MG Midget
Alternator Conversion

I had a plan, long before I finished the restoration of the car, to fit an alternator. I bought the alternator and the bracket for it in 2017. However, I had also decided to get the car working as it was with the dynamo, and to then start upgrading / tinkering. Two years on from getting the car back on the road, I had all the bits and bobs required to get an electrical tachometer working too. It was time. I did the tachometer conversion first, then tackled the alternator. The trickiest part for me was to understand the wiring requirements for both jobs, and to confirm that what I was about to do would not see the car bursting into flames! I have Dr JE Davies of the MASC Club to thanks for guidance and support. Note: as a reminder, my car is negative earth.

Dynamo in-situ

This has been a trusty thing, serving me well since the late 1970's when I first took on the car.

Existing wiring to the Control Box / Regulator

The dark arts of the control box wiring still elude me, and I believe I fitted it around the wrong way...

Control Box Wiring Detail...

For posterity, and as a reminder should I ever decide (need) to go back to a dynamo and control box

Existing Wiring Diagram

as seen in the car ( the wiring should be the same for frogeyes apart from the dynamo wires, which are both plain yellow). The thick brown wire from "A" actually disappears off into the loom, reappearing under the dash. This is then pushed through the bulkhead and attached to the battery terminal.

Bus Bar for the 3 x 9.5mm spade connectors and 1 x 6.3mm connector

Most of the guides I found would have had me cutting off the spade connectors and soldering everything together. I didn't want to do this to my new loom, so came up with the idea of a "bus bar". I located an old stainless steel kitchen knife and took to this with my dremel-like tool. This image was the first attempt, it required further adjustment for the final fit. I intend to make a little case for it to live in, for additional protection and insulation.

Alternator, bracket and fan belt

These have been sitting in a box on the shelf in the garage waiting for me. Soon to be replaced with a box containing a dynamo.

Before starting, disconnect the battery at the earth terminal, make sure the car is "dead" before proceeding

Dynamo removed

Easy out...I am always surprised as to how heavy this thing is. I need an elbow connector for the oil cooler pipe!

This is where all the head scratching started. None of the guides seemed to match my wiring, until Dr John pointed me in the direction of the MGA Guru pages, which seemed to have the best fit for what I was about to attempt. There is also a helpful guide from Moss Motors (USA) The diagram below is what I planned and what I used.

New wiring

You should be able to see the "bus bar" with the three heavy wires and the lighter gauge brown wire attached. I made up a short patch lead with two male spade connectors to join the two wires for the ignition warning light. I have "earthed" the black wire, not sure if this is needed. I have work to do to properly arrange and fix all this wiring, target area below the flasher and the hinge. or all up on the bulkhead.

I followed the Moss Motors guide for the fitting and installation of the alternator. The "1275" fan belt I had purchased was much shorter than the one I had on the car, I wanted to use it so had to fabricate a new adjustment bracket (the existing one was far too long). I also found it easier to get the fan belt over the pulley before loose fitting the top mounting bolts.

Order of assembly (loose fit everything)

  1. Fit rear mounting bracket

  2. Attach adjustment bracket to lower alternator mounting point

  3. Alternator into engine bay, and attach the adjustment bracket to the pillar mounting

  4. Work the fan belt over the pulley

  5. Fit water pump top mounting bolt

  6. Fit rear top mounting bolt

Place a lever (crowbar) between the alternator and the block to get tension on the fan belt

  1. Tighten the pillar nut (once this is done you can remove the crowbar)

  2. Tighten lower alternator mounting bolt (with adjustment bracket)

  3. Tighten water pump top mounting bolt

  4. Tighten rear top mounting bolt

  5. Tighten rear mounting bracket

Check for alignment, and use washers etc. to assist with adjustments. Do a final tighten up all round

Connect the two cables to the alternator

Alternator in place - close up

Alternator in place

Check your handiwork against the wiring diagram again, making certain that you have everything in the correct location, connected to the correct thing.

At this point I dug out my fire extinguisher from the boot, and grabbed an old towel and wetted it.

When you are happy, reconnect the earth battery terminal. "Nothing" should happen, but wait a little, check for any smoke, both in the engine bay and under the dash. Check the wires for heat etc. All should be well....

Finally, moment of truth time.....

Turn on the ignition. Again, nothing should happen, but do your checks again.

Start the car!!

If all has gone well, the car will start, the ignition light will go out, and there will be no smoke or other electrical dramas, but do your checks again.

I tested the voltages of the car (at the battery terminals) at tickover (800 rpm) for the dynamo before removing it, and for the alternator:

  • Dynamo: 12.3V

  • Alternator: 14.02V

I ran the car for a while, gave it a few revs, but didn't test drive as it was raining and I had the roof stowed in the boot.

Switched off, did checks again, and checked all the mounting bolts for tightness, and fanbelt.

There we are then, all done, and completely reversible. The dynamo and control box will be wrapped up and put in a box for safe keeping.


MGA Guru Alternator Pages

Moss Motors Alternator Conversion

MGB Stuff Alternators

MASCOT - Apr 2019 issue p23-24; Jul 2011 issue p14-15