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MG MGB Technical - BGMGT Heated Rear Window

Can someone please help me with determining whether my heated/tinted (Triplex Sundym) rear window from my 1977 V8 conversion is going to do what it's meant to do.
I have tested it for continuity by placing multimeter terminals on opposing spade terminals from the heating element but the meter did not register. The strips which run up on either side of the glass connecting the heating wires which traverse the windom test out ok along their lengths. I am confused as I would have thought that at least one of the heating wires would be intact and thus provide continuity between both sides.
I am unsure about probing the actual heating wires in case I do damage.
Am I missing something?
Peter M
Peter M (member)

Sorry - meant to head the thread MGBGT Heated Rear Window
Peter M (member)

You do get zero ohms i.e. full scale deflection when you touch the probes together, I trust?

I've not actually measured the resistance of mine before but it is low, in the order of a few ohms I'd expect, and should register on any typical meter. I've just tried to measure mine but it came up as zero even on the low ohms scale which has graduations down to an ohm, then I discovered it reads the same on the low scale as the high scale when measuring other things, so looks like that analogue meter is going the same way as the two previous digital ones!

Is the glass on its own i.e. no rubber? I guess so if you have been able to test the vertical strips up the sides for continuity. Were these wires or track? I'd consider using a pin to test from each side into some of the elements a little way, say the top or bottom ones just in case you do damage them.
Paul Hunt

These might help:

http://www.ukcar.co.uk/features/tech/others/heated_rear_window/index.htm

or this:

http://www.popularmechanics.com/automotive/how_to/4221212.html

Good luck
Maurice Alston

Thanks Paul and Maurice for your comments.
The multi meter tests out ok when the probes are touched together and the strips up the sides are flat (copper?) plate.
I have again tested for resistance across the whole element (a probe on each spade terminal)and I get a reading around 0.56(ohms?) - I originally tested for continuity using the audible warning device on the meter but apparently there was insufficient resistance for it to sound.
Not being technically very good with the finer points of electronics I can only assume, as I got a reading, that all is perhaps ok.
Regards
Peter M
Peter M (member)

Ah, if you were using the *continuity* test then no sound would have been because there was too *much* resistance (even at half an ohm) not too little. 0.56 ohms is lower than I would have expected, at 14v that is 25 amps! As the fuse is only *rated* at 17 amps (35 amps blow) I would have expected it to be no more than that i.e. 0.8 ohms. In fact higher even than that as it shares the fuse with everything else on the green circuit including brake lights, reversing lights and indicators. With all these going they are taking about 7 amps, so if the HRW took the remaining 10 amps that would make it about 1.5 ohms. I know the HRW does take a lot, on the original wiring of mine the high current resulted in a significant volt-drop in all the wiring and connections going back to the brown circuit and only left 7v at the HRW, and slowed the flashers right down as well. I added a relay which itself was powered from the green circuit, but which powered the HRW from the brown directly via its own fuse. This increased the voltage at the screen to 10v and made it much more effective, as well as reducing the effect on the flashers. This is on a V8 with twin electric fans that are *also* powered off the green circuit and the same fuse, it's wonder the wiring didn't melt when everything was on. I powered the fans directly off the brown in a similar way and got much better cooling from those as well.
Paul Hunt

Paul - thank you very much - I'll check the reading again but I'm quite confident that the screen is in working order now.
Kind regards Peter M
Peter Malkin

There has been some discussion on how to fix these - I thought I would share a tip I discovered while investigating one that wasn't working at all.

My first thought was to check the power & earth connections on the screen - these turned out to be fine but, while one probe was still hooked under the clip on the left hand side I noticed that if I ran the other probe along the wires painted on the screen I got a voltage reading.

So - to fix your broken wires you just need to run the probe along the wire (with the power turned on!) assuming the fixed probe is on the left, a battery voltage reading means the break in the wire is to the left of the current position, no volts means the break is to the right of the current position. Just work along slowly and put a dab of repair paint on the spot where the voltage goes from 0 to 12 or vice versa.

Once a wire is repaired you will get half battery voltage in the middle of the wire (and proportionate readings along the length).

This method certainly works on 1975 cars (with brown coloured wires) - I don't know yet whether other years had different types of rear screens that used different types of wire which don't give a voltage reading because they are insulated in some way.
Chris at Octarine Services

This thread was discussed between 10/11/2008 and 12/11/2008

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