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GTV Rear screen demister fix.

60404 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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