Tachometer / Alternator help please

watermark

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13 Apr 2005
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I hope some one can help my slightly fuddled brain.

The Tachometer in the standard volvo instrument panel is massively under reading on my MD2030 after several years good service.

It still "kicks" on power up, does show revs as the rpm builds, but is showing a max of about 1,200 rpm at cruising speed (realistically 2,500 ish) The batteries are still charging apparently properly and I am getting 14.0 volts according to the sailing instruments (B&G Wind).

The sensor cable on the back of the alternator was badly corroded and I have attached a new piece of cable to replace the last 2 inches or so of this which did improve things. From 800 rpm to 1,200 rpm max.

Any thoughts gratefully received!

Also....how does a tachometer workl and what is it actually measuring?

Thanks
 

david_bagshaw

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Firstly welcome on board


If replacing the last bit of wire & connector improved things, there is probably more corrosion inside the alternator, so it is a strip & clean up job, however see if you can clean the terninal tang first , unless it is a bolt & eye type, which will still need cleaning.


My understanding of how the tacho works is that there is a tapping onto the fields which measures the frequency of the voltage change , which is then "adjusted "to display the frequency of rotation of the crankshaft.
 

boatmike

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The clue may be in your partial solution. Before you do anything more take a length of cable and run from the alternator terminal to the Tacho to see if this restores things. High resistence in the cable or any connection is the first thing to rule out. If you have a plug connector for the electrics in the engine compartment pull the plug and see if this is corroded. If you are sure there is no problem here and you have a digital tacho with a liquid Chrystal Display for distance etc. the tacho may have lost its programmed settings. Turn it over and you will see a button hidden beneath the casing. This changes the tacho range. I cant tell you the right sequence of button presses for your engine but the effect of pressing it will cause the range to alter.
 

spannerman

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The tacho counts the pulses from one of the output stages on one of the windings in the alternator, and because the crankshaft pulley to alternator pulley ratio is known this converts to engine RPM, if it still doesn't work after you have tried with new cable let me know and I will send the reprogramming procedure and code for your particular engine. regards Steve
 
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