Frozen rear demister?

[marcusus]

So I've recently noticed on cold days (less than 5 degrees or so in the morning) my rear demister seems to not work.

After driving around with the heater on for a while, it comes back to life. Admittedly, I haven't tried driving around with the heater off to see if it comes back to life irrespective.

My best guess is that there are metal contacts somewhere which have shrunken due to the cold, and that the warm air expands the metal so they contact again, thus bringing the demister back into action.

Anyone ever experienced this before?
When I have time, I'm gonna try checking the plug that sits behind the passenger seat and perhaps also the fuses for tightness. Are there any other connection points that go from the button to the demister?

[Fuzzlet]

Maybe its so damn cold it just cant get the heat into the glass, and needs the main heater to take the initial chill off?

[marcusus]

Maybe its so damn cold it just cant get the heat into the glass, and needs the main heater to take the initial chill off?

It's not a problem of applying the electrical heat to the glass, because the actual demister doesn't trigger the relay. The demister light doesn't turn on, and nor do I get the usual audible relay click when it works.

So no one else has come across this? I'm surprised I'm the only one... unless it's a problem specific to my car, in which case it'll mean pulling everything apart again to check connections...

[Craig]

It's not a problem of applying the electrical heat to the glass, because the actual demister doesn't trigger the relay. The demister light doesn't turn on, and nor do I get the usual audible relay click when it works.

You may have answered your own question there, try replacing the relay perhaps?

[marcusus]

You may have answered your own question there, try replacing the relay perhaps?

Hmmm... depends whereabouts the relays are located. Any clues? I have some doubts that a relay would sporadically fail like that. Usually that type of stuff is very discrete (and I should know, considering I work with control systems all day, albeit not in these situations) and the only thing that would cause it would be in the relay not contacting.

I just find it funny that it works once it's warmed up. Bit of a lose lose situation... if it's too cold and my rear window is frosted up and I can't defrost it, but if it's warm enough, it doesn't have any frost on it.

So this is just me then?

[Craig]

Be under the dash with all the other ones!

What about the spade connector that goes onto the back of the hardtop...might be loose and/or the solder that holds it onto the rear window might be dodgy.

[marcusus]

Be under the dash with all the other ones!

What about the spade connector that goes onto the back of the hardtop...might be loose and/or the solder that holds it onto the rear window might be dodgy.

No hard top. This is the soft top demister. Anyway, I've already tried jiggling the connector (disconnecting it, reconnecting it, shoving it in as hard as I can etc) and it hasn't seemed to make a difference. Although it's been a bit warmer lately, so it's been working without incident.

As for the location, I suppose it must be somewhere near the fuses. I can always hear it clicking over around there, so it might be worth checking around there to see what I can find.

[Alex]

hmm when it is working just keep pressing the button and follow your ears to the relay

[marcusus]

hmm when it is working just keep pressing the button and follow your ears to the relay

Yeah, was gonna give that a shot when I had a spare weekend. At least it's been warming up quite a lot lately so the demister's been working alright.

[Craig]

What about a hairline crack in the metal strips running across the rear window? Grab a multimeter and test it's continuity when it's cold and then again when it's warm. Just remember to unplug the plug first to take it out of the circuit.

[marcusus]

What about a hairline crack in the metal strips running across the rear window? Grab a multimeter and test it's continuity when it's cold and then again when it's warm. Just remember to unplug the plug first to take it out of the circuit.

Hmmm... That actually sounds like it could be a good option. The biggest problem is waking up early enough on a weekday to have time to get out there and put the multimeter on it

Either way, I'll keep it in mind and hopefully I'll remember to give it a shot when it's cold enough.

[Craig]

Just sleep in all the time then until it's warmed up...problem solved!

[marcusus]

Something tells me work won't look too kindly on that type of arrangement

[Alex]

I don't think it would be a crack in the heating elements, my other car has that problem but the rest of them still heat up so I normally have a single line of frost in my rear window

[marcusus]

It could be across the return wires or something, causing all of them to fail in the cold.

Still haven't had a chance to look at it because I've been away this past week. Maybe the coming weekend...