Diesel Tacho

[smithy010]

Hey all,

I heard brackie on the Mk1 forum asking about whether GlDgti had a tacho on his diesel, and i thought that might make for a good thread.

First question: Does anyone have a tacho on their diesel?

I have been thinking about this a bit, and since i'm doing a diesel conversion, and will be keeping the same old dash, i obviously wanted the tacho to work...

So a little bit of thinking later, and i reckon i can do it in one of two ways.

Preamble: The current tacho takes a 12V pulse signal and turns it into a reading on the tacho (unconfirmed)

Solution One (least favourable)- You could mount a small magnet on some rotating part of the car- it only needs to be tiny, so you may not need to worry about balancing, though balancing would be easyby means of simply sticking an equivalent mass on the other side.
Using a sensor called a hall effect sensor, and some pretty simple electronics, one can make it put out a 12V pulse signal whenever the magnet passes the sensor. As i said, this is the least favourable.

Solution two- There is a company that makes a thing called a "tiny tach". It is basically a small tacho which gets its signal by clipping a sensor onto one injector pipe. Unsure as to how they do it, but the problem with this, is it comes with its own digital readout- less than ideal. I want my original tacho to show the signal...

Anyway, what i plan is to get a hold of one of these sensors, hook it up, and work out what signal its putting out, then, hopefully without too much trouble i'll be able to make up a little amplifier circuit, and bob will be my uncle.

Hard to say whether it will work, but i welcome your comments.

[peter_j_g]

My mk1 GTD had a tacho as standard - same place as the tacho in the petrol versions, only visual difference was the scale and the redline position. It was driven by a feed from the alternator. I've also seen aftermarket kits that put a small tacho in the centre console that work the same way.

[brackie]

The feed came from the "W" terminal on some Bosch alternators. This provides the signal needed by the tacho.

[syncro]

I fitted a tacho once to a truck that sensed the ripple frequency of the alternator output. Easy fit, but I did not do the calibration.

[smithy010]

of course- i overlooked something.....

[Golf Loon]

Can you use the gauge you have smithy? I still have the clocks from that diesel if you want the gauge.

[smithy010]

Yeah i'm pretty sure i will be able to use my current gauges.
Cheers

[gldgti]

not because its bad..... but because they are laggy, so it might not suit what you want it for.

i have tried one option.....


i used an ABS brake sensor mounted on bracket infront of the water pump pulley to produce a low voltage signal at 3 pulses per rev. i did build a circuit to try to mp that signal and feed to my aftermarket tach, which was had a setting for 6 cyl cars (3 pulse per rev) but my electronics wasnt good enough apparently as it did not work.

i have seen a working application of the ABS sensor where one was mounted inside the timing belt cover pointing at IP sprocket, and got a signal from the bumps. this lead to a counting circuit which then fed the signal to the tach, worked perfectly but takes a bit of electronics nous.

do a search for "diesel tach" or "dump W terminal" on the www.vwdiesel.net forum, you might find a good thread.

[smithy010]

I'd doubt the tiny tach sensor would be laggy- the lag on a tiny tach is just due to the sampling rate of the electronic bit. If i am constantly sampling, that shouldn't be a problem.

I just had another idea- what type of sensor is on the TDC marker which goes in the flywheel plug? If it's a H/E sensor, you could just hook the circuitry straight up to that. Not sure how it works, but part of the flywheel must be magnetised.... maybe.