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MG MGB Technical - Hazard light

Any idea why my Hazard light on the dash would stay on all the time, key in key out? Runs down my battery.
78 B rdstr
Cleve Crews

Check the hazard flasher. Not sure about a USA '78 but on earlier UK models it is clipped to the central stiffener, behind the centre console.
Also the wiring to the multi-plug on the hazard switch is prone to breaking.
Allan Reeling

Allen,
Just the dash light is on and not the outside lights
Cleve Crews

The flasher units are dependent on current draw for them to work properly. Which is why converting to led bulbs, which draw tiny currents, renders traditional flasher units useless.
.Do the normal indicators work?
Presumably your B has the addition of side repeaters,so check all bulbs and connections.
Allan Reeling

Hazard flashers work on voltage, not current. It is the indicator flasher unit that relies on current, and that way gives the driver a warning that one of the bulbs has failed. Hazard flashers by contrast are required to flash even if one or more corners might have been damaged in an incident, and also to be left on for some time with the battery voltage dropping, both factors causing large changes in current, but the flasher still flashes.

Which hazard light - the tell-tale in the bank between the speedo and tach? Or the tell-tales either side of the speedo and tach?

If the former then either the hazard switch or its wiring is incorrect. The hazard switch should have six wires - two greens that are normally connected together by isolated when the hazard switch is on, and four other that are normally isolated but all connected together when the hazard switch is on. Of these green/white and green/red red go to the corners of the car, light-green/brown comes from the hazard flasher unit, and light-green/purple goes to the hazard tell-tale in the centre of the dash. I know North American spec required this but it is superfluous as the two green tell-tales are flashing when the hazards are on. It might be easier just to disconnect this light-green/purple.

Paul Hunt

Paul,
It's the one in the middle that says HAZARD that stays on.
I just got the car home yesterday, so I'm going to check it out further today if the weather holds out.
Cleve Crews

Paul, I thought both flasher worked on the same principle. Is the hazard flasher unit then not a bi-metalic, heater coil type? If you put LED bulbs in the indicators the hazard flasher still won't work. That's current drop but not voltage!
Confused of Telford!!!
Allan Reeling

Guys,
I fiddled with the hazard switch, turning it off and on and the light went out.
Now for a little different problem, the head lights and running lights don't come on. Pulling back on the turn indicator turns them on as it should, but not with the light switch.
Keep in mind, I just brought this car home and it has been stored inside since 1992.
Cleve Crews

Allan - the original hazard flasher does have a heated bi-metallic strip, but perhaps the current of even four LED 'bulbs' is below that of one incandescent and not enough. Normally if you use LEDs for indicators you have to add a load resistor each side (which means the 'bulb failure warning' is then simply a 'resistor failure warning'), which should then mean the original hazard flasher would still 'see' the same current as before and so work normally. If you use an electronic i.e. 3-pin LED indicator flasher so you don't need the load resistors, then you don't get the failure warning at all, and in that case hazards might not work either without a similar 3-pin electronic unit.

Clive - the main-beam flasher uses a different voltage supply to the main light switch. With neither coming on you need to check you have 12v on the brown wire at the switch. Bear in mind that unlike the headlamp flasher supply the main lighting switch supply is not fused.
Paul Hunt

This thread was discussed between 08/03/2015 and 10/03/2015

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