GTV Rear screen demister fix.

[stoyce]

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.

Remedy:
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.

Note;
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

Installation;
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!

[DuckNCover]

Great stuff! Nice instructions!

/goldstar

[dan653]

thanks, job for this weekend, weather permitting

[Mitch166]

Excellent post, well done that man

[Rewe]

Very, very good explanation. Thank you.

[dan653]

so what happens to the existing heater relay?

[itacarboy]

Very well explined I think this I common fix that has or need to be done, were the alfa design fails is that the cable to the rear demister has not got the current carrying capacity of 30 amps or 20 amps on later models,

[r100gspd]

Thanks for this.It would explain the burning plastic smell when I turned switch to max the other day.

[LozGTV]

excellent write up, looks like ive got a fair bit of work to do this weekend now....lol

[stoyce]

Originally Posted by dan653

so what happens to the existing heater relay?

The existing heater relay still works in exactly the same way, but you have transfered the load from the wire running through the car to the short run of wire running from the battery to the new relay.

If you were designing it from scratch, you'd do it differently, but this is a sure fix that doesn't involve changing anything else

[Mitch166]

Originally Posted by stoyce

The existing heater relay still works in exactly the same way, but you have transfered the load from the wire running through the car to the short run of wire running from the battery to the new relay.

If you were designing it from scratch, you'd do it differently, but this is a sure fix that doesn't involve changing anything else

So the existing relay output voltage, that went to the element, now is used to switch the new relay. Therefore the long run of cable has a very small load on it and the high load is only on the short run from new relay to element. Have I got that right?

[stoyce]

Originally Posted by Mitch166

So the existing relay output voltage, that went to the element, now is used to switch the new relay. Therefore the long run of cable has a very small load on it and the high load is only on the short run from new relay to element. Have I got that right?

Spot on

[dan653]

what gauge wire did you use?

btw, it free delivery this weekend at maplins (from friday morning)

[stoyce]

Originally Posted by dan653

what gauge wire did you use?

btw, it free delivery this weekend at maplins (from friday morning)

10 or 12 AWG should do the trick. They can carry over 40A, (thats 30% above rated, so quite safe enough).

Free delivery - no reason for anyone not to fix theres then

[kakahara]

Wow, nice. Needed this, so tagging for near future

Please sticky!

[dan653]

just done this, nice and easy as luckly(?) previous owner had put a 20a fuse in, it had been completely fired but had saved the fusebox and rest of the wiring

took about an hour and a half, most difficult and time consuming thing was putting the fusebox cover back on, little plastic fasteners

[monkeybrain1234]

This is what this site is all about. Stoyce, the word "legend" springs to mind.

I get the burning smell when its on for more than 5 min. I will be doing this asap. Will let you know how it goes.

[stoyce]

I'm glad this is of use to others.

I've found some very useful information on this site, and it's only a pleasure to be able to offer some in return!

As for the rear window demister, I really don't like the idea of 'burning wires & plastic' in the car.
Safety first!

[Mitch166]

It is an excellent post. I am going to ask the Mods to make it a sticky as it is an importaint issue.

[pswillies]

Good post

I tried using my heated screen for the first time a few weeks ago and noticed it wasn't working... looks like I'll be giving this a go shortly!

[monkeybrain1234]

Right all done. And it works!!! A few points and a question:

The heater wire on my GTV was on the passengerside, not the battery side. (incase your looking for it)
The 30A in the footwell had no green Plastic left on it! Only the bare metal 30A fuse was left! The plastic had been melted away! Good job I have done this fix!

And the question I have, when I went to maplins and asked for the 2.5mm wire he gave me this instead:

24/0.2 Wire 10m Black, guy in maplins said this was equvialnt to 2.5mm! Is this true? - I have used this wire for the earth connection and the live from the battery. When I tested the system eveything works but the live from the battery seems to get warm. Is this wire thick enough or should I get a thicker one?

Thanks

[DuckNCover]

Looking at American wire gauge - Wikipedia, the free encyclopedia it looks like that is around 1mm in thickness.

Maybe someone with experience can confirm this?

[monkeybrain1234]

DuckNCover thanks.

The wire I used is about 1mm diameter (physically). If I put a thicker wire will it heat up less?

[Mitch166]

Originally Posted by monkeybrain1234

DuckNCover thanks.

The wire I used is about 1mm diameter (physically). If I put a thicker wire will it heat up less?

Yes. The thicker the wire, the more load it can carry, so less heat.

[monkeybrain1234]

Right new wire it is. Lets go and raid the maintainence department at work!!!!!!