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MG MGB Technical - Calling Electrical Gurus

Im trying to change from the plastic rocker type HAZARD switch (1969 pillow dash) to an English Dash type. Problem is there are only 4 contacts on the ENG switch and 5 on the plastic one.

Can any of you see a way to rewire it so I can get the ENG type to operate my Hazard lights?

I have copied the existing wiring diag and added a pic of the ENG switch I would like to use. (its a simple 2 position SW)


Innes Bint

A diagram of the internal contacts of the switch is needed to determine if it will work. Which contacts are closed, if any with the switch off? Which contacts are closed with the switch on? Knowing that information may help.

Clifton Gordon

What's an ENG switch, and as Clifton asks how are its terminals interconnected in each position?

A hazard switch does two things, and has two completely separate and different circuits inside the switch.

A standard hazard switch has one on/off contact which is closed when the hazards are off and open when they are on. Its two terminals (1 and 2 on your diagram) conenct the green (fused ignition) 12v supply to the indicator flasher unit, and ensure the indicators can only operate when the hazards are off. The second circuit involves either three or four terminals (3, 4 and 5 on your diagram), which are all isolated from each other when the hazards are off, and all connected to each other when the hazards are on. These go to the hazard flasher unit, the two circuits which light the lamps at the corners of the car, and the console tell-tale where provided.

You can achieve the same effect (with the exception of a console tell-tale, and who needs one when the two indicator tell-tales will be flashing on the dash anyway) with a switch that has two separate changeover circuits. This is wired differently to the original and does involve cutting into existing wires. A diagram can be provided if required.

Anything else shouldn't really be used as it could cause the ignition and fuel pump to be powered when the hazards are on but the ignition off (and the key in your hand) which could cause a fire.
Paul Hunt

The ENG is my abbreviation of 'English' and refers to the SW I photographed for reference. I probabally should have put LUCAS?

I will have to inspect the SW and see which contacts are interconnected to see if it can be done. Yes a diag would be helpful thanks Paul. I certainly dont mind cutting the wiring if I am able to replace the horrible plactic toggle switches with the older 'lucas' ones?!

Innes (NZL)
Innes Bint

Diagram attached, based on the original you posted so hopefully self-explanatory. The dotted line between the two switch contacts indicates they are *mechanically* linked i.e. they operate together, but they are not electrically linked. I don't claim to have invented this way of using a convential two-pole changeover switch in this way as I saw it here or elsewhere some time ago, but it is a neat way of getting round the issue of finding a 'proper' switch with the unique contact action in this role.

However whether you can find a Lucas toggle switch with this configuration is another story, but that is easily resolved by getting a relatively standard 12v *relay* with the required action, and using a standard on-off switch e.g. the one you photoed to operate that. In that configuration the on-off switch would need to be fed from the brown terminal on the hazard flasher, the other side of the switch to one side of the relay, and the other side of the relay to ground, to ensure that it operates under the same conditions as the existing hazard system.

Paul Hunt

This thread was discussed between 25/05/2008 and 26/05/2008

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