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GTV Rear screen demister fix.

60402 Views 234 Replies 72 Participants Last post by  HFStuart
GTV rear screen demister repair:
The problem with the early GTV’s rear screen demister is down to poor basic electrical design.

The power feed to the heater element travels along the full length of the car, from the fuse box under the driver’s side dash, along the dash to a connector behind the glove box, along the passenger sill, across the rear seat and up to the heater element, ironically just passing the ‘power source’ (battery).

This wire carries a high current load. Anything that carries a high current load gets hot.
The timer on the relay can fail and cause the heater to stay on.
When the heater stays on, the wire carrying the load heats up continuously.
This heat melts the flimsy plastic fuse holder and, in most cases, the plastic connector block behind the glove box.

The safest way to fix this, and ensure there will be no more problems in the future, is to remove the power from the wire running through the car, by fitting a power relay in the rear, close to the battery & heater element.

If the fuse box and / or connector under the glove box are damaged, it will need replacing first.

What you need:
• Single pole normally open 12V, 40A relay (£2.70, Maplin code NO2AW).
• Inline blade fuse holder (£1.50 from local auto parts centre).
• Bag of insulated spade connectors (£1.49, Maplin code JH82D).
• 6mm insulated round connectors
• AWG 10 or 12, (2.5mm Dia) wire, approx 0.5m
• Insulated crimp tool
• Solder & iron
• Self tapping screw
• Drill (suitable size for self tapping screw)
• 5A blade fuse

1. Remove the boot trim section covering the battery and locate the wiring loom to the rear window.
2. Locate the Br/Wh wire in the loom.
3. Trial fit the relay in a suitable position (behind the right speaker, above the battery), and check there’s enough slack on the wire to enable you to connect to the relay prior to cutting the wire.
4. Cut the Br/Wh wire & wire up the relay as follows;

85 – earth - (switching coil)
86 – Br/Wh from front of car - (switching coil)
30 – Br/Wh to heater element - (power contact)
87 - +ve direct from batt (via 30A fuse) - (power contact)

I attached the fuse holder to the relay, by means of super glue, to keep it nice & compact, but as long as it cannot touch the chassis (earth), fit it anywhere between the battery & relay.

Drill a small hole where you want the relay to fit and attach it to the chassis using a self tapping screw. Secure the earth wire from pin 85 under that screw when fitting the relay.

Use 2.5mm wire for the power feed, crimp a round terminal to one end to attach to the battery +ve, (but DO NOT attach it until last thing), routing the wire neatly to the relay. Crimp a spade connector (or solder) the other end to attach to the fuse holder.
From the other end of the fuse holder, use a short piece of 2.5mm wire to connect to pin 87.
Crimp a spade connector to the heater element side of the Br/Wh wire & connect to pin 30.
Crimp a spade connector to the car side of the Br/Wh and connect that to pin 86.

You can now substitute the 30A fuse under the dash for a 5A fuse & fit the 30A in the fuse holder in the rear!

Last thing; connect the wire from the fuse holder to the battery +ve.

Job done! You have now turned the current loaded wire running through the car into a no current switching wire!


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thanks seadart, i think that was the problem. i managed to download a pdf of the wiring diagram from the forum and found that the resistor was in the wrong place. i have moved it over and it is now all working.

just a quick question, should there be a relay where i moved it from for something else? why would someone have put it there ?



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thanks for this info... I've pulled the fuse for now pending investigating connector behind glove box...
This does sound like a rather alarming trait, a vehicle which could potentially set itself on fire if you use it's rear screen heater!... anyone had dealings with Alfa over it?... or had the job rectified by a motor/elec specialist?

The above description is the best fix & easy diy. I think garages just replace/bypass the burned out wire, so it could happen again. I'm no electrician but I did this myself with no problems but if you doubt yourself then just print out the relevent posts and take them to an autoelectrician - should be a doddle for them.

Not sure if it has been brought to Alfa's attention (they won't be interested now the cars are out of production) but my specialist - an ex-Alfa dealer - said that 164s also featured this "quirk" ...

I have asked the Mods to reinstate this thread in the How To sticky, if not as a sticky in its own right, as it used to be ...
..yeah...I think it SHOULD be a 'sticky' in it's own right... could be a life saving sticky at that!
Thanks for the advice..I'll take it to my trusted mechanic (then I can have someone to blame if it goes wrong :))))
Just as this happened, the radio aerial has also given up the ghost, and digital display on CD players gone 'well-blurred'... laugh a minute eh?... having said that, the new thermostat has made a massive difference to performance and fuel economy...things are on the 'up'..I can feel it...:cheese:
Just done this fix to my GTV after nearly 3 years of the demister not working.

I used wire described as 4mm 27amp. Is this likely to be ok - I didn't realise until after I fitted it that it wasn't 30amp, seems a bit odd since it is much thicker than what was already there.

At the moment the demister works but I don't want to leave it in if its going to go wrong.
It'll be OK. The normal circuit is fused 30A ( actual consumption will be lower obviously ) and your mod isn't feeding the door mirror heaters so that'll reduce the current as well. It's also a much shorter run of cable.

I used 44/0.30 rated at 25Amps on my car ( but I did drop the fuse to 25A ).
This should definitely be a sticky. Who was it on here who only got out of their GTV just in time before it burst into flames?
This is all great, but... I wrong in thinking that the heated rear window element negative cable (black) should now be connected to a good earth or the battery?
Correct, I earthed in the rear wing behind the aerial. I wrong in thinking that the heated rear window element negative cable (black) should now be connected to a good earth or the battery?
You need to earth the coil of the new relay. The window is itself already earthed*. Nothing to change there.

* Earthing point G63b on the GTV and G407 on the Spider with hard top.
...anyone know of an auto electrician in Leicester area who may be able to do the rear window demister mod?...I just haven't got the time or technical skill required (half developed amphibian syndrome).

If I had some mists at the back, how long does it generally take for it to get cleared? I had some the other day after having pretty must everything sorted and the rear demister didn't seem to clear anything - maybe until about 3 to 4 mins after it was on. Not sure if this is normal but the guy who owned the garage said it is. When the demister works, does the mist slowly go away from where the 'bands'? Because that is how the demister works on my rx8. I recon it was the heating which dispersed the mist at the rear window for me
mine demists in a minute or so. It melts ice no problem too.

You can actually feel the heat coming off it if you put your hand over the bottom corner.

After not using the demister I started using it recently for only a minimum amount of time so I could at least see Something out of a few lines out of the rear window. As soon as i could see something I turned it off straight away without letting it clear the whole window.

Hope that will hold out and not blow up :cool:
Strongly advise you to do the fix. It is easy enough even for a non-techy or just take these instructions to an autoelectrician. There are issues with the fusebox getting hot and gradually melting/deforming over time even if you avoid a full failure.
bumping this as it quite approriate for this time of year.
And mine, although I wish I had a pet PKR in the garage as I am not fond of elictricals myself...,
quick one ..
if there's anyone who, like me, did this mod about four or five years ago .. it is an idea to check the high current terminations on the mod. relay once in a while .. a wiggle and a blast of WD should do it ..
Hi guys (and gals) !

I have finally taken a closer look at mine after 3-4 years of demistifier not working. Wasn't exactly on the top of my "to do" list :lol:

BUT... It is turning out to be a bit more tricky than I thought. My car is a left hand drive TB version from 1996 (Italian market version) and I have a service manual from ebay. According to the wiring diagram mine has, or should I say should have, a timed relay? It is hard for me to know since the relay is missing. Also the location of the fuse is different from even a 1997 ts model I saw a picture of recently.

Here is what I have found out so far:
I thought it would be a breeze to fix; Plug in a relay and then do the mod. But I have not been able find the type it should be. I have tried the circuit with a "normal" basic relay and light comes on & 12v at demistifier as long as button is pressed. As soon as I let go it turns off.

So basicly what I'm asking is: Do you know if maybe early versions had a timer in the relay, or should I find a separate controlbox somewhere? Where?

If it is a timer relay then how the heck do I find one not knowing the code. Local shop looked at the schematic and said it is a timer relay according to the drawing (thats what it looks like to me as well), but he has no idea of the type or where to get one. Thanks!:confused:

Here is a picture of the manual, hope it is ok to post it here?

I would appreciate any help you can offer:)
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