BT-H Magnetos Ltd.

Wiring Notes for the BT-H Mag-Alternator.

Many British bikes were fitted with a Lucas Magdyno.   The generic Magdyno equipped wiring diagram is provided as a general guide only to what you may have on your machine.  There are too many variations to be able to give specific wires/colours.

The dynamo output and voltage on these systems was regulated by a control box.  The control box had four connections labelled “F”, “A”, “D” and “E”.  (They are usually in that order but not always.)  The “F” and the “D” were connected by wires to the dynamo.  The “A” connection was the output and was connected to the ammeter / headlight switch.

The “F” and the “D” wires are not used when a BT-H Mag-Alternator is fitted.

Many early machines used a positive (+) earth system.  Your BT-H Mag-Alternator can be wired to work with a positive or a negative (-) earth.  Make note or label which wires on your machine connect  to “A” and “E” on the control box.

The control box should be removed or all wires disconnected.

Connect the two yellow wires from the BT-H Mag-Alternator to the two yellow wires on the rectifier regulator. (It doesn’t matter which yellow goes to which yellow as it is A.C. current.) The red and black wires from the rectifier regulator provide the D.C. current.

(N.B.  The makers of the rectifier regulator that we supply as part of this kit recommend that resistor (5K) plug caps are used on ignition systems as electrical interference can affect the correct functioning of their unit. )

Decide if you are going to use positive (+) or negative (-) earth.

If you are going to use positive (+)  earth connect the red wire from the regulator rectifier to a good earth point.  If you are going to use a negative (-) earth then connect the black wire to your earth point.

The opposite colour wire from the rectifier regulator should be connected to the “A” wire in your original circuit. (e.g.  If you connected the red positive (+)  wire to earth then the black (-) negative wire will connect to the wire going to the ammeter.)

Replace all your lighting bulbs with 12 volt unit and fit a 12 volt battery.  Make your battery earth lead the same polarity as the earth lead from the rectifier regulator.  (e.g.  If your rectifier regulator has the black (negative) lead to earth then your battery must be negative earth.)

We recommend that a fuse be fitted in the battery supply line if possible.

If your ammeter reads the wrong way then swap the leads over on the ammeter or change your wiring from positive earth to negative or vice versa.

The generic mag-alternator wiring diagram example  is provided as a general guide or starting point and is not a definite circuit.

Copyright © 2008 E. Grew