Tachometer FAQ


Is the conversion right for me?

Problem after installation

More Tachometer Board Information

General Electronic FAQ

Sink or Source

Can your tachometer conversion board be used on a positive earth vehicle?
The Conversion board is designed to work with negative earth vehicles, however, in ceratin circumstances it can be made to work. One option is to use a 123Ignition module. The 123 drives the coil in a similar way to a negative earth vehicle and our tachometer board is triggered correctly. However, you will either need to electrically isolate the tachometer from the chassis, or better still, use our external conversion kit which is already isolated.

Will your tachometer conversion convert all RVI tachometers?
Electronically the conversion board is compatible with all RVI tachometers, however there is one model of tachometer fitted in an external pod and supplied to Volvo as well as an aftermarket accessory that just doesn't have enough space inside. For this model we recommend our external kit.

Will the tachometer board work with the output from a Megasquirt, Emerald, EDIS or other electronic system?
The Conversion board has three signal inputs of varying sensitivity. The most sensitive will work at input signal voltages from around 2V to 12V and therefore is suitable for direct connection to most electronic ignition systems, ECUs etc. with a logic level output. The middle sensitivity will respond to signals from about 5 to 20 Volts and is suitable for the 12V output of some systems as well as picking up the intermediate signal between (for example) an accuspark trigger and a stealth module. The least sensitive input is suitable for direct connection to a coil -ve terminal where the voltages can exceed 60V

Will the conversion board fix my broken RVC tachometer?
Electronically the board is compatible with the RVC tachometers, however, the internal board is designed to mount on the RVI movement and therefore will not physically fit the RVC tachometer. If you wish to rejuvenate an RVC tachometer, use the external module and hide it behind the dash, The external module gives the same benefits as the internal board such as range of input and output adjustment..

Will the conversion board work with any other tachometers?
The both the internal and external versions are compatible electronically with (AFAWK) all moving coil tachometer movement. The external module does not need the special mounting points of the internal board and this makes it suitable for updating a wide range of tachometers. The module has been tested on tachometers with coil resistances from 10 to 800 Ohms with no problems reported.

How do I know if the movement in my tachometer is OK?
99 times out of 100 an old tachometer movement will be fine, it is usually tired capacitors or a transistor or chip blown due to a momentary reverse of high voltage that is to blame for failures. Usually just a clean and installation of a new electronics module and the tachometer will work just fine. However, for the more technically minded it is possible to check the function with simple tools. The movements need to be both mechanically and electrically sound for the conversion to work, If the tachometer was a know working one then it is likely that it will be fine. The most common problem is dirt or metal particles obstructing the free movement of the coil around the magnet. This can be fixed by cleaning with compressed air or washing in an ultrasonic cleaner. The movement should move freely without ant tight spots.

The electrical characteristics can be checked against those of known movements for example

RVI low impedance 10 - 11 Ohms fsd 0.8V 80mA

RVI high impedance 74 - 80 Ohms fsd 1.7V 23mA

RVC 200 - 230 Ohms fsd 2.2V 10mA

However these are only approximate values and the module will work with other movements

for movements that require more than 1.5 Volts for full scale deflection, a 1.5V battery is a good test.. the movement should move proportionally to the voltage, so for example a 1.7V movement will move just under 90% of the full scale

The low impedance movement is harder to test as a 1.5V battery is too high a voltage to see if the movement is properly working.. however, an inexpensive silicon diode, such as an 1N4001 in series with the battery will drop the voltage by about 0.7V and the movement will move to almost full scale.

One thing to remeber is that the movements do not need to be lubricated, oil will attract dirt and form a goo that will cause friction and ultimately cause the movement to jam.

How do I know if my tachometer is an RVI?
There are three giveaways

  1. it says "RVI xxxx" on the front ! ------ sorrry had to put that one in ;-) .
  2. the two small black slotted screws either side of the spindle, are just off centre by a couple of millimetres.
  3. the four brass or aluminium slotted screws on the back (two of which hold the insides to the case) form an "Isosceles Trapezoid" (not quite a rectangle) (the Rover P6 is like this)

Will the conversion board work with my Points Replacement Electronic Ignition?
The board should work with all Points Replacement electronic ignitions. We test every board that leaves the workshop on an Accuspark module, but they have been tested on every system we have access to. One issue reported is that some older systems (such as optronic) require the signal wire to be connected to the middle pad instead of the high voltage pad, so if you have an older design points replacement, and the tachometer is not responding properly, try moving the input wire over.

Can I use a rolling road to calibrate my tachometer after fitting?
The adjustment screw on the internal board is at the rear of the tachometer so you will need to have the tachometer removed from the dash to access the screw while adjusting, other than that it is a perfectly reasonable way to calibrate.

What methods can I use to calibrate the tachometer after fitting the board / module?

  1. use the calibration cable to supply an audio tone to the tachometer, if at first the tachometer does not respond, try a different audio source
  2. use a signal generator to supply a square wave to the tachometer input
  3. use the rpm figures generated direct from an ECU monitoring program or a vehicle analyser
  4. set a constant fast idle and use a smartphone app to determine the actual rpm

I have fitted the board to the tachometer but can't get it to repond to an audio input, what should I do?

  1. If you are working on the bench, use a battery or well smoothed power supply to power the tacho, mains interference from a battery charger of unsmoothed power supply will prevent proper operation.
  2. The low voltage input of the board is just low enough to respond to the audio output of common devices such as smartphones and MP3 players.. if at first you get no response, try another device.. and if it has tone control, use maximum bass and maximum treble.. in fact turn everything up to max..
  3. Double check that the wires to the movement coil are not the wrong way round, The original colours in the tachometer do not always seem to be the same polarity as the wires on the modules, try switching them anyway.

the tachometer worked when calibrating on the bench but not now its fitted to the car?
There are two common reasons for this

  1. The wiring connections for an RVC (voltage sensing) tachometer are different to an RVI (current sensing) tachometer, check that you have made the necessary alterations, diagrams can be found here.
  2. The new electronics require a better quality ground than the old electronics, try running an extra ground to one of the screws that mount the movement to the inside of the tachometer case.
  3. Interference in the power line can affect the tachometer, try a temporary power connection direct to the battery.

The tachometer works but won't go above a certain rpm?
There are two common reasons for this

  1. At higher rpms, even though the peak voltage may be well above the trigger voltage of the module, the signal voltage is not high enough for long enough to trigger the module, the answer is to use a more sensitive input, or on older models reduce the size of the series resistor. This increases the amount of time the voltage is above the trigger threshold voltage
  2. The movement is physically obstructed. This is more common than it sounds. The disturbance of changing the circuit board can dislodge fine metal or other particles that can get down into the gaps between the magnet and the movement. The solution is to carefully inspect and blow out with an air line.

The tachometer is accurate at one rpm but not at others?
The movement is surprisingly linear when set up correctly, when it was calibrated at the factory the linearity was set by calibrating at more than one point of the scale. Changing the electronics should not change this calibration and the new board has a linear output so the modified tachometer should still be close to linear. If for some reason the setting has been changed, it must be reset. It will require playing two audio files and setting the tachometer at the two appropriate points on the scale.

  1. Calibrate at the higher rpm (higher frequency audio file)
  2. Check the reading at the lower rpm (the lower frequency audio file)
  3. Adjust the position of the fibre spring mounting plate that is just behind the dial.
  4. Reset the pointer position to zero
  5. repeat several times as above adjusting the fibre plate whichever way makes the error less...

Don't expect miracles, it will never be as accurate as a modern stepper motor tachometer, the movement is an old design and will have suffered wear and tear over the years. It should however be able to get a reasonable degree of accuracy at the cruising and redline rpms, which is where it is important.

The tachometer needle sometimes shoots to full scale (sometimes sticking there)
The electronics in the conversion board need a reasonably clean 12V supply. Occasionally, on older vehicles, there can be sources of interference that can upset the circuit. The most common of these is a negative going pulse from a solenoid or relay that does not have a flyback diode . This still only occurs in around 1 in 200 installs, so we don't as a matter of course include the components in the module.

It is possible to prevent these pulses getting to the tachometer by adding a 1N4001 diode and 100uF capacitor
they should be installed in parallel from the power terminal on the tachometer to the case screw that is connected to ground
The line on the diode should go to the power terminal, the "-" on the capacitor to the case earth screw

The tachometer seems to be too sensitive, I cant adjust it low enough (GM tachos etc.)
Some tachometer movements are more sensitive than others. The range of adjustment might not be enough for some sensitive movements such as GM tachos from the late 60's. The fix is to desinsitise the movement with a couple of resitors. Fit a 200 Ohm resistor in series with the movement (close to the movement itself), and a 27 Ohm resistor in parallel with the movement.

Who makes the conversion boards and how?

Every part of the manufacture takes place on our own equipment on-site so that we can control it. We design and manufacture the circuit boards using an etch process. We drill each board on a small CNC drilling machine (larger holes are drilled by hand) , we apply solder paste using stencils. The components (all the SMT components) are automatically placed in position by our pick and place machine. The boards are then inspected before being heated in our reflow oven. After reflow, the boards are individually inspected under a microscope and any corrective actions taken. The through hole components are then hand soldered and the board is ready for testing. After testing, Boards are ultrasonically cleaned, any leads are soldered on and the board is treated to a conformal coating before packing.

what testing takes place on the boards before dispatch?
Besides visual inspection, we test every board on a custom test rig. The boards are connected using a "pogo pin" array and are tested in the following way

  1. Power applied, check current draw
  2. Apply 2V 115Hz signal to the LV input and adjust output to read 0.35V
  3. Apply 5V 214Hz signal to the LV input and check that the output is 0.64V
  4. Connect coil -ve to HV input and apply test signal generated by accuspark ignition module, check that the output is 0.64V, run for at least 1 minute.
  5. every 50 units we run a board at high voltage for at least an hour

We test at both low voltage and high voltage and for at least a minute to minimise the possibility of faulty boards leaving the workshop. Despite this it is possible that a board may prove faulty, but it is much more common for the installation to be the problem, so please treble check the FAQ for possible causes before assuming the worst!

what is a "Sink" output?
Many of our modules act as switches using a transistor to turn on or off a load, be it an LED, Relay, Buzzer or Bulb.There are two ways to connect transistors as switches and they are called "Sink" and "Source". The terms refer to the current flow, a "Source" is where current comes from and a "Sink" is where current flows to.

Connecting to a "Source" output - A "source" output supplies the +V to your load, for loads that are polarity sensitive such as LEDs, this should be connected to the + side of the load, the other side of the load should be connected to ground.

Connecting to a "Sink" output - A "Sink" output connects the load to ground, so the other side of the load should be connected to +12V.

There are several advantages to using "Sink" outputs,

  • Because the control circuitry is more isolated from the load they can easily be seperately fused,
  • The electronics to drive a transistor that is grounded on one terminal is easier to design
  • The electronic design makes it easier to get rid of unwanted heat from the switching transistor
  • Multiple control circuits can be connected to the same load without interfering with each other.
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