by Paul Dierschow & Tom Ehrhart
Originally Printed 6/87
Shown below is a wiring diagram for a Mk I Tiger with the tachometer circuit highlighted. Although the harnesses are different for Tigers and Alpines, the principle of that circuit is exactly the same. This is also true of most other British cars with electric tachometers from the 1960’s through the early 1970’s.
In Figure 1, the tachometer circuit has been removed from the factory wiring diagram and rearranged, showing the simple concept of this design. Several items are important to remember in understanding this circuit:
- The ignition coil receives its power via the tachometer loop wire (white).
- The tachometer must also receive constant switched power (green) and must have the housing grounded (black).
- The tachometer is simply a voltmeter that measures the voltage generated by an internal printed circuit corresponding to the number of pulses produced by the coil in the low-tension circuit.
- The counting of these pulses is affected by variation in dwell angle or point gap, especially with dual point and after-market distributors.
- The inductive loop of wire on the back of the tachometer must be properly oriented for the system to function.
Tachometer Wire Hook-Up
This impulse tachometer cable installation is for negative ground; reverse A & B for a positive ground system. The ground wire must be attached under one of two knurled mounting posts. The other end of the wire must be connected to the car chassis.
As shown, Wire A goes to SW terminal on the coil or to the 12 volt terminal of coil. When using a ballast resistor, this lead must be the input lead to the resistor with the resistor output lead going to the coil.
Wire B goes to No. 2 terminal of ignition switch, or to a 12 volt source that is turned off with the ignition switch.
Since the tachometer is simply a pulse counter, all tachometers of this type are basically the same, differing only in calibration, type of ground, and face design. It is therefore, possible to use the unit from an Alpine or an MG, change the face and recalibrate it to have a suitable replacement for a defective Tiger tachometer. To replace the face, remove the bezel, glass and glare ring. Remove the needle with its hub from the tiny shaft by very carefully prying both sides gently and evenly at the same time. Then, the two small black screws are all that hold the face to the body. Since the unit is simple to recalibrate, it is also possible to put in a unit with a higher range of reading to accommodate the capabilities of a high performance engine. A tachometer without any indicator jewels on the face would be preferable, since light from the face illumination bulb would show through unless it is covered. A Series V Alpine has a 7000 RPM tachometer with the proper face style to match a Mk II Tiger.
Recalibration procedure is as follows:
- With the tachometer removed from the car, remove the chrome bezel, glass and glare ring by turning and aligning the tabs with the gaps in the housing. It may be necessary to bend up the tabs slightly to be able to loosen the bezel enough to turn.
- Remove the two screws from the back of the housing that hold the assembly to the can. Look carefully as two other screws do not release the guts but are part of the internal assembly.
- Now a 5/8 ” diameter black (or yellow) plastic variable resistor is exposed. It has a narrow screw slot in it that is easily turned slightly for exact calibration. It can be turned enough to change from a four to eight cylinder reading.
- The tachometer can now be rewired temporarily in the car and adjusted while it runs parallel to a good quality dwell tachometer and matched to that reading. The tachometer calibration will ensure accuracy over a small range around the point of calibration. For example, if adjusting the tachometer to 3000 rpm it will likely be accurate from at least 2000 to 4000 rpm. Although possible, in most instances, do not expect your tachometer to be accurate at 1000 and 6000 rpm if it was calibrated at 3000 rpm. the required calibration by a professional instrument re-builder.
- If necessary, for future adjustments, you can drill a hole in the proper location on the back of the housing to make recalibration easy without removal from the car. Use the following template:
It is easy to convert your tachometer should you want to use a positive ground tachometer or change your Series IV Alpine from positive to negative ground. The procedure is:
- Remove the guts of the tachometer from the housing as listed from above.
- On the rear most board, find the large brown resistor that connects to the terminal for the large green wire (power) from the harness and the small green wire soldered to the silver button adjacent to it (ground).
- Carefully unsolder these connections from the board.
- Reverse these two connections and re-solder. You can cut the long lead from the resistor or bend it backwards to fit properly. CAUTION; Items 1,2,3 & 4 apply only to tachometers where the printed wiring board ([PWB) circuitry do not come in contact with the metal meter frame. Examine carefully. It will be necessary to modify the circuitry if it comes in contact with the meter frame.
- Reassemble the unit.
- If you are changing the car from positive to negative ground, you must also do the following:
- Mark and cut the white induction loop wire a couple of inches from the plastic block.
- Reverse the connections and re-solder and tape them. Don’t forget that this is your ignition lead, so be sure the connections are good.