With a proposed dynamo / alternator swap in the offing, I was also going to need to change the tachometer from mechanical to electrical. I got my hands on a positive earth electrical tachometer, which needed a new induction loop block. My car is already converted to negative earth - done a few decades ago to allow for a “modern” radio, which meant I would have a few more steps to follow:

• Do my research...

• Convert the positive earth tachometer to negative earth

• Understand the wiring requirements

• Test and calibrate the electrical tachometer

• Installation

Research.

I found a wide range of sources to help me in my task: John Twist- University Motors, MGA Guru, MGB Stuff, Moss Motors (USA), Triumph Club of New Zealand. I am also most grateful for the support and help provided by Dr John E Davies of this club, who helped to validate and confirm my findings and intentions.

Convert the Tachometer to Negative Earth.

Now if you have a positive earth car, you won’t need to do this part. It requires the dismantling of the tachometer to get at the innards, and the desoldering swapping round and resoldering of a (green) wire and a capacitor (resistor?). John Twist video most useful for this. I also blacked out the “posi” on the face of the dial, and put in a little “-” sign. Back of the case was also marked “-ve earth”. Gave the thing a good clean up, fitted new rubbers to the bezel, and while the case was still off, drilled an extra hole in the back, to allow for calibration.

Understand the Wiring Requirements.

There were plenty of resources telling me how to do this task, but none actually showed the wiring requirements. With the help of Dr John, I finally unravelled the mystery by working with the wiring diagrams from my Sprite and Midget Workshop Manual Issue 3 65194 and MG Midget and AH Sprite Haynes Manual 265 1982. I was then able to draw out the wiring diagram showing the specific components.

The main change is to “develop” the wire coming from the coil (SW +), and to extend this so that it then passes through the induction loop on the back of the electrical tachometer and from there on to make a new connection at the ignition switch. The electrical tachometer also requires a 12V + feed. There was a lot of “noise” in the resources I found about getting the wires / current flow the correct way around. I created the patch lead that wound around the induction loop, using this diagram:

Noting that SW was coming from the coil, and A3 was going to the ignition switch.

Here is the electrical tachometer all wired up:

I used slightly longer wires than probably required, but this was to give me a bit of wiggle room when installing under the dash. The earth lead (black) is just for show (and used for testing) there is an earth connection in the loom. The 12V+ lead is the green one. You can see I have marked up the wires coming from the induction loop: “+” heads off to the coil, “⏺” goes to the ignition switch. You can see the hole(s) drilled for access to the calibration slot.

Testing and Calibration

I jury-rigged the electrical tachometer in the engine bay for the test. I made up a patch lead to run from the ignition switch, and used a piggy back spade connector to connect up the two conjoined white wires (thin one to warning list, thick one to fuse box) and my patch wire. The 12V+ lead was connected to the fused side of the fusebox, along with all the other green wires there. A suitable earth was found and secured. The induction lead marked “+” was connected to the coil lead that emerges from the loom at the fusebox, and the induction loop lead marked “⏺” was connected to the patch lead coming from the ignition switch. No sparks or smoke yet!

Started up the car and let it settle to tickover. I had a reading!! It was way off at @ 3500rpm (tickover was @ 1000 rpm on the mechanical tachometer). If you get no response on the tachometer, you may have the coil and ignition switch leads round the wrong way. Switch them over and see if you have results. If still no joy, could be that your tachometer is broken in some way). I used my digital tachometer to get a truer reading from the car.

This works by placing a piece of reflective tape to the crankshaft pulley, then pointing the tester’s light beam at the pulley. The output produces up to three readings which I can then average. I then turned the dial on the calibration wheel to match. Worth noting that the mechanical tachometer was running some 250 rpm high. I then turned up the engine speed to 2000 rpm and tested, adjusted calibration again. Will do more testing/calibration once fitted correctly to the car.

Installation

I didn't feel the need to disconnect the battery for this one, because all the wiring is ignition live, but you can if you wish.

First job is to get the mechanical tachometer and cable out.

1. Unscrew drive cable from the back of the tach, unbolt drive cable from the back of the dynamo, then remove the cable into and then out of the engine bay

2. Pull out the ignition warning lamp and the illumination/panel lamp from the rear of the tach

3. Undo the two thumbwheels holding the clamp in place

4. Release the earth cable.

5. Remove the clamp, then draw out the tach into the car

Next up, is to measure up and create the two patch wires needed. This first is from the induction loop to the ignition switch:

Then make up another wire to go from the induction loop into the engine bay to connect up with the wire coming from the coil. I fitted a new pass through grommet in the hole in the bulkhead left by removing the drive cable. I used this for the coil wire and the 12V+ wire coming from the fusebox.

Use heatshrink tubing to protect and insulate connections.

I found that the holes in the original clamp were in the wrong place for the electrical tachometer. I located a clamp in my parts bin from a Triumph Dolomite which fitted perfectly. Instead I could have just drilled new holes in the original clamp.

1. Loosely position the electric tach in place.

2. Connect the new piggyback wire coming from the induction loop with the two white wires at the ignition switch.

3. After passing it through the bulkhead to the engine bay, connect the other wire from the induction loop to the coil lead, at the fuesbox.

4. Connect the 12V+ wire to the tach then pass through the bulkhead to the engine bay, and connect to the fused side of the fusebox, with all the other green wires.

5. Fit the illumination / panel lamp and the ignition warning lamp to the tachometer

6. Fit the clamp, refit the earth lead, and tighten down with the thumbscrews

7. Tidy up and secure new and existing wires to prevent movement.

Things should now look like this. All that remains is to start the car and check that your tachometer is again working correctly. Do safety checks to ensure all wires are correct and nothing is getting hot or melting! The benefit I appreciate the most is the smooth and steady motion of the needle. The mechanical tachometer needle would wobble and flutter about, rarely, if ever steady.

The mechanical tachometer, gearbox and cable will be wrapped and put in a box for safe keeping, should i ever feel the need to return to the original setup.

Resources for Electrical Tachometer Conversion

John Twist University Motors - Convert Positive Earth Tachometer to Negative Earth

MGB Stuff - Tachometers

MGA Guru - Electrical Tachometer

Triumph Club NZ - Smiths Tachometers

Sprite and Midget Workshop Manual Wiring Diagram

Moss Motors - Alternator Conversion

Credits where due to all contributors....