Swinging rev counter

tmtracey

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I have a problem with my tachometer. The dial arm goes to the correct revs when the engine starts, but after thirty seconds it starts to swing back and forward.

I have just installed a new alternator, so I don't know if this is an issue. I initially accidentally bought an insulated-earth one by accident, so initially when I started the engine there were no revs at all. I then fitted an earth cable between to the alternator to engine so I wonder if I could have damaged the alternator possibly. The old alternator had a battery sensing wire which I have removed (the electrics shop said to do this).

I've done a few checks
- the batteries are definitely charging.
- from W on the alternator to the Tacho positive lead, it was swinging between 18 and 28 Volts AC
- from the engine earth to either of W and D+ on the alternator, it gives zero VAC (I don't know if that matters)
- the belt isn't slipping
- the contacts are all good
- W is the cable connected to the tacho

Any advice would be brilliant.
 

VicS

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Some questions / observations

Did the engine you fitted come complete with wiring harness and control and instrument panels or did yo use the original panel and wiring. If the latter what engine did they belong too

Did the tacho work OK before you fitted the new alternator

If you measure the volts at the alternator output while the engine is running do you get a steady reading around 14 volts or so.

I dont understand why you have taken the readings you have.

When switched on you should see the battery voltage applied to the + terminal of the tacho The W terminal should be connected to the W (or S+) terminal of the alternator

Can you measure a DC voltage at the W terminal when the engine is running ?
 
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tmtracey

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Hi Vic- I didn't mention the reconditioned engine, you've got a good memory! It was a reconditioned Bukh- from a company who specialises in them. It came with it's over wiring harness and switch panel (include diodes etc).

I decided to install a new cockpit engine panel at the same time- and replicated the wiring in the switch panel I received with the engine. All electrics / lamps seem to work correctly. I am not using the Bukh control panel or diode plate at all.

The alternator is the same wattage as previous. The tacho worked fine beforehand.

Looking back, when I was tidying up the old wiring I removed what I thought was just a straight connector inside a shrink seal - but guess this might have been a resistor...? The cable changed colour half way along- someone may have put this in to regulate the pulse?

I'll get back to you with those readings. Thanks.
 

tmtracey

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Update - I've taken the alternator in to auto electrics store. They say it is fine.

On the boat with the engine running, I tested the DC voltage from W terminal to ground = this was fluctuating between 1.8 to 6.3 V.
AC voltage was also fluctuating from 3 - 14.6 VAC.

Alternator is charging the batteires - the volts don't fluctuate.

I wonder if these are actual normal readings and it is just my multimeter isn't appropriate to read them. If they are normal , then I probably need to replace the trachometer.
 

rogerthebodger

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Update - I've taken the alternator in to auto electrics store. They say it is fine.

On the boat with the engine running, I tested the DC voltage from W terminal to ground = this was fluctuating between 1.8 to 6.3 V.
AC voltage was also fluctuating from 3 - 14.6 VAC.

Alternator is charging the batteires - the volts don't fluctuate.

I wonder if these are actual normal readings and it is just my multimeter isn't appropriate to read them. If they are normal , then I probably need to replace the trachometer.

This thread may help you as the output from the W terminal

http://www.ybw.com/forums/showthread.php?116230-Alternator-W-terminal
 
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