MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - MGBGT HEATED REAR WINDOW

How can you tell if a heated rear window works when it's not in the car? I have two windows to pick from.
TOM TREPAC

Use an ohm meter to check continuityreal easy-Ric
Ric

Test for continuity, as Ric says, but test from the center of each line to each of the opposite edges. (Center to right, then center to left) This tells you that each line has continuity to the "power" strips on each side where they draw their power, without being confused by tester current running through other lines. That could happen if you only tested from one side (power strip) to the other. Many used rear defogger windows have broken elements (lines) at this point in time, and this will help you find them, if there are any. If you find a good window, be grateful.

Your last check should be for continuity between the two spade lug connections on each side (edge) of the glass. This is where the juice comes into the grid, and if you don't have continuity here, you may need to fix the connection between the spade lug fitting on the edge of the glass and the side strip that distributes the juice to the individual elements (lines). There are repair kits available, http://www.frostfighter.com/

Good luck!
Bob Muenchausen

Bob, that will work for some defrosters but not all. My GT has the grid embedded in the glass and is composed of nearly invisible wires. I'd expect all you could do for that one is to test continuity at the wire connectors. Of course I'd also hope that it's less likely to be damaged...

Could you not mist the glass with a plant mister and then hook the defroster up to a battery?
Rob Edwards

Rob,

You could could use a mister, or you could chill the glass with some ice for awhile, and then let natural condensation show up. Then try the grid out with some voltage applied to it. My understanding is that the type of rear window you mention was only available for a short while when they first offered a heated rear window. This type was replaced by the more common type of surface mount grid setup.

Note that these grids take a fair amount of power (they are usually one of the biggest current draw items in a car) so use a 12 V source that has some amps behind it - like a battery or one of those quick start battery chargers.
Bob Muenchausen

Continuity will only tell you if at least one of the lines is operational unless you know exactly what resistance you are looking for. Bit of a bummer if you buy and install it then only find one is working ...
Paul Hunt

True, but in this case Tom said he had two to pick from. Given that, he could test both and pick the better of the two by picking the one with the lower resistance. But as you rightly point out, this doesn't guarantee *all* of them are working.....

Rob Edwards

I have a spare deck embedded grids in the back-light. It came from a parts car so I have not tested it for operation, however, it measures 65 ohms. Don't know if that's a good figure. They are fuse protected with a 25 amp line fuse according to my owners handbook. FWIW, Clifton
Clifton Gordon

My post should read, "I have a spare deck lid with embedded grids in the back-light". Clifton
Clifton Gordon

Hi..

A resistance of 65 ohms will draw about 200mA (0.2A) from a 12 volt supply, resulting in power dissipation of 2.4W, which isn't nearly enough.

I would be expecting a figure closer to 1 ohm, which would draw 12A, giving power dissipation of 140W.

(all figures approximate).

Don
Don

Don, Thanks, I should have fiqured that one out. In any case it looks like I have a bad heater in s good back-light for a spare. Clifton
Clifton Gordon

Hi..

The HRW on my GT measures 3.2 ohms, BUT ..
I have never used it, it may not be fully working.

The HRW on my Land-Rover (which does work) measures 1.7 ohms.

Hope this helps... Don
Don

This thread was discussed between 09/04/2003 and 14/04/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.