Electrical Tachometer Conversion
Electrical Tachometer Innards AFTER negative earth resoldering
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.
Existing Wiring with Mechanical Tachometer
Proposed Wiring from GAN2/3 diagrams
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:
Induction Loop Wiring
(found in many Smiths publications)
Electrical Tachometer Wiring
Testing the Tachometer
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.
Digital Tachometer Tester
Mechanical Tachometer ini-situ
Front of mechanical tachometer
Rear of mechanical tachometer
Gearbox on rear of dynamo for mechanical tachometer
Next up, is to measure up and create the two patch wires needed. This first is from the induction loop to the ignition switch:
Piggy back connector to fix new induction loop wire with existing wires to 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.
Loosely position the electric tach in place.
Connect the new piggyback wire coming from the induction loop with the two white wires at the ignition switch.
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.
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.
Fit the illumination / panel lamp and the ignition warning lamp to the tachometer
Fit the clamp, refit the earth lead, and tighten down with the thumbscrews
Tidy up and secure new and existing wires to prevent movement.
Original tachometer clamp (top) and replacement (ex Triumph Dolomite - bottom)
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.
Electrical Tachometer in-situ
Resources for Electrical Tachometer Conversion
Credits where due to all contributors....