Auto Electrical

Re-wiring a Mark 3 Tachometer
Mark lll Sprite Tachometer
A brief history, I purchased the Austin Healey Sprite Mk lll with the tachometer not working.
After dismantling the tacho and conducting simple resistance tests I found that one of the movement springs was fractured. I searched around and found an old 100kPa pressure gauge that had a similar spring in number of turns and the strength of the spring action.
With magnifying glasses I replaced the spring and linearised the movement by applying a calculated input (mV) to simulate the range.
After more tests I found that both transistors were open circuit and was not surprised since the broken spring had left arc marks on adjacent components.
I set about constructing a modern circuit board to update the components without the need to change the original wiring to the tacho. The finished board with components cost $AU20.00.
I included two adjustment points, one is a simple calibration adjustment and the second is a rejection circuit to minimise coil current oscillation that would double or triple the signal count.
Fig 1
The frequency to voltage IC I chose, has a threshold input of 10 to 15mV that can be used to our advantage. See fig 1; as the coil oscillates on contact point operation, the tachometer internal induction coil receives a signal that is depicted in the diagram fig 1. By using the variable resistor as a divider network you can trim down the mV output from that induction coil (created by the current HT coil oscillations) to below the threshold. This reduces the input signal and has no effect on its operation, since the first IC is only interested in the frequency. I then use the second variable resistor as another divider network to trim the value into the second IC as a calibration point. The second IC is a dual op amp single supply rail unit where the first portion is used to create a base gain of 3.13 (10000/4700+1) and the second half of the op amp is a buffer to support the current required to drive the analogue meter.  See fig 2; I fitted a diode in the supply line to protect against accidental reverse battery connection.
Fig 2
Because I changed one of the hair springs and caused a minor mismatch in spring tension the gauge was antilog in response. By using a pulse generator to simulated input I adjusted tensions of one spring until I was able to create a linear response. This was just me being pedantic since the error only affected to 5 to 7k rpm range. If you have original springs then this step is unnecessary.
Fig 3
                                                                                                         Fig 4
I experimented with the placement of the board on the tacho frame and settled on the position depicted in the photo herein. See fig 3; Consideration for the illumination and the generator lamps also directed me to fit the board adjacent to the power and earth terminals, these terminals also served as mounting points along with two metal plates screwed into the holes vacated by the original circuit board


Frequency (Hz)















Table 1
For the novice; If you expect two pulses per single revolution of the motor per minute, then with the formula (rpm x 2)÷60 will return the frequency (Hz per second) relevant to the rpm. This is tabulated above in table 1
Since I was unsure that the meter required a frequency to current output rather than a frequency to voltage output, I was left to believe that the antilog response was due to the spring failure and the mismatch of the spring that I replaced with the lower original spring. See table 2 and board layout fig 4
Good luck with the project and if you need help email me on


Value or part number

Resistor  x 4

Resistor x 1

Resistor x 1

20 turn pot  x 1

20 turn pot x 1

IC x 1

IC x 1

Capacitor x 1

Capacitor x 1

Diode x 1

16 pin socket











Generic part

No more lights left on flat battery PDF Print E-mail
Ever been caught out by turning the spridget headlights on and then forgetting to turn them off. It’s certainly happened to me usually just after a spot of rain Inevitably it showers on you while your trying to get a push start after realising you left the lights on.

sprite_tec_headlight.jpgWouldn’t it be grand if like modern cars, when you removed the key the lights would turn off! The sprite headlights can normally be turned on when there is no key in the ignition a typical cause of flat battery and resulting feelings of stupidity.

I made a simple discovery (don’t know if its 100% technically accurate but it worked on my Mk3 and have had no problems).

The sprite standard ignition has 3 positions OFF – ACCESSORIES – START
The main problem is the headlights are not linked to any of these switch positions, they are wired into the fuse box at Battery Power which is on all the time!

1) I had to work out which wire at the fuse box took power to the headlights. You can look up the wire colour codes in your workshop manual.

2) once you isolate the headlight power wire you needed to find which terminal on the fuse box is only active when the key is in the ignition (ACC) position. I used my multimeter to detect this, you can also use a light bulb on a wire attached to an earth point and then probe each of the fuse box terminals until you find a suitable connector. Usually one main power wire comes in one side of the fuse box, goes across the fuse and the power is then send off to a number of other wires)

3) connect the headlight wire to the ACC powered position  (make sure you connect to the side which is fused usually the side with many wire already attached) in the fuse box and you’re forever saved from the lights on flat battery.

In this photo the left set of terminals has power coming in at the top – goes across the left fuse and out to 2 wires at the bottom.
On the right set of terminals power comes in from the bottom, across the fuse and out to green and black wires at the top. You should connect any wiring on the fused side for safety, generally identified where there are many wires on the fuse side [in this photo they are the bottom left set of top right set].
Thats It – it was that simple and from memory it was just one wire moved from the lower set of terminals to the above set of terminals.
NOTE:If you’re unsure or not confident with electrics you should consult an auto electrician.

Replacing the coil in your Sprite PDF Print E-mail
In these times we are often temped to go the major discounters of motor parts and accessories. This is fine for some items, such as tools, polishers, batteries, but with parts for your Sprite be careful. There staff won’t know what a Sprite is, they think it is a can of lemonade.

So if you are looking for parts, be very sure what you are buying. Coils have caught a few members out including me. Modern cars ignition spark is different to our Classics. So often you will buy a coil, which comes with or requires a resister built in. If you fit a coil requiring a resister, without the resister, it will run, but misfire terribly. If you fit the resister this solves the problem. Well I thought so. Over time my resister broke down and started to draw large amounts of current. Eventually it got very hot and failed completely. Another member had the porcelain crack, and the car fail to start, and some of the wiring was burnt.


So what is the answer? Just make sure you buy the old fashioned coil that does not require a resister, whichprobably means not shopping at the major auto accessory discount store and possibly paying a little more.

Bosch GT40 coil is a common replacement

Catering for Austin Healey Sprite and MG Midget Enthusiasts