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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...
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...
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