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Electrical Instructions for Replacing the Generator with a GM Alternator in a Spitfire MKI, MKII, MKIII, or a GT6 MKI

by Dan Masters,

Important Note: this entire article assumes a previous conversion to negative ground!

Method One (Utilizing existing wiring)

You are finished!

There are two things, however, to be aware of:

  1. Triumph did funny things when they built these cars, so your car might not match the configuration above, and it may well have been modified by a previous owner.
  2. You now have an alternator capable of more than twice the output of your old generator, but the wiring capacity has not changed. If you add heavy loads, driving lights, high power stereo, etc, you can exceed the capacity of the wires. Also, If you let the battery discharge completely, the alternator can possibly recharge with enough current to overload the wires. If your battery is completely discharged, recharge it with a charger rather than push starting the car and letting the alternator recharge it. Under all other usages, the wiring should not present a problem. The main advantage of the higher output is the ability to provide a higher charge rate at low RPM and idle. The standard loads on Spitfire or a GT6 do not require a higher charge current at normal engine speeds, so the alternator will not be called on to provide enough current to overload the wires.

Method Two (Upgraded wiring)

If you wish to upgrade the wiring to take advantage of the higher output, it is really quite simple.

As above, except:

Cut off both ends of the large Brown/Yellow wire - at the old generator and at the control box - as close to the wire harness wrapping as possible (or, unwrap the harness, and remove the wire all-together).Connect the three Brown wires together at the control box.

Instead of connecting the large Brown/Yellow wire to the new alternator, add a new wire of at least 10 Ga (8 Ga preferred). Connect one end to the screw terminal at the alternator, and the other end to the terminal on the starter solenoid where the main cable from the battery and a 2 Brown wires are now connected. Leave the existing wires at the solenoid connected. Very carefully route this new wire alongside the existing wiring harness, and use cable ties liberally for support.

Now, the alternator can provide full charging current without worrying about burning up the wiring.

If you wish to add extra loads, such as a high power sound system, connect them directly to the battery, properly fused, of course. Loads connected directly to the battery will not be indicated on the ammeter.


You may wish to add a voltmeter. If so, simply connect the plus terminal of the voltmeter to any convenient Green wire, and the minus terminal to a good ground point. One side of the ilumination lamp inside the voltmeter should be connected to the most convenienet Red/White wire, and the other side should connect to a good ground point (unless the lamp is grounded to the meter case. If it is, ensure that the case is properly gronded to the metal dash structure).

You may also choose to use a different alternator , rather than the GM unit. If so, it will be wired very similarly to the above, only the connections at the alternator itself will be different.


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