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Bad Astronaut

Rear Window Defroster / Demister

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I've got a 2005 Corolla Colour Collection - I've noticed recently the rear demister has stopped working.

The light comes on, I've checked the 30 fuse that lives behind the glove compartment and I've also run a multimeter over the wires on the window - from what I can tell these are all fine.

One thing I notice is the light for the demister comes on even with the fuse out - but maybe this is just because it gets its power from somewhere else. I can also hear the relay switching - whether fuse is in or out - I'm pulling the 'defrost' one.

Has anyone else had a similar problem?

Cheers

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Have you checked the voltage of each of the wires that go across which should go from around zero to around 12 volts from one side to the other if unbroken? Are any wires working?

I recently fitted a fuse to mine (It was missing!) and also replaced the blown warning bulb behind the dash. However only two wires are working and all the rest have tiny breaks in them.

I've got some conductive paint to repair them.

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Theres also a slow burning fuse for the rear demist too

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I'll double check if there's voltage - I was only seeing if the meter could read itself.

Where might I find the slow burning fuse? I did a bit of googling but afraid I don't quite follow.

Many thanks

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It's just the fuse for the demister is a 'slow burning type' which means it allows for any voltage surge when you switch it on.

I would imagine if no wires are working then no current is getting to it at all so can you check where wire connects to demister?

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Yep, those are the ones I found. I tested the one for defrost and it's fine.

Looks like the Corolla was originally built with the left-hand drive model in mind (unlike my old Celica), as it's actually behind the glove box and mounted upside-down (as the manual describes). I pulled some other ones to check I was in the right place.

I pulled back all the trim on the rear window and tested the wires that attach to the glass - there's a circuit. I also tested the wire that attaches to the screen, up to a plug in the wiring loom - again seems fine. No voltage when I turn the screen on.

I can only think it's wiring further down or maybe the switch? Perhaps it worth getting a replacement switch and testing that before I really start tearing things up.

Thanks for the advice so far chaps - if anyone has some other ideas I'd be really grateful.

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Hi,

According to the diagram in the Haynes manual the Rear heater switch activates the DEF relay which connects the power from the 30A fuse to the rear heater.

On that same power line a spur is taken off by means of a 10A fuse to power the wing mirror heaters.

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Interesting - I'm definitely hearing the relay click when I switch it off.

My model doesn't have the heater wing mirror heaters - but I'll double check if a fuse was left in and if it's working.

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From the diagram the switch is powered from a different line / fuse from that 30A line.

The relay may be clicking but when its activated do the contacts actually pass the voltage ok, they can wear out.

My 1.6 is a CC and has heated mirrors , yours a 1.4 or just a spec change for 2006 models ?

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I think I need to track down where the relay actually is - I can only guess it's near the fuse box in the cabin.

I was fairly sure the heated mirrors was just an option (now you've got me thinking!) - no mine's a 1.6 CC - definitely the facelift model too.

Thanks for your help - looks like I need to have another dig around later!

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Hi,

Just looked in the T handbook, it should be on the dashboard fuse box, the middle one of the three relays.

There is also a fuse and relay box by the Battery.

The heated mirrors are easy to miss as they do not have thier own switch, being combined with the rear window one.

If you acivate that switch, engine running, you should feel some gentle heat on the mirror glass in about a minute.

However if like mine, someone has been replaced a cracked glass its often done using just a plain glass as the heated glass version is only available from Mr T at about £60

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I can see a white box marked 'relay integration' - but looks to be part of the fuse box.

Looks like I'm out of my depth, thanks for all the hints - at least I know it's not something really obvious - so if it costs, it won't be like they just changed a fuse or something!

I think I'm going to have to get my mate to look at it (assistant manager at Halfords auto centre - yeah yeah I know).

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I managed to get it looked at. Turned out the solder on one of the contacts on the window glass had failed, it came away with very little effort. It's working again after being re-soldered! Very pleased.

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I'm trying to figure out if there is a fault with my rear demister, it appears to work but takes 45 minutes to clear... Is that normal or have i got a problem starting?

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Hi,

Yes I would say so, under typical condtions it should clear most the rear screen in about 10 mins, though perhaps a bit longer on these recent cold nights.

I assume you are operating it while the engine is running ?

The first things I would want to check, with a steamed window, when you start the demister, you can see the clear lines open up over the element lines as they heat up.

Are all the lines there that match the actual elements on the window .

Another point to check, does you car start easily in this cold weather and do your headlights seem dimmer than usual ?

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Hi,

Yes I would say so, under typical condtions it should clear most the rear screen in about 10 mins, though perhaps a bit longer on these recent cold nights.

I assume you are operating it while the engine is running ?

The first things I would want to check, with a steamed window, when you start the demister, you can see the clear lines open up over the element lines as they heat up.

Are all the lines there that match the actual elements on the window .

Another point to check, does you car start easily in this cold weather and do your headlights seem dimmer than usual ?

Hi, yep i'm running it with the engine on, headlights seem fine the bulbs were replaced by the previous owner to me (only last year) Eventually there are lines that match the elements on the window, but only after the demister has been on for at least half an hour.

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Hi,

Well from what you describe it sounds like everything is functioning, eg the dashboard switch, the demister relay and the elements.

My thoughts would be that somewhere between the relay and the elements you have a bad contact/ high resistance which is limiting the voltage getting to the elements, so slowing down the demisting.

This is where you need a voltmeter and knowledge of testing things.

Testing for the voltage across the actual window element connectors is whats needed first.

Probably the two most likely points would be either a partly faulty demister relay or bad /corroding contacts where the wiring connects to the actual window elements.

Another possibility is the wiring partially fracturing where the tailgail hinges up if your is a hatchback.Don't know if you can do any of that yourself or if you know of someone who is good with a test meter ?

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Thank you, i'll find someone to help me with that i know engines not electrics :( it's possible with the tailgate wire fractures mine is the 5 door hatchback version, the previous owners had kids in it so they may have caused some kind of damage

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