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MG MGB Technical - rear screen heater

My rear screen heater does not appear to work and the warning light wired to the switch only works intermittently. Not checked yet but switch might be faulty. Reason for this question is that wires to and from the switch seem very light; is this correct?
R E Merrall

Depends whether there is a relay in the circuit.
Allan Reeling

from my own previous enquiries, seems it draws about 150w so a huge wire not really needed. but a relay would be wise.
mine is bust too, tried those conductive paint kits to no result and an electric fan heater (12v) which was hopeless.
a new screen is i think the only real solution, for me anyhow,
G
Graham Moore

The factory wire for the element is thicker than what I would call the normal gauge for MGB wiring. That for the warning light would be thinner i.e. 'normal' gauge. I measured my HRW at 1 ohm which implies 14 amps at running voltage and equates to nearly 200 amps. However with the inherent resistance in the wiring and switch at loads that high the voltage at the element and hence the current drops, before I fitted a relay I was losing nearly half. With a relay in the engine compartment the voltage at the element increased to about 10v, or 100 watts. With the relay at the rear somewhere and tapped directly off the battery (it must be fused) you would probably get higher still.

If the warning light is intermittent the voltage to the element is probably intermittent as well, hence it may not appear to work. You can tell by watching the interior light while turning it on and off, the light should definitely dim slightly with the load of the HRW.

If not you need to do some tests with a voltmeter. Measure both screen terminals against a good earth from somewhere else. The 12v supply should be on the drivers side and the earth on the passengers. If there is no 12v on the 12v connection there is a bullet connector by the hinge, and another down by the rear light cluster I think, and another in the engine compartment where the white/black wire from the main harness joins the rear harness. These last two are convenient places to insert a relay.

If you see 12v on the earth connection then the earth is bad. There is a bullet connector by the hinge that side, then it should be screwed to the body nearly.
Paul Hunt

Thanks for all the advice. I have checked the switch which appears to be faulty; sometimes works but mostly no reading. I have ordered a new switch. The wiring diagram I have, for a 1975 rubber bumper bgt, shows 2 outputs from the switch, 1 to the warning light and the other to a relay. I am assuming that is to the relay under the bonnet near the fuse box but haven't had time to check it yet.

Rod
R E Merrall

Rubber bumper cars until the 77 model year did have a dedicated relay, which would explain the thinnish wires at the switch.

77 on had an ignition relay which took on that job as well as the rest of the ignition circuits ... until 1979 or when because of sticking relays most of the circuits including the HRW were moved back to the ignition switch!

There should be two relays by the fusebox, the other will be the starter relay. The HRW relay should have purple from the fusebox which is the 12v supply to the screen, black (earth) and a third wire that comes from the switch. This is shown as green on the diagram, but is logically incorrect as that is the colour of the wire carrying the 12v ignition supply to the switch i.e. its input. The other wire on the output of the switch is red/brown to the warning light, one of two joined together on the output, so check what colour that is.

Bridge the switch wires, and see if you can hear the relay clicking. If not you will need to do some voltage tests on the relay, to see if you are getting 12v on the mystery wire and have an earth on the black - these two should go to W1 and W2 on the relay but it doesn't matter which way round.

If the relay clicks see if the interior light dims slightly. If it does then almost certainly the HRW is being powered to some extent at least. If the interior lights doesn't dim then with the relay operated check you have 12v on the purple and the white/black, which should be on C1 and C2 of the relay - again it doesn't matter which way round.

If no 12v on the purple then there is a break back to the fusebox - although this is probably the shortest wire on the car.

If you have 12v on the purple but not the white/black then the relay is faulty.

If you have 12v on both then you need to check the bullet connectors mentioned earlier.
Paul Hunt

This thread was discussed between 16/11/2015 and 20/11/2015

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