Testing tachometer

SvenglishTommy

SvenglishTommy

Well-Known Member
Joined
22 Sep 2005
Messages
688
Location
Spain
sailingawayforayearandaday.blogspot.com
The tachometer for my MD2010 hasn't been working ever since I bought the boat, and I'd like to see if I can find out where the trouble is. Does anyone know how I can

a) test that the output from the alternator is correct - is this just a matter of the voltage between W and negative?

b) test if the tachometer is working by bypassing the little electronic gubbins in the control pannel (which I think is what is broken) - i.e. can I just take out the tach and connect it directly to the alternator?
Thanks,

Tommy
 
If the alternator is working OK then the output to the "W" terminal will almost certainly be OK unless it has become disconnected internally. You could check with a multimeter on the 20 volt AC range.

No you cannot check the meter by connecting it to the alternator directly if its electronics are external to it. It is probably just a milliammeter and doing so will blow it. Someone with knowledge of a little electronics may be able to test it for you though.

You might find the instruments section on this web site interesting and the rest of it useful as well.
 
You should have a a 5v output from W on the alternator, check you have this at the rev counter, and a plus and minus. Not sure what you mean by electronic gubbins as the wiring should go direct to the rev counter.
 
[ QUOTE ]
Not sure what you mean by electronic gubbins as the wiring should go direct to the rev counter.

[/ QUOTE ]
In my volvo control panel, most of the wires go into a very small device which holds the bulbs for the warning lights. The wires to the tachometer come out of this box so I was wondering if it actually did anything to the output. I know this box is buggered as the neither the warning lights nor glow light work properly and the alarm test fails, even though the alarm itself works fine and the glow plugs are working correctly.

I'm not sure I fancy paying out £300 for a replacement box, as it looks like it probably cost in the region a ten quid to manufacture. I was hoping I could bypass this meanwhile so I can still monitor revs.
 
The meter itself will have a movement which operates on about 1 milliamp (from .1 to 15 milliamps depending on design) However it is normal for the signal processing to be done inside the meter case.
If you can open up the meter case you may find integrated transistors and circuiit board which would be the signal processing circuit.
If you can't open the case then the presence of at least 3 wires ie ground (negative), positive 12v power and a signal wire would indicate the circuit is in the meter case. If it has just 2 wires then these perhaps go to the meter movement direct with the signal conditioning in the panel box. Beware there may in both cases be a wire for panel lights to light the meter.
If you have just the meter movement in the case it can be tested with a digital multimeter on ohms low range and you should get some meter pointer movement.
If the signal conditioning circuits are in the case then yes you should be able to connect it directly to the alternator W connection.

If you check the alternaor output you should measure about 5 volts AC on your digital multimeter.

These might give you some ideas but really the chances of fixing it are pretty slim without knowledge test equipment or spare parts. good luck........ olewill.
 
Also watching with interest. My rev counter has becoime erratic, maybe it needs a good rev in neutral when I start up, sometimes this works and sometimes not. There is a TWC charge controller in the system, would that affect the workings of the rev counter?

Having forgotten to take a multimeter with me last weekend I couldn't check anything:-(
 
If you're experiencing faults with the tacho and the warning lights maybe it's worth investigating the multiway plug and socket which connect the instrument panel to the engine. Its a round plug with about 15 or 20 pins (can't remember exactly) which all the connections go through, maybe it's suffering from some corrosion. I can't see any problem in experimentally connecting the Tacho W input terminal directly to the alternator W output. For info The W output is a connection to single one of the alternators three 12 volt ac output phases, (taken before the rectifier diodes) and the tacho gives its RPM reading according to the frequency of the ac voltage that it recieves, as this frequency is directly related to the speed at which the alternator is spinning. Since the alternator rpm is a mutliple of engine rpm set by the relative sizes of the crankshaft pulley and the alternator pulley there is sometimes an adjustment on the back of the tacho (look for a little hole to put a small screwdriver into) to cope with fitting to different engines /alternator combinations.

Finally the Volvo warning light modules sometimes turn up on eBay, and go for £25 to £40 ish if you do need one
 
Hello Bill it might have been a slip of the finger but I think the W terminal goes to the centre of the 3 phase star winding of the alternaor. This gives then 6 half cycles of AC per revolution when measured in reference to ground negative. The ground negative being the output from 3 diodes feeding negative pulses from each of the 3 phases to the negative terminal. So expect a fairly high frequency AC to drive the tacho. olewill
 
You must log in or register to reply here.

Share this page

Top