Tiger Tach Rebuild
Nov18, 2002
Back to the Front Page First Page  Second Page

Wiring the Tach
Parts of this section apply to the stock Tiger tach as well as the modified unit discussed in the previous pages. If you have a near-stock Tiger that uses the current-trigger setup to trigger the tach, and the tach absolutely doesn't work, you might first check the underdash wiring against the following schematics, before embarking on a full rebuild of your instrument.

The following arrangements are discussed:

Stock Tiger underdash wiring

The factory tach connections are depicted above. Not counting the lighting, there are only three connections to be made: Firstly, the tach case is grounded using a (usually black) daisy-chained ring lug connection that is sandwiched between the tach hold-down post bracket and the thumbscrew used to hold the tach in place.The tach is powered using the green spade-lug connection. This connection is fused to the main ignition circuit using one of Joe's two fuses, so if you have tach issues, check the voltage on this connection with a voltmeter when the ignition switch is ON. You should have 12 to 14 volts here depending on whether the engine is running or not. The last connection is the inductive pickup loop that the tach uses to sense the current pulses going to the coil as the points open and close. In order for this to work properly, you need to loop the wire through the little metal U bracket that sticks out from the back of the tach. This means the wire passes through the U twice, going in the same direction, as shown in the diagram above. If the tach works intermittently you can try either running another loop of the white wire through the core, or you can try running the wire in the other direction (i.e. counterclockwise instead of clockwise).
It is important that no other electrical accessories be connected to the white wire after it has passed through the transformer coil. Any current pulses on this segment of the wire will be measured and counted by the tach.

Rebuilt tach underdash wiring

The rear view of the rebuilt tach is shown here. The external additions are a white wire protruding through a rubber grommet, and a hole in the case for access to the calibration adjustment. The tach is operated in voltage-trigger mode by connecting the white wire to an appropriate signal source (as depicted in the following sections), with the current-trigger transformer disconnected (i.e. no wires running through the metal U strap), or the tach can be operated in current-trigger mode by wiring it as depicted in the previous section, and leaving the voltage-trigger wire disconnected. It would be a good idea to insulate the voltage-trigger wire and tape it up out of the way in this case.

Engine compartment wiring - standard points

Not much to say about this one other than to make sure there isn't something wired to the 'downstream' side of the current sensing loop. This would add extra noise to the pulses being sensed by the tach and possibly give erroneous readings.

Engine compartment wiring with voltage trigger

The modified tach can be wired in voltage-trigger mode if so desired. This would be similar to any of the popular aftermarket tachs such as the AutoMeter units.

Engine compartment wiring with Pertronix or other points amplifier (includes Mallory Unilite, etc.)

Current trigger
If you're using a points-replacement electronics module such as the Pertronix or Mallory Unilite, then the tach can still operate in current-trigger mode as long as all the coil current is routed through the wire that's looped through the current-sensor ring on the back of the tach. When starting, the ballast resistor is bypassed and the tach will not be triggered, but as soon as the starter is disengaged, the coil switching pulses will be sensed by the tach.

Voltage Trigger
If so desired, the tach can be operated in voltage trigger mode by wiring the ignition as depicted above. In both cases, it is essential to NOT wire anything to the ballast resistor bypass circuit, since it's only really connected to 12 volts when the starter is engaged. The rest of the time, any current drawn from this circuit has to flow through the ballast resistor, and any extra current draw (besides the coil) can lead to overheating of the ballast resistor, as well as unreliable operation of the electrical accessory that's connected there.

Engine compartment wiring with MSD or other multi-spark or computerized controller

The essential things about MSD connection are to wire the heavy-duty power connection to the starter-relay battery connection or to the fusebox using  good-quality ring lug or spade connectors, and to wire the ground connection to a bare-metal spot on the chassis. Keep the wiring for the distributor sensor away from the coil-trigger wires if at all possible - do not run the coil wires parallel to the distributor-sensor wires under any circumstances.
The tach trigger wire gets plugged into the side of the MSD unit. As with the distributor wires, keep this wire away from the coil-trigger wires.

Back to the Front Page First Page  Second Page
Questions or Comments?
Images and text copyright 2002 Theo Smit. All rights reserved. Information presented on this webpage is presented for educational purposes only and I make no claim about its suitability for your particular situation.