Tacho erratic function

Information related to the Harris Matchless (also known as Devon Matchless)
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Tacho erratic function

Postby oldjexy » Wed Feb 24, 2016 4:30 pm

I have a Harris G80 that displays erratic tacho readout. It is steady up to around 2500 rpm and then swings to max rpm.
I suspected this was noise interference on the signal line so rerouted the cable on the opposite side to the HT. No change.
I then disconnected the output from the regulator and hey presto the taco behaves normally, great so the noise is from the regulator or alternator. I then fitted a voltage regulator chip(LM2940) in the 12volt feed to the tacho which drops the voltage from 14.5v to 12 exactly and is supposed to smooth out spikes, but it makes no difference. This machine is not running with the original voltage regulator, a CEV 6704, apparently unobtainable, I have tried. The CEV is a 6wire unit and the one being used is a 4 wire from the club spares.
Does anybody have any ideas if this might be the regulator? Or maybe the alternator is very noisy, the voltage outputs are normal........Help

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Re: Tacho erratic function

Postby Stuoyb » Wed Feb 24, 2016 5:14 pm

I had an Armstrong MT500 (same Rotax engine as Harris) and got a replacement regulator from one of the MT spares specialists (LMS, Force Motorcycles).
This was 6 wire as you describe.
Not sure that the 4 wire regulator/rectifier from club spares is suitable for your usage as it was only tested on the old AMC machines with Lucas or Wipac systems.

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Re: Tacho erratic function

Postby Les P H » Wed Feb 24, 2016 11:30 pm

Would ferrite rings on the sensor wire snub out the spikes?........also try some Supression caps on the supply leads to the unit....BTW... it would be worth checking over things with a 'scope to see what was going on.....Les

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Re: Tacho erratic function

Postby oldjexy » Thu Feb 25, 2016 9:28 am

Thanks chaps, I will try if the Armstrong spares guys have a suitable regulator.
Like the idea of ferrite beads, simple to fit, but am unsure the meaning of BTW?...Dave

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Re: Tacho erratic function

Postby Les P H » Thu Feb 25, 2016 9:32 am

BTW....means: By The Way.....I tend to use it without thinking...saves a few seconds in typing time....
Other common ones are IMO.....In My Opinion.....or IMHO.....In My Honest Opinion

Just to add that the capacitors for filtering are usually 50V 100nF ceramic discs or similar but the output from the voltage regulator should be pretty smooth so perhaps fit the ferrite rings on all the leads to the tacho and as said before, a quick check with a 'scope will reveal all.
Les

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Re: Tacho erratic function

Postby Reynard24 » Thu Feb 25, 2016 10:52 am

As a long shot, is the bike fitted with a resistive spark plug cap (NGK or Champion) with around 5k resistance?
I fitted an electronic tacho to my M18 short stroke and with a straight through spark plug cap fitted the tacho behaved very much as you describe. Champion 5k resistive cap cured it.

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Re: Tacho erratic function

Postby ushbert » Sun Mar 06, 2016 6:37 pm

I expect you have already checked the earth connection. It has been reported elsewhere that the cause of erratic tacho readings has sometimes been traced to a poor earthing, but you probably know that.
Do let us know how you get on, David.

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