Thread: VDO Tacho wiring

I went to the Speedometer shop to get a wiring diagram for the VDO Tacho.

They can check the calibration for $0-15 which is pretty reasonable.

First though I need to gather the following information :

• Alternator Pulley Diameter
• Crank Shaft Pulley Diameter
• No. of Pole pairs on Rotor
• Single phase or Star point pick up for signal
• Tacho F.S.D. (Full Scale Deflection)

This seems to suggest that if the alternator was replaced with a bigger / different unit, the tacho may need recalibration.

Can anyone tell me what the standard alternator & crank pulley diameters are? And answer any of the other questions.

I suppose if I can determine that my alternator has a non standard sized pulley, then I can narrow it to an alt. mod.

While I was there I found some Brand New VDO Volt Meters, old stock for $20.
They've got 5 left. They were an upmarket meter in their day, but their "chunky" surround means they don't match any other gauges in style. They come with green & red sleeves for the globe and the guy says they light up behind the numbers like a modern gauge.

Ringwood Speedometer Services (03) 9874 2260

No affiliation, I just thought someone might need a cheap Volt Meter.

I suppose I should post this on the Extra Gauges discussion too.

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
http://www.sso.siemensvdo.com.au/htm....asp'dsb=4

and these instructions as most newer tachos are very similar.
http://www.sso.siemensvdo.com.au/htm...d=image&id=326


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.

Thanks zwitter -for a very informative & well composed explanation.

Maybe we should start a new Forum on auto electrical issues and you could be a moderator / on-line consultant.

I'll give them a call back and see if they have an optical tacho, as that's going to be a lot easier than pulling things apart. Mine's got the W plug.

- Michael

Hi Michael

No worries.

I too have a 96 defender, mines a 130. I also have a VDO Tacho but not fitted as yet. Got it NOS on ebay.

I too will be looking for an optical tacho to dial it in. Used to have one where I worked many years ago. Problem is I do not want to talk to the guy there to borrow it if i can avoid it.

cheers
james

Thanks James.

I went & got the Tacho calibrated with an optical tacho. It was about 100rpm out at idle, which would translate to about 500rpm at 100kph. At least now I can give it a few more revs without getting worried!!!

They got it to within 36rpm at 2,000rpm (1.8%) which is close enough for a Land Rover.

I appreciate your good explanation, which saved me trying to crawl under the car or pull the alternator off.

- Michael

Hi Michael

Ok I wired mine in today.
Found the W terminal had a rubber cover but clearly marked with a W.
Shifted the Fuel and temp guages over to the right and removed the clock.

Connected it up and works but reading way high. Now I need to find an optical tacho to calibrate it.

FWIW I found an old post in this forum that said
Alternator pulley 49.2mm
Crankshaft pulley 162mm
12 poles (6 pairs?)

Did the calculations but no test equipmwent to set it anyway!

james

You could just adjust it at idle with what you know the idle should technically be. I think 750rpm. That will get it in the ball park. As you drive it, check the speedo (or GPS speed if you can) and calculate how far off it might be.

hi guys, if you want to check your rpm calibration goto http://www.grimmjeeper.com/metric_gears.html and input your specs (yes land rover is covered). it will tell you what your rpm's are at any specific road speed in any gear. use a GPS to get your correct speed and you will know if your tach' is correct. don't trust the speedo though, mine reads 107 km/h at a true 100 km/h in my defender with 285/75/16 BFGs on it, i would hate to know how far out it would be with std size tyres!