf, on page 164 of which shows the internal fuse panel. 

 

@nige8021: thanks for the heads-up on the (lack of) relays; I've just been pulling my hair out wondering why I couldn't find any information on them!

[SDIclassic]

Well the switch is working. I done a bulb test on the wires in the hatch I made a good earth and as you are looking at the hatch there is 1 wire on the left side & 2 on the right side the top wire on the right side lit the bulb up when touched so that must be the live feed & the other two must be earth's. I will check it again when the screen is misted up. Other than that I don't no where else to look. Anyway thanks for the replys I will keep you posted cheers Colin.

[yo8aiv]

anyone has or found a sollution for this issue ? 

i find myself in the same situation.

- fuse ok, wiring ok. 

- measured 12v coming at the rear resistor (demister) for a few seconds then turning off (green led inside demister button is blinking).

i assume it must be some can-bus check wich turns off the demister.

weird because last winter worked even with 5 of the lines broken / i'm in the process of repairing them.

 

[jareid]

On 09/11/2022 at 22:20, yo8aiv said:

anyone has or found a sollution for this issue ? 

i find myself in the same situation.

- fuse ok, wiring ok. 

- measured 12v coming at the rear resistor (demister) for a few seconds then turning off (green led inside demister button is blinking).

i assume it must be some can-bus check wich turns off the demister.

weird because last winter worked even with 5 of the lines broken / i'm in the process of repairing them.

 

 

I know this post is some time ago, it is always worth opening the switch up and cleaning the contacts with some emery board.
They can be very intermittent, (wondering about you observing the indicator in the switch flickering)
I am not aware of any CAN bus involvement with the switch. 

[yo8aiv]

thanks. ill have a look. 

testing via VCDS seems its working .. dont know if the fact that half of the traces being interrupted might affect the working condition.

[makymak]

@yo8aivI see on your profile you have a Fabia mk1. Here is the Octavia mk1 session and Octys have a lot of differences. On Octavia only the heated mirrors are monitored electronically. The rear demister is a plain electrical circuit (even not a single relay is used, only the switch). You could have more luck trying the Fabia mk1 session, instead. 

 

Edit: yes, the interrupted traces could have an impact at the Fabia since the central electronics module could monitor the whole resistance and switching off the circuit if any major difference is found. But I'm not sure about that. 

Edited 28 March, 2023 by makymak