MGB GT Heated Rear Window

MGB GT Heated Rear Window

I’ve just purchased a 1974 MGB GT and noticed that the rear window does not defog when the switch is turned on. My question is what should I be looking for? Other then the switch and the window grid, are there any other components? Does the system work on a simple resistance principle in which the heat is generated by resistance in the grid? Without resorting to expensive equipment, how do I check it out?

Tom Rawlings

Fortunately the GT HRW is a simple affair with just a switched power feed to the window and an earth on the other side. Power is supplied via a green wire that is fed directly from the main fuse box. This goes to the switch on the dash where it then passes to the rear window element. In the switch is a spur from the switched side of the power supply that goes to the warning light, which has a separate earth. If the lamp illuminates when you switch the HRW on then at least you know you have power to the switch. If not then it means a slightly deeper check procedure.

Switched power feeds the elements on the window and the resistance in these lines creates the heat for demisting. It is almost unheard of to find a break in every one of these element wires, so if power was passing through the elements then at least some should be demisting.

Check all visible connections for a good clean fit.

Connections
Assuming that the switch tell tale lights up then get a multi-meter and locate the wires on each side of the rear window. You will find a spade connection between the element common ends to which is connected a wire. White with black tracer is feed side and black is the earth path. Select a volt scale that can read up to 16 volts. Connect the positive lead of the meter to this terminal and the other to a good earth position. With the HRW switch on, there should be 12 volts (or close to it) on the scale. If there is, then go to the other side and slide off the spade connector and using the continuity scale place one probe in the detached wire end and the other to that good earth on the body. You should have a good clean continuity showing that the earth path is sound.

Elements
If the earth path is sound then remove the other spade connector on the other side and place the probes on each spade terminal of the glass. If all cross elements have been broken then you will have no reading. If only one element is good then you will get a reading, and that element should be giving you demist. If not then the demist elements are broken. Breaks can be found by very detailed examination of the elements and there is an expensive electrical ‘paint’ that remakes electrical contacts on HRWs. Note that this contains gold, so a very, very small volume costs in excess of £10. The paint does not have a long shelf life once opened, but is still far cheaper than a new rear window!

Tell tale light
Assuming that no light comes on the tell tale you should go to the fuse box and note the terminals with several green wires attached. Check both the fuse. the holder and the fusebox for damage, corrosion and other obvious fault. You can replace the fuse with another or check this one out with the meter. Rectify any faults, which often occur due to the quite heavy loads of this circuit leading to corrosion, fuse failure and even some heat damage of the holder.

Switch
Once power is being fed through the fuse to the green wire you need to go to the switch and check the light again. If still off then you will have to extract the switch and start to probe the wires at the back of the switch, red probe to wire and black to another good earth on the body. Probe the green wire first and see if you have 12 volts. If not then there is a break in the wire somewhere and the simplest solution is to lay in a new wire. If you have 12v then check the white/black wire and see if that has a switched12v as you operate the switch. If it does, then there is a break in the wire to the HRW and whilst this is easier to dig out than the green wire it is still easier to lay in another new wire. If the tell tale light still isn’t working then there is a broken bulb or bad earth connection for that alone.

Once you have achieved power to the HRW and with the earlier checks showing the rest of the circuit to be good then when the damp strikes you should be able to switch the HRW on and see a rear view emerge through the gloom!

Roger Parker