Alternator Conversion

By Tommy Sandham

I recently carried out this conversion on a 1965 G.T. Cortina MK1. The usual magazine articles imply it is a straightforward job. I might be thick, but I encountered a few minor problems along the way. This information should save you repeating my mistakes.

There are many suitable alternators to choose from. Mine came from a MK5 Cortina estate we were scrapping. It is a Lucas alternator, probably pushing out 34 amps. This is a big improvement on the standard 22 amp dynamo, (Lucas C40) or the heavy duty 25 amp dynamo (C40L).

  1. You must convert to Negative Earth. Swap the battery round. This may involve changing the battery leads. My leads bolt on to the battery, so there were no problems. If your leads clamp round the battery terminals, you may have to purchase new leads.
  1. Swap the LT and IG terminals on the coil.
  1. If you have a radio, it needs to be changed to Negative earth. This may involve changing a link or changing a setting on the back of the radio.

Rev Counter

The standard Smiths rev counter (the one with "Positive Earth" on the face plate) can be converted. You need a soldering iron and a steady nerve. See the circuit diagram below!

Remove the chrome ring from the front of the rev counter, by rotating the chrome ring until it comes away from the body. Remove the glass and rubber ring.

At the back, remove two screws (the ones with washers). This will release the instrument from the body. Treat it with care as it is a moving coil instrument which does not like rough treatment.

Identify the following components:

A resistor goes to the 12v tag.

A short green wire goes to the earth tag.

Swap these connections. The resistor now goes to the earth tag. The short green wire to the 12v tag.

My 15W iron was not man enough for the job. I had to borrow a 40W iron. Take care when soldering not to overheat the resistor.

Reassemble carefully. Label the rev counter on the back, clearly, NEGATIVE EARTH.


I disconnected my ammeter. The normal 30-0-30 will not work well with a 34 amp alternator!

Converting the Alternator

You may find that your alternator was built to run on the left side of the engine block, whereas on the MK1 Cortina, you want it on the right side of the block (where the dynamo is now!).

Remove two small headed screws holding the plastic cover on the end of the alternator. You will now have access to three long screws which hold the two halves of the alternator together. Remove these three screws.

Gently rotate one half of the alternator against the other until you get two mounting holes to line up (these will match with the dynamo bracket) and one mounting hole for the fan belt adjustment bracket. Replace the screws, replace the plastic cover and the conversion is complete.



Remove your old dynamo and control box.

Mount the alternator on a new bracket, or in my case a modified dynamo bracket. You will need a long bolt and some spacers and/ or washers. I ended up cutting my dynamo bracket so that I was left with two right angled pieces of steel. This may not prove to be ideal, see below.

Make sure the pulleys line up properly. In my case they didn't, so some time had to be spent sorting this problem.

Next, the fan belt. I had to use a small bracket to extend my dynamo adjustment bar. I used a Ford Escort fan belt, part number ??

This keeps the alternator nicely away from the four-branch exhaust manifold.

Use the cable connector which comes with the alternator. Mine came from a MK5 Cortina, so I cut away a chunk of wiring loom with the alternator. If buying from a scrap yard, discuss before cutting or they might want to charge you for destroying a wiring loom.

There are three connections on an alternator. These are:

  1. Positive output,
  2. Sensing connection,
  3. Ignition warning light connection.

I messed up here. I assumed the thin blue wire in my wiring loom was the sensing connection. WRONG! The thin blue wire is for the ignition warning light. If in doubt, check with a meter or continuity tester.

The sensing connection on some alternators is a straight connection to the Positive output connection. In other cars, such as the MK5 Cortina, the sensing connection consists of a foot or so of heavy gauge wire crimped into the Positive output wire. This is revealed when you peel back the insulating tape.

When you plug the connector into the alternator, I managed to get mine in the wrong way round! I don't know how, but I did. Be very careful to get the connector in the right way round. Then fit the wire clip which holds the connector in place.


I got really upset here, as the standard magazine article makes it sound so easy. Follow my advice and you will have a much neater installation. NOTE: My car had the Regulator box mounted on the inner wing. If your car has the Regulator box mounted inside the car, next to the glove box, you will have to modify the following information.

  1. A branch from the wiring loom takes three cables down towards the dynamo. Two are for the dynamo, the third is for the water temperature sensor.
  2. Cut and peel away the grey insulating tape from the dynamo end of this wiring. Cut back to the main wiring loom. Disconnect the heavy Brown/Yellow cable and remove it. It is not needed.
  3. Remove the Brown/Green cable as well. It is not needed.
  4. Remove a short Black earth cable. It is not needed.
  5. This should leave you with a heavy Brown cable, and a thinner Brown/Yellow cable.
  6. Tape up the Brown cable. It will be Live, but is not needed.
  7. The Brown/Yellow is connected to the Ignition Lamp cable from the alternator. Crimp or solder this connection.
  8. The main Positive feed from the alternator is run to either the Battery Positive terminal OR the solenoid Positive terminal. The cable must be crimped with the appropriate end, in my case a ring. I ran the cable round the front of the car, using existing metal clips and the lighting cables to build a neat "loom". My cable terminates on the solenoid, as my battery is in the boot.

Now tidy up the wiring loom. Use grey electricians tape, or spiral plastic wrapping, or combinations of the above and tie wraps.

The water temperature sensor cable can be wrapped in with the alternator cables, or run neatly and tie wrapped in place.

Draw a wiring diagram of what you have done. Keep it in a safe place.

Identify cable colours etc, or the colour of the insulating tape used to tidy your loom.

NOTE: You must keep cables tidy, well insulated and with the correct ends crimped on. I have had a Volvo go on fire (not through an electrical problem I hasten to add!) but it is an experience I would not like to repeat.

Most amateur electricians use taped joints, or just wrap bare ends round connectors. They also run wires directly from point A to point B instead of routing them neatly. This WILL NOT DO.

Double check everything, connect the battery and switch the ignition on. The generator/ ignition warning lamp should glow. Start the engine. The warning lamp should go out. Your conversion is complete.

 Smiths Tachometer Diagram

tacho.gif (29769 bytes)


 Used by kind permission of Glenn Wallace.

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