4. The Filter Unit. (not applicable to newer models)
4.1 Purpose
The Tacho Filter Unit is a simple filter contained in a small plastic box that may be required when
using a Farringdon RCA5x Tacho with contact points ignition systems. It may be of use with
some electronic systems as well. Essentially the unit filters the signals taken from the negative
(on negative earth systems) side of the coil. These signals carry considerable noise from the
arcing inside the distributor cap. Many other tachos have this filtering included but this prevents
them from working with very short pulses that can come from directly triggering from a sensor on
the flywheel.
4.2 Fitting
Mount the unit at least 25cms away from the distributor. The components in the plastic box
are “potted” so it can be mounted in the engine bay.
a) Connect the violet wire of the filter unit to the violet wire of the tacho.
b) Connect the black wire of the Filter Unit to ground.
c) Connect the negative of the coil to the green or blue wire. The blue input provides more
filtering than the green so start with the green wire.
Temporarily insulate the unused blue or green wire.
4.3 Commissioning
Set the tacho filter setting to 8. See Section 3.4 for details of how this is done. Try the tacho and
check that the needles do not jump about (+ or – 500 rpm or so) when the engine is held at a
steady 3000 rpm. If the needle is steady, try at full speed under load.
If either needle makes jumps to higher rpm when the engine is running at a constant speed,
increase the filtering by connecting the trigger wire to the blue instead of the green filter wire.
If the needles are steady, then try changing the tacho filter setting to 7. This will make the tacho
more responsive.
Warning DO NOT rely of the tacho reading until you are certain that you have set it up
correctly and that it is showing the correct rpm..
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