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I knew that, so I re-worded it.
Anyway, that eliminates the BM and points to the Regulator.
But , before changing that, make sure you have the Var V at that third wire, as that is what triggers the Reg.

I mention that wire as yellow, but it may be a different color b/c I do not have the schematic for your model. The 124 uses yellow.

That 3 wire feed plug should have a ground, [ neg] a 12V Hot [pos+] and the third will be the Var V from the Control Panel.

If you have 12 V from the plug pos and ground and Var V on the other wire
and ground..then you need a regulator.

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A Dalton