Tacho connections

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Graham_Wright

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IN the "BAJWTBF" pile are two tachos;- one VDO the other anonymous.

The VDO tacho has two terminals on the back labelled "-" and "G". There are two other holes in the connector block labelled "+" and "Z". I think I could guess at the functions for all four but, am I right in thinking that the "-" goes to battery negative and the "G" to the alternator output or should it be to an unrectified phase? My 130A alternator has all three phases brought out but the 40A starter battery alternator has not. Neither has it a "W". terminal.

It seems naïvely that any ripple on an output connected directly to the battery would be swamped by the DC component.

The anonymous tacho has "+", "-" and "W" connections.

+ to +
- to -
and W to?

I understand some alternators have a "W" output for a tacho. The tiddler does not the bigun has an unlabelled third (+ and - as it is isolated ground).

Any help appreciated.
 
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The tacho, if it is intended to be triggered by an alternator, will be expecting a signal from one of the poles. But lots of tachos operate instead from a flywheel sensor so they will not work off the back of an alternator..

The W is the tacho signal wire. D+ or IGN or L or 61 is the ignition light connection though some modern alternators dont have this at all.

So the tacho with the W has the W connected to the corresponding output of the alternator. Where the G and the Z on the other one go, I do not know. Suggest you ask VDO.
 
The G terminal on the VDO tacho should be connected to the W terminal on the alternator. If it doesn't have a W connection, any reasonable auto-electrician should be able to add one. There's a diagram here showing where to connect a W terminal.
 
pvb said:
The G terminal on the VDO tacho should be connected to the W terminal on the alternator. If it doesn't have a W connection, any reasonable auto-electrician should be able to add one. There's a diagram here showing where to connect a W terminal.
Click to expand...

Doddle to do. Pull the back off, add a screw terminal, wire to one of the diode inputs. 15 minute job and the tacho works!

Now - how to calibrate it.

I have a mechanical tachometer but insufficient room on the end of the crankshaft to insert it. I guess a strobe will be easiest.

eBay let me down!

Mains lamp (fluorescent or neon if I can find one);- 1 mark would indicate 3000 rpm or 2, 1500 (which would shake the hull a bit less).

Sums correct?
 
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Graham_Wright said:
Mains lamp (fluorescent or neon if I can find one);- 1 mark would indicate 3000 rpm or 2, 1500 (which would shake the hull a bit less).

Sums correct?
Click to expand...

Doesn't really matter how many marks - you'll know whether it's doing about 1500rpm, so use that as the calibration speed.
 
earlybird said:
IIRC, the flicker rate of fluorescent lighting is twice mains frequency, so factor by 2.
Click to expand...

Yes - that makes sense (but not all Googled responses agree). The modern ones use a high frequency converter also.

I guess the crankshaft would synchronise at sub multiples also. As we speed up a lathe, we can see static chucks jaws at several speeds.
 
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That's China for you- but I suspect those days are numbered as their wage bills are rising fast. I bought one last year, higher cost, less postage, same approx overall price. Works fine. best if you can set the tacho roughly first on basis of tick over & /or max revs if known, so just leaving finer adjustment on the tach to do on the basis of the laser revs read out.
 
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