Tiger Tach Rebuild: Getting one for your car
Dec 3, 2002
Updated: May 3, 2011
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MG MGA/MGB Tachometers
Price and Ordering Information:
Assembled and tested board: $100 US, shipping included.
Rebuild service for your tach: $180 US, plus shipping by your
Prices may change without notice.
You can buy the boards, modules, or get your tach repaired by the
107 North Valley Bay
Calgary, Alberta T3R 1H9
5712 San Luis Ct
Pleasanton, CA 94566
Tom's email: firstname.lastname@example.org
I accept cash (sent at your risk), money orders, personal cheques
day hold), or PayPal.
This circuit is not a cure-all for your tach or your car. If the
of your tach is impaired by a mechanical problem with the meter
or if the movement coil or the transformer coil are broken, this
can't compensate for that. The calibration procedure described in this
document sets the meter display to be correct at a single engine speed,
within the tolerances of the meter movement's mechanical friction and
The testing and verification of the tach accuracy at other RPM values
up to you. This tachometer circuit has no function as an engine RPM
If you think that you might be exercising the upper end of the rev band
with your engine, invest in a quality rev limiter.
Schematic Diagram and Circuit Board information
Installation in Your Tach and Calibrating Your Circuit
The picture below shows the required connections on the board. The
colours follow the scheme used by Smiths. The intent is that you'll be
able to hook up the circuit without permanently altering anything
the tach, just in case you'd ever want to restore the thing to it's
Installation begins by disassembling your tach.
- First, clean the outside of the case. Any crud that isn't on the
isn't going to fall inside.
- Undo the front bezel by firmly gripping it in one hand, the case
other, and twisting it until the tabs on the bezel line up with the
in the case. Prying the tabs up will permanently mark the outside of
bezel, so DON'T do this unless you absolutely see no other way to
- Remove the bezel, outer O-ring, glass, inside O-ring, and the
The backside of the tach has four screws. Loosen and remove only the
two that hold the case and the innards together. As you do this, the
movement and the electronics will fall out the front of the case so
that end of the tach with your other hand. Don't inadvertently push on
any part of the needle during this process, either. It's actually
robust, but the best way to keep it working properly is to not tweak
Assembling the circuit board into the tach requires some soldering
and a pencil-tip electronics soldering iron of 25 to 35 Watts. Do NOT
a transformer-type solder "gun" like the 100/140W Weller, because the
field set up by these things will affect the tach's field magnet, and
it's way too big for the job.
The best solder for the job is 60/40 rosin core electronics solder.
Don't use acid core solder under any circumstances, and "lead free"
is meant for plumbing, not your tach.
First remove the OEM connections to the meter movement and the current
- Unsolder the red and black meter connections from the OEM
as well as the red and black current transformer connections. Don't
cut the wires, because then you'll have to re-strip the ends, and
tricky, especially the enamelled transformer wire.
- Unsolder the green power wire from the tab on the back of the
the free end back on itself and solder it to the OEM circuit board so
out of the way.
- Push the red and black meter movement wires through the
in the new circuit (from the backside, so they stick out the front),
and solder them in place.
The circuit board is installed by ziptieing (?) it to the meter
movement's magnet bridge.
- Cover the back of the circuit board where it will touch
the magnet with electrical tape, then run a ziptie through the two
so that the tip extends leftward out the bottom of the board (referring
to the above picture).
Bend the free end of the ziptie under and through
the gap between the magnet bridge and the meter movement body, then
it up so the board tucks up under the face plate. Make sure the two
meter movement wires don't snag in the movement.
The power connections are made next.
- The black ground wire is soldered to the lug that connects
to the OEM power resistor on the back board.
- The green power wire is soldered to the spade lug on the back
board (the one from which you removed the OEM green wire).
Finish up with the current transformer wiring.
- Tighten the ziptie a little more to make sure the board
freely (it doesn't have to be stretched taut), then run the red and
current transformer wires through the corresponding holes in the
board, and solder them in place. Note that each of the red and black
actually consist of two strands of enamelled wire, and take care to put
only the tinned part of each pair through each of the holes.
Calibration is required to correct for tolerances in the electronic
in the magnetization of the meter movement, and the mechanical
of the meter movement. Like the OEM circuit, the new tach circuit has a
single calibration adjustment, which allows you to set the scale
to a reference at one RPM. I recommend that you do the calibration at
to 4000 RPM. Lower than this, and any errors made in the calibration
multiply by two or more at the top of the scale; Higher than this, and
you run the risk that the lower and middle range accuracy is seriously
compromised due to mechanical or magnetic errors in the meter movement.
Besides, if you've built a high-revving killer motor for your car I'd
that you'd protect it with an electronic rev limiter, as opposed to
the connection between your eyes and your foot. A Smiths tach is NOT a
racing instrument of the 21st century.
- If you intend to run the voltage-triggered mode of
operation and/or have
the calibration adjustment externally tweakable, you will need to drill
holes in the case in the appropriate places. The grommet supplied with
the voltage-trigger wire is sized for a 3/16" (4.8 mm) hole. The best
to mark the case for the calibration hole is to measure the distance
the calibration adjustment pot to the two nearest screws in the back of
the case, then mark off the arcs corresponding to these distances on
back of the case. The hole location falls on the intersection of the
arcs. Measure twice or more times... The location for the
wire hole is not too critical as long as it doesn't interfere with
on the backplate of the tach.
- To reassemble the tach, push the voltage trigger wire through the
and pull in the slack as you feed the guts into the case. Snug up the
screws on the rear of the case, then reassemble the bezel parts in the
proper order and twist the bezel back on.
To install the tach in the dash, refer to the appropriate diagram on
wiring page. If you're not using the voltage trigger mode, insulate the
end of the white trigger wire, and tape it up out of the way; if you're
not using the current trigger connection, don't run any wires through
current sensor transformer loop.
- To calibrate the tach, connect it to your reference signal, then
the potentiometer until the tach reads the right value. Make sure the
is upright and rotated like it would be in the instrument panel. Wiggle
the tach around and disconnect/reconnect the test signal a few times to
ensure that the movement isn't sticky.
Tach Rebuild Service
The tach rebuild consists of
If you decide to ship your tach to me, please observe the following
- disassembly and mild cleaning of the inside of the tach (no major
of tach faces, gaskets, etc.)
- check of meter movement and current sensor coils
- installation of the circuit board
- drill two holes in the back of the case for the voltage trigger
calibration access (OPTIONAL)
- reassembly, calibration, and testing
1. Tape off any holes on the back of the tach case (the light hole)
2. Enclose the tach in a ziplock baggie or similar air-tight bag.
3. Use foam, packing peanuts, or bubble wrap to COMPLETELY fill the
box you're shipping it in. If the tach can't rattle around in there it
stands a much better chance of survival.
4. I live in Canada. Customs regards anything that comes across the
border as something that I've bought, and therefore they reason that I
ought to pay GST (7%) of whatever the value of that thing is before I
it in my hands, unless there is clear evidence that I didn't buy it. So
by all means, insure your tach for whatever you think it's worth, but
the contents as being "used antique car tachometer", and that the value
is "for customs
only, no commercial value". The Harmonized Tariff Code is
near as I can figure, or you can try to find a better match in the
doc at: ftp://ftp.usitc.gov/pub/reports/studies/0205htsa.pdf
5. You can figure out what the return shipment cost will be by choosing
your method (Canada Post, UPS,
or another courier service), hitting up their website, and finding
their rate calculator.
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reserved. Information presented on this webpage is presented for
purposes only and I make no claim about its suitability for your