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MG MGB Technical - heated screen

hi all.
on the cgt iv just bought the rear heated screen isn't wired up. switch on dash with no wires connected. my question is, is it just a live to the screen through a switch then earthed on other, or does it go through anything else.
thanks bob.
bob taylor

No, that's how it's wired up. Make sure the power feed is on an ignition key controlled circuit, and make sure it is fused. The window is quite a large electrical load and if you leave it live when the engine isn't running it will drain the battery in short time.
Mike Howlett

thanks mike.
bob taylor

Personally Bob i would put it through a relay.
Allan Reeling

Good point Allan and something I should have mentioned since mine is powered through a relay!
Mike Howlett

hi all
not that clued up on wiring. so what route does it take. wire from fuse box to switch then relay then window.?
thanks bob.
bob taylor

Bob,
I'd be surprised if the wiring wasn't there. B's and C's shared similar wiring and unless someone has fitted the wrong (roadster) loom at some time it should be there.
One white wire from your top fuse goes to a line fuse then emerges white with black going to the switch then to the rear screen via one of the o/s bulkhead bullet connectors.
Check first before adding a circuit.
Allan Reeling

If I remember correctly the wires go up inside one of C pillars and emerge between the roof lining and the metal roof so they can go round the hinges and inside the rubber window seal to the contacts on the edges of the screen. If you can gently pull the roof lining down a little at the rear, you might just see the wires in there.

If you use a standard 4 terminal relay, wire it up like this.
Feed from switch goes to terminal 85
Power from fuse box goes to terminal 30
Feed from relay to window goes to terminal 87
Terminal 86 is earthed (grounded).

This way the relay handles the large current and all the switch is doing is turning on the relay, so the switch contacts are preserved.
Mike Howlett

If a new purchase, I'd be inclined to check the screen works first....
Michael Beswick

Why bother with a relay? No chance a little defroster circuit will heat up and pop. Medium draw and a fair amount of current needed to operate, but not like lights or heater fan. Relax and enjoy the ride.
Urney Yak

Why bother with a relay? It makes it a good 50% more effective for a start, and also takes the load off the main fused ignition circuit and ignition switch.

A little defroster circuit? It's by far the biggest single load on the car at about 1 ohm i.e. 14 amps or 200w. By comparison the heater fan takes about 3 amps. You would have to power all four headlight filaments to get more than the HRW.

It looks like the HRW was optional until the 1971 model year, i.e. post-MGC, before that the wiring would quite likely only have been provided with the HRW.

When Mike says 'power from the fusebox to terminal 30 of the relay' that should be taken from the purple which is fused (or from the brown via a separate in-line fuse), not the white or green. The feed to the switch should come from the green, so the HRW will be isolated with the ignition off.

Rubber bumper 4-cylinder MGBs (but not V8s) did have an HRW relay until 1976, then it was replaced with the ignition relay which basically did the same job of taking the load off the ignition switch.
paulh4

car had hrw when i got it but didn't work, tailgate was unrepairable so fitted another one with screen that works. original switch was in dash but no wires to it, could have been disconnected when screen packed up. will have to pull radio out and see if any wires there.
regards bob.
bob taylor

The easiest thing is to look at the wires in the boot, to see if there is a white/black in the harness, or an additional wire somewhere else.
paulh4

This thread was discussed between 11/08/2016 and 13/08/2016

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