Thread: How to tachofy your Mk 1 diesel

Part 1-Search for a signal

The Mk 1 diesel usually has a perfectly reasonable clock where the tacho should be. Fitting a tachometer to the diesel is not too difficult but there are a few things to do to make one work in the absence of an engine speed signal like that found on a spark ignition engine.

A pulsed signal that increases in frequency with engine speed is required. On later model diesels, and some not found here, the W terminal from the bosch alternator is used. This is a terminal that is connected to one of the three stator windings inside the alternator, providing a
single phase alternating current signal with a frequency proportional to engine speed.

I am yet to find a bosch VW alternator that has a W terminal but it is an easy mod to add. Heres how I did it.

Heres the VW alternator, clearly no W terminal here:


First, remove the brushes:


Remove the four long bolts that hold the front and rear assemblies together. I used a large screwdriver and a shifter to increase the turning moment to undo the suckers:


Once the four long bolts are removed, I gently levered the stator away from the front housing using a smaller screwdriver:


Apart, the alternator looks something like this (note the spring washer):


Check the slip rings, they look good here, also, check the front bearing for play and smooth rotation. Refer to your workshop manual for other tests you can perform at this point. If all is well, put this aside until reassembly:


The stator, rectifiers and rear cover:


These are the ends of the three phases of the stator windings. A wire connected to one of these would have unrecitified alternating current!


To be continuned...

This is the other side of the diode board, showing one of the stator ends. Using a hot soldering iron, i enlarged the hole so that i could run a wire in to this point:


This shows the hole and a wire inserted through the hole:


This is a bad solder join:


This is a good solder join:


Reassembly of the alternator is the reverse of dissassembly. Get to it!

The reassembled alternator with W terminal output.


Next - how to modify a petrol tachometer to accept a W terminal input

The tachometer itself:

Find yourself a petrol tachometer. When at the wreckers, check the no. of points on the terminal on the back of the instrument cluster. Try to get one with a 10 point connection. There are clusters out there that only have 8 points and this will make things difficult when installing it in a diesel.
The diesel instrument cluster has a 14 point connection but only uses 11. The 10 point petrol cluster uses all 10 so one connection, the glow plug light, will be connected separately. More on that later.

Remove the instrument cluster and facade as a whole unit and also cut the terminal off the end of the wiring loom and take this too.
This is the plug to get, remove this with as much loom as you can:


Remove the tachometer from the cluster and disassemble until you have the circuit board on its own. I measured the values and wrote what they were on the board before mods were made:


The tachometer motor. Note the connections of the wires for reassembly:


To modify the board, follow instructions found here using the wiring diagram and instructions for the westmorland tachometer:
http://www.vwdiesel.net/forum/index.php?topic=3741.0
The component referred to as RV2 is actually a capacitor, replace it with a 47nF cap.

Heres mine modded:


To test the W terminal, and to provide a reference for calibration, I connected a programmable VDO tachometer to the W-terminal. This tachometer has a digital hour meter and can be programmed using pulses per revolution. Pulses per crank revolution can be determined as follows:
The rotor in the alternator has 12 poles, 6 positive and 6 negative. Per revolution of the alternator there are 6 positive pulses. The ratio of crank to alternator pully determines how many times the alternator revolutions per crank revolution.
I connected the VDO programmable tachometer and the petrol tachometer to the W terminal, +12v and earth....


and started the car. After some adjustments to the trimpot on the petrol tachometer and a bit of a rev, i took this photo:


It took me a while to stop grinning.
Next - fitting the petrol tachometer into the diesel dash

Awesome write up mate. You will be a hero to the oily brigade with that mod.
Nice to see some grassroots engineering on here. Keep up the good work and the updates.

Thanks Matt,

one problem is that there arnt so many mk 1's at the pickaparts anymore.... I need another petrol cluster for our project car too. The parts for the tachometer mod can be sourced from jaycar etc so the only special item is the petrol cluster.

Originally Posted by hiho

Thanks Matt,

one problem is that there arnt so many mk 1's at the pickaparts anymore.... I need another petrol cluster for our project car too. The parts for the tachometer mod can be sourced from jaycar etc so the only special item is the petrol cluster.

man, because you are a true legend, i will happily give you an old petrol cluster....

....after i check i have one

Great write up!

I didn't know there was so much involved!

it looks like more work than it is. The W terminal takes about an hour the first time you do it. I've done three now. The mods to the tachometer take a while longer, but if you can solder its straightforward. Resistors and capacitors will cost you less than $1. This weekend im installing the tachometer in the car..... More writeup to follow.

I have heaps of petrol clusters including some in bits if it helps the cause.

I have installed the petrol cluster into my Mk1 diesel and now have a working tachometer! It isnt very exciting to watch, the needle can go all the way to 7000 but rarely moves past 2500. Photos and a description of the process to come.....