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Posted: Tue Jul 29, 2014 9:14pm

Post Subject: Volt Meter

Hi does anyone have any ideas? Returning from Newark to Shardlow on a very hot day on our narrowboat volt dial suddenly went over to the far right showing a very high charging rate. We stopped the engine looked at both alternators and belts, all looked fine. Boat is six years old with one alternator already replaced and without further checks wondered if the other alternator was faulty or just a faulty dial, any help please before I call in the auto electrician. Many thanks Rob


Posted: Tue Jul 29, 2014 10:14pm

Post Subject: Volt Meter

You say "dial" so I am going to assume it is an analogue (needle type) voltmeter and in that case unless the hair spring in it has broken I doubt its a faulty gauge. However it is easy enough to test if you have a digital multimeter set to DC volts and you connect it across the relevant battery bank whilst the engine is revving. If it reads 15 volts or more the meter is probably OK. I also note that you give no information re a possible external advanced alternator controller or other similar equipment. If you have one of those then maybe its sensing wire has fallen off or the battery, charging circuits terminals and connections or the relevent master switch has gone resistive and(bad connection). If not and the voltmeter is OK then I suspect a problem field diode in the alternator. This can be tested by connecting the multimeter set to DC volts between the main alternator output terminal (B+) and the warning lamp terminal (D+) and revving the engine. If you get a reading of close to a volt or more then you have a problem diode and any auto-electrical specialist should be able to sort it for you if you take them the alternator. However double check all the terminals etc. including cleaning the battery posts and clamps. Tony Brooks

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