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MG MGB Technical - Heated screen not working

I'm trying to get the rear heated screen to work, console switch and light come on & power gets to the sceen connection. Also checked the earth and this is OK. Does anyone know what resistance I should be getting from the screen element itself? I would think it should be low if it is to draw 8+ amps. Tried to measure mine and it appears open circuit. Anyone ever changed over one of these screens? DIY job or off to the local screen shop?
Roy Soper

Install a 3 bar fire running directly off your alternator.
Toasty warmmmmm
phil

What! and disconnect the coffee machine.....?
Roy Soper

You will probably see where the circuit is broken if you look closely at each line. There is a cheap fix available of conductive paint, sold by car parts places. I've had little success with it (it falls off) but it may be worth a try. If you have an open circuit it suggests that there is a break on every line, and a new screen may be a better bet. If it's like the front screen, DIY is possible but a bit fiddly.
Steve Postins

What year? Early cars had vertical embedded wires whereas later ones had horizontal surface-printed tracks, although I'm not sure how much difference this makes. If it really is open circuit one of the main connections is broken, the individual wires/tracks are in parallel so if one is broken only that element fails to work. Although if the car is new to you or the screen has never worked in your ownership then I suppose all individual tracks could be broken as Steve sez, although personally I'd consider this unlikely without it being the later type and obviously damaged. Did you measure the voltage between the supply and ground connections at the screen with it switched on? Just checking resistance of the ground can be misleading. If you measure 12v between the two connections both at the wire ends coming off the glass, and on the glass itself (which I think is possible with the later type at least, under the rubber) then there is a major break somewhere on the glass. If you can't get to here the wire is bonded to the glass I'd stick a pin through the insulation as close as possible to it and look for the voltage, it's possible that the conductor has broken inside the insulation and hence probably repairable.

Glass replacement is feasible but a bit of a pain with the bright trim, you might like to have a look at http://www.mgb-stuff.org.uk/wn_bodyframe.htm and click on 'GT Screens'.
Paul Hunt

The car is a 72 with the horizontal lines. I thought the supply fuse would be blown when it didn't work as the live wire has some insulation missing and at first I thought this must have earthed and fused the supply. Disconnected the bullet connector from the screen fly lead and it measured a healthy 13/14 volts with the engine running. I'll have a dig round the rubber trim to see if I can see where it bonds to the screen as I agree it seems unlikly that all the lines are broken so I need to look for a common break.
Roy Soper

I thought the same but unfortunately the trace has 3 inch gaps in places on mine. It looks like a combination of aged materials and overfilling the boot has wrecked it. Easy enough to see though.
Steve Postins

You *must* measure the voltage with it connected. If you measure it disconnected and there is a bad connection (i.e. not a full open-circuit) back towards the supply or earth you will most likely see the full voltage shown on most voltmeters. With it connected the load (if any!) of the screen will cause some or most of the voltage to be 'dropped' at a bad connection and there will be little or none left for the screen, or the voltmeter connected as close to the screen terminals as possible. This low voltage tells you there *is* a bad conenction somewhere. With the load (i.e. screen in this case) disconnected there is only the load of the voltmeter in the circuit, which is microAmps, which means negligible voltage lost in any bad connections. I would also expect to see about 14.5v with the engine running at a fast idle with minimal electrical load. Compare the alternator brown, and fusebox brown, white and green.
Paul Hunt

This thread was discussed between 27/09/2004 and 29/09/2004

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