VDO Tachometer (VP2003 circa 1990) : Bench Test Signal ?

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RIBW

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After a fair amount of searching the web I can't find anything specific to define the characteristics of a signal to inject into the tacho on the 'W' input to test the Rev needle deflects (getting the revs right will be a refinement). As far as I can tell it will be positive pulses of up to 14V at a frequency of ??Hz.
My spare 'Basic' Instrument Panel (bought as an unknown entity i.e. never seen working) is on my bench and I do not currently have access to the boat (and cannot therefore put an oscilloscope on the running system).

My question is, does anyone know :-

The maximum (for damage avoidance) and minimum (for operating) peak voltages
The maximum trough voltage
A sensible pulse rate to try

Thanks in advance
Bob
 
Dont know whether this is of help but from memory the Volvo alternator is a 6 pole jobbie so frequency wise I would think up to 400 cps. I wouldnt worry about the minimum - try zero. Max 14v as you say. But all guesses so if you can get some decent info, use that.
 
As far as I can recall, the W terminal generally outputs an unrectified half-wave AC voltage, the frequency of which is proportional to the engine speed. If you try around 7v with a frequency of around 150Hz, you should see a tacho reading.
 
Thanks Both,
Based on your thoughts I'll knock up a 12V 50/50 ratio pulse genny and take a feed off it with about 7V to try.

Regards
Bob
 
Great!
My first cut indicates the tachometer is working well and smoothly!
I need to refine the drive signal to 'W' but at first sight, a 12V circuit at 400Hz indicates at 2000rpm.
When I get more time I'll try to refine this information
Thanks for your inputs
Bob
 
After a little refinement, the results are for a pulse of +6V 50% duty cycle
Approximate readings are:-
1000rpm indicated = 200pps
2000rpm indicated = 400pps

Using the most accurate settings available to me,
2700rpm indicated = 500pps

Theoretically, using Wotayottie's 6 poles and a ratio of Crank pulley to Alternator pulley (from a stored VP2003 engine) of ~135/80,
the pulse rate would be (135*2700*6)/(80*60) = 455pps
I am happy the tacho works (well) and even happier that measured is close the theory

Thanks again for your thoughts
Regards
Bob
 
nigelmercier said:
Thanks for the info. But can't the ratio of this tachometer be adjusted?
Click to expand...

There are no obvious/visible adjustment points on the outside of the tacho.
It has 4way 'AMP' style connector for W, G and power, a press fit tube for the replaceable lamp fitting, 4 retaining screws and 4 small holes, two of which are blanked off with a dab of red resin?. The other two (not blanked off) are very small (2.5mm dia.)and nothing is visible through them. For obvious reasons I don't want to poke anything in there!
Hopefully someone who has done an alternator change might have the information.
 
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