Boat Fault Finding - Marine Electrical Problems
Tony Brooks is an expert on Marine Diesel Engine & Boat Maintenance matters. Thanks to him and TB-Training for permission to reproduce these materials.
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Starter/Battery Fault Finding
All readings assume a 12volt system – double for 24 volts.
Clip a volt meter across the battery terminals and take the following actions:-
1. Take a reading immediately:
More than 12 volts = battery PROBABLY OK – higher the better up to 13.2 volts
Less than 12 volts = battery PROBABLY flat or faulty – see battery checking below
2. Get someone to hold the stop control in the stop position.
Spin the engine on the starter and after about a second note the reading. 10 volts and above = probably a starter fault if the engine is not spinning as normal. (I would accept 9.8 volts). Less than 10 volts = starter, battery, or seized engine fault.
3. Run engine at a good speed - Assuming the engine can be started. (say 1500 to 2000 rpm) and watch the voltmeter. It will probably read 13 volts plus, but watch for a while and see if it starts to creep up, if it does wait until it stops before taking a reading. 13.8 to 14.2 volts (plus or minus about 0.2 depending on make) = alternator PROBABLY satisfactory. Less than 13.8 = faulty battery or alternator (Check/rectify battery BEFORE condemning the alternator! More than 14.2 volts = Charge controller in use or faulty alternator voltage regulator.
Testing the alternator for more reliable results requires the use of a high current ammeter – you drop a lead and you will probably set fire to your wiring.
Testing "Normal" lead acid batteries.
ACID IS DANGEROUS – TAKE GREAT CARE
1. Make sure that battery has sufficient acid in all cells to take hydrometer readings.
If not, top up and charge for several hours – then let the battery stand to cool.
2. Note the following:-
Colour of the acid – if one or more cells are brown the battery is probably at or very close to the end of its life.
The actual reading from each cell – a difference of more than 0.05 indicates a faulty cell (but I would charge it and recheck before condemning it).
3. The state of charge (on the opposite side of float to the numbers). More than half charged = probable faulty starter. Less than half charged = slow charge at 1/10th of the battery’s amp hour capacity (or less), taking hydrometer readings every hour. When they stop rising over two consecutive hours the battery is charged.
If the case or top looks as if it is swelling up, you can be sure the battery is at or close to, the end of its life.
EXIDE UK informed us that there are no end user tests for Gel or "sealed for life" batteries. They stated that the batteries must be taken to a battery specialist for testing.
Starter Circuit Testing
Do not start this unless you are confident in your use of a voltmeter, you can again cause a fire or get burnt!
All tests assume negative connection to starter case or engine and pre-engage starter (with the solenoid as part of the starter)
1. Ensure the battery is serviceable and sufficiently charged.
Connect voltmeter across battery, operate stop control, spin engine on starter and take reading. This is your "base" reading.
Less than 10 volts = faulty starter or seized engine.
2. Leave negative connection on battery, connect positive to main, battery, terminal on starter (the bolt connection farthest away from the starter body). Operate starter and take reading.
More than about 0.5 volts less than "base" reading indicates faulty connection between battery & main terminal.
3. Move pos. connection to other big terminal, operate starter.
More than 0.5 volts less than in 3 above = faulty starter solenoid.
4. Move positive connection to starter body, operate starter.
More than 0.25 (I say 0.5) volt reading when starter is operating shows a bad connection in the negative circuit back to the battery.