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MG TD TF 1500 - Heater switch wiring

How does one wire in the illuminator bulb in a 50's heater switch? I have the switch wired to acc power and it works correctly but there are only two connections, so I'm not sure how to wire the bulb so that it lights and I'd like to light up with the dash lights as well. The two connectors on the switch are M and B.
Geoffrey M Baker

I would guess M is motor (fan motor) and B is Battery.
That would mean the lamp is internally connected to the switch
However there should be a ground (earth) connection. Perhaps the switch body?
If what I postulate is true the lamp will always be on when the fan is running.
If you try to tie it into the dash lights the fan will only run when they are powered.

Jim B
JA Benjamin

My heater switch is wired as Jim said and the bulb illuminates when I turn the switch to the ON position.
Jim Merz

Jim is correct. The heater switch must be grounded (usually through the bracket) in order for the lamp to illuminate.
kmclemore

Thanks, that was what I suspected... Which means I can't wire it to the dash lights. I will ground it.
Geoffrey M Baker

That's correct, Geoffrey. The light comes on only when the heater is on... it's not illuminated when the heater is off.
kmclemore

I did the same thing! Ground the bracket and you will be golden! :)
Rich King TD 8732

I'm grounded and golden Rich :)
I had hoped I could wire the bulb to the dash lamps so it would only come on at night, but it was not to be...
Geoffrey M Baker

I suppose you could wire in a relay that only closes when the dash lighs are on, and in closing it delivers ground to the switch.... seems like overkill, though.
kmclemore

This thread was discussed between 11/03/2015 and 13/03/2015

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