Adding a Tacho to the diesel

**hiho** · 04-06-2007, 09:47 AM

Hi dieselers,

I have a VDO tacho with alternator signal input and according to the internet (US) it is simply a matter of connecting the W terminal of the alternator to the tacho, setting the pulse rate and youre done. The concern that i have is that none of my alternators have a W terminal. There is a + terminal that is not used, is this the same as the american W therminal? Id rather not fry my tacho by trying.

Thanks!

brackie · 05-06-2007, 07:44 AM

The W terminal is connected to a signal generator that goes to the tacho. I don't think any old tacho will do, it has to be able to count and read the pulses. You can buy a converter if you have the $$$s
Diesel alternators with the W terminal are hard to get (I believe) as they were not imported to Oz.

http://www.dakotadigital.com/index.c...rod/prd128.htm

**smithy010** · 05-06-2007, 09:15 AM

Hiho,

If you want to wait, i'm planning on setting up a tacho for my diesel when i get it in the car. It will probably involve a hall effect sensor mounted near some rotating component, possibly a magnet on the flywheel and a HE sensor in the bell housing hole. Then, the required circuitry to bring it up to a 12V pulse. Shouldn't be too hard i don't think.

Cheers

Smithy

**hiho** · 05-06-2007, 09:09 PM

Thanks guys,

the tacho that i have is alternator pulse compatible so its just a case of tapping one of the coil phases before the rectifier in the alternator and putting the right diode in series to convert the AC wave into a half wave pulse. I have two to experiment with so i will let you know how easy this turns out to be. Smithy, let me know how you go with the hall effect sensor on the flywheel, this could be easier than the alternator, however, tacho's are available that are designed to work from the alternator as described above.

**gldgti** · 06-06-2007, 04:15 PM

Originally Posted by hiho

Thanks guys,

the tacho that i have is alternator pulse compatible so its just a case of tapping one of the coil phases before the rectifier in the alternator and putting the right diode in series to convert the AC wave into a half wave pulse. I have two to experiment with so i will let you know how easy this turns out to be. Smithy, let me know how you go with the hall effect sensor on the flywheel, this could be easier than the alternator, however, tacho's are available that are designed to work from the alternator as described above.

where from?

syncro · 06-06-2007, 05:08 PM

There are tacho available that work off the ripple frequency of the alternator. They just connect to the D+ (or it may be B+ as it was a long time ago that I fitted one.

Does a '76 Golf with the VW Diagnosis socket have a flywheel sensor?

**hiho** · 06-06-2007, 08:50 PM

I bought it on ebay, its a VDO gauge that also has an hour meter so its probably intended for a stationary, marine or other industrial diesel engine. If i didnt have it i would use the item that Brackie put a link to so i could use a VW petrol instrument cluster instead of having the tacho mounted elsewhere.

http://www.dakotadigital.com/index.c...rod/prd128.htm (Brackie's link)

**Gigitt** · 20-06-2007, 01:41 AM

I still have my VDO diesel Tach from my Mk1 GLD.

I took my Alternater back then to an Autoelectrician and for $15 he spliced a wire off of one of the rectifier diodes so I can have an A/C Tap (W point). The alternator had a 15cm wire poking out the back and cable tied out of the way. There was a Female Spade lug on the end and all I had to do was connect a wire to the tacho.

It worked flawlessly.
Do it. It is Better than spending huge $$$ on hall effect and other sensor adapters.

I have the Instructions at my parents place somewhere. It has the diode tap instructions.

The Tach will be installed into My 4x4 but I have been Told it already has a W point on the alternator making my job a hell of a lot easier. this time.

The closest Installation Manual I could find ATM is:
http://www.egauges.com/pdf/vdo/0-515-012-044.pdf
it tells you how to hook it up but not how to get an A/C Tap.

**smithy010** · 21-06-2007, 05:27 PM

From what i have read, it's just a matter of connecting in parallel with one of the stator coils. Seems to make sense...

Is the pulley ratio 1:1?

**hiho** · 25-06-2007, 04:44 PM

I just spoke to a chap at Ringwood Spedometer (in Ringwood Melbourne, nice chap) and the W terminal is an unrectified tapping of the W phase stator coil. To make your own W terminal you can tap any phase (U, V, or W) before the rectifier. The W terminal is therefore an AC waveform. Im going to have a crack at this tonight and will inform you on the result.
Matt