View Full Version : connecting tachometer

29th May 2008, 02:51 PM
I want to connect my tacho up. Does anyone know where the tach wire comes through the firewall? And more importantly what does the tacho run off, EG a voltage pulse on off? Hopefully I can simulate the pulse or whatever the tach requires to test if its working.


29th May 2008, 02:54 PM
Might need a few more details on your vehicle...

Most / many Land Rovers use a sensor wire from the alternator as the pulse (signal) wire for the tacho.

29th May 2008, 03:08 PM
sorry mate, its a 1987 RR.

and i just found this which may answer my question after I read through it!

Lightbulb Tachos 101 -hold the sour cream
Hi Michael2

FWIW here are my answers.
lets start with the basics
Tachos display rpm. More correctly they are voltmeters showing a voltage relative to the engine RPM and marked in graduations of rpm.

Next wiring diagrams. The tacho will have 4 wires. One is ground (chassis), One is positive 12 volt in our case and will be connected to ignition power (on when engine running), One is for illumination and is obvious as it connects to the base of a bulb holder usually. The last wire is the signal wire. The signal is a voltage that Oscillates (rises and falls) in a frequency relative to the engine speed. the tacho has circuitry to convert this signal frequency to a voltage for display. Internally that voltage will be referenced to eliminate voltage fluctuations in the electrical system.

In a petrol engine this signal is connected to the wire between the coil and points (or equivilent on higher tech engines) In this case the tacho is factory calibrated and has switches to select the number of cylinders (or can be fixed). The incoming frequency is effectively divided according to the number of cylinders. on 4 stroke petrol thefrequency of the signal will be half the number of cylinders times the RPM. Ie in a 4 cylinder engine there will be 2 pulses per revolution of the crankshaft. 4 for a V8, 3 for a v6 etc if it is 2 stroke the pulses per revolution will be equal to the number or cylinders.

In a diesel there are 4 common ways to source a signal for the tacho.
1) Transducer on one of the injector pipes on manual injector systems. Often a piezo crystal that creates a voltage when distorted so the injecton pressure pulses cause the crystal to give out spikes. Signal ratio is pulses=half number of cylinders per revolution
2) Direct connection to electric injector driving wire on electronic type injector systems. Signal ratio is pulses=half number of cylinders per revolution.
3) Directly from the ECU on computerised engine management systems. Signal could be any ratio.
(for the above ifthe diesel was 2 stroke then ratios are doubled.)

4) When all else fails. Connected to one phase of the alternator. Often called W terminal (other phases can be called X and Y). Alternators generate alternating current (AC) internally and then is rectified by a diode network to output direct current (DC) or voltage around 14volts. the W terminal is the out put of one of the phases producing AC voltage so it is a signal rising and falling as each pole of the rotor passes the coils of that phase. Most alternators have 3 phases but it really is not important. Many alternators do not have W terminals but most fitted to diesels do and if not can be modded by a skilled auto electrician. The ratio is pulses = number of poles on the rotor per revolution of the ALTERNATOR. So for ENGINE RPM you need to know the ratio of the crankshaft revolutions to alternator revolutions. This is often displayed in car/engine specs but not in the literature I have here for LR defender 300TDi.

Back to the questions
The tacho "counts" pulses (and converts to voltage) not rpm directly. basically you will end up with so many pulses per engine revolution. This could be something like 17pulses so at 1000rpm the signal would be 17KHz

Yes if a different alternator was fitted then this could change the number of pulses. The number of pulses is dependant on the internals of the alternator. ie number of rotor poles. The number of pulses per engine revolution is proportional to the ratio of crankshaft and alternator pulleys. All the other things on the belt can be ignored. The pulley size for an alternator is chosen so that the alternator will not be driven too fast at engine red line and is almost always much smaller than the crankshaft pulley so the alternator spins much faster as they are more efficient at higher speeds. The same alternator on a small 4 cylinder car may have a larger pulley than on a slower revving V8 or diesel.

The offer from the Speedometer shop seems to be a very good deal. If your tacho works ie moves relative to the engine rpm then they do not need to know all the things you listed. they should have an optical tachometer and stick a small reflective sticker on the crankshaft and rev the engine up to about two thirds of red line and while reading their optical tachometer rpm compare it with what your guage displays. In the side of nearly all VDO guages there is a small hole with a small adjustment screw that can be turned to "calibrate" the display. So id they rev engine to say 2500rpm using their optical tacho, then you turn that screw until your guage shows 2500rpm.
The other way would be to read the signal directly from the altrnator and compare with theoretical value obtained by dividing the crankshaft pulley circumfrence by the alternator pulley circumfrence and multiplying by the number of rotor poles. They could then set the tacho using test equipment inside their workshop.

The pulley diameters can be measured. they can lookup the specs on the alternator to find number of poles and signal pickup type. The tacho FSD may be able to be set using switches. The FSD will be a maximum frequency of the signal connection to show maximum rpm on the tacho. It is often displayed as pulses input frequency coresponding to a display of 1000rpm.

New tachos will have switches to set range but older ones may have a set range.

See this website

and these instructions as most newer tachos are very similar.

I think yours will just need a tweak of the adjustment in the side of the tacho.

OK is there a limit on posting length?

I hope that makes it clear as mud.