Smiths Tach Interworkings

By "Brian Scally" <>

If we start with how the tacho works we may understand why we have so many problems. Iíve done some ASCii art so courier is the font to view this in...

There are two types of tachs. Current sense and voltage sense. In Smiths parlance, RVI and RVC.

To my knowledge no RVC was fitted to a Elan in production. RVI's are no longer in production.

RVIs are connected in series with the coil....

Pwr--(ign switch)--(RVI)--(Anti theft SW)--(coil)--(points)-óGnd


Pwr--(ign switch)--(Anti theft SW)--(RVI)--(coil)--(points)--Gnd

RVCs are connected in parallel with the contact breakers.

Pwr--(ign switch)--(Anti theft SW)--(coil)--|--(points)--Gnd


Internally both tachos work the same way. As the contact breakers open and close they trigger a bit of circuit called an "Monostable". This produces a pulse of a fixed length on the monostable output for each opening OR closing of the points.

This means that the variably timed ignition (the dwell period) is converted to a series of fixed length pulses. One pulse for each spark. This pulsed output of the monostable is then used to drive the tacho needle mechanism. Each output pulse moves the needle a small amount per second (~0.02 deg./s - 4 cyl) (The meter effectively integrates this signal)

In the RVI a coil of wire in the ignition circuit forms part of a simple transformer that is used to generate a pulse for the input of the monostable. In the RVC the voltage input is taken directly from the points.


________ ______ Pwr

Pwr )( | _______|______ _____________

( ) |____| |________________| |

)( | Monostable |_______ | Meter |

( ) | | Cap | |

________)(__________| |_______| | |

To Igt. Coil | |____________| | |___________|

| | Adj. Res |





_______________ Pwr

To Coil neg/ | _______|______ ______________

Points Pos |____| |___________________| |

| Monostable |_______ | Meter |

| | Cap | |

____| |_______| | |

| |____________| | |_____________|

| | Adj. Res |




So why is there a difference between the RVI and RVC? and what is better about RVCs

1)In the early days of the electronics in the tach the "input impedance" of the monostable was low (impedance is like resistance). So directly connecting it to the points upset the operation of the ignition circuit.

2) The "transformer" link is very susceptible to the characteristics of the ignition circuit. A change of ignition coil, or points (eg going to electronic) may make it ineffective and give false or no readings.

3) RVCs are less susceptible to changes in the ignition circuit, and use less wiring. Advances in semiconductors made them able to withstand the fluctuations of voltage around the points.(300V are regularly seen across points - 1000V occasionally)

So what are all those components and adjusters and things...

In an RVC there are many more components since it works in a far harsher environment. However most of them are to do with the monostable. As time went on the original RVI developed and the monostable circuit developed to be a "One shot monostable" these were a lot less susceptible to point bounce and they have more electronics and have more internal components.

There are two important components from the timing point of view.. The electrolytic capacitor and it's discharge resistor. These control the time of the output pulse of the monostable and thus the deflection of the needle per rev. This discharge resistor (In the example I have seen) is the adjuster on the back panel. The electrolytic capacitor is the largest electronic component in the unit.

So why does my tacho read wrong.

This would account for a systematic error... eg 10% out all the time

Point bounce
If the points are bouncing at high RPM the monostable can double trigger giving high readings at high RPM. Always a good thing to check any way

Bad point capacitor
This would give a low reading at high RPM, however this may cause point bounce, giving a high reading.

Sense coil fallen off
The loop of wire (White) can fall off giving no reading.

Bad electrolytic capacitor
The electrolytic capacitor's performance will change over time. These capacitors are now ~30 years old and probably 300% of their design life. Symptoms could be meter inaccuracy at high speed, low speed and maybe erratic behavior over bumps.

Tired transistors
As the monostable triggers the transistors are both switched on together giving a short term short circuit. This will tire the transistors degrading there performance. This would generally give pore pulse definition and high and low rpm errors

Aged resistors
The resistor values may change over time, effect would be to move the transistor out of operating range, most likely this will give no reading.

Changed points to electronic ignition
Really the only solution is to rewire and change to an RVC internals