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Post by Clinton Cool on Aug 14, 2017 18:15:47 GMT
I'm trying to help a fellow boater while I'm stuck here in Middlewich. His boat has a BMC 1.5, the batteries aren't charging. The voltage of his starter battery was painfully low, 11.8. Firing the engine up, via a generator, didn't change this. My clamp meter showed negligible ampage going through the wire from his alternator. The voltage at the positive terminal of his alternator with the engine running was 7.2. I noticed that his v belt was very loose so tightened that up but it didn't make any difference to the readings.
I've told him I think his alternator needs to come off and potentially repaired, or replaced. Just checking that I gave good advice? I'm mystified as to how the alternator could produce 7 volts. Thought it would be 14.4 or thereabouts, or nothing?
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Post by naughtyfox on Aug 14, 2017 18:30:49 GMT
How old is the starter battery? They don't last forever. What make starter battery?
How old alternator? Make & model? (not that I know these things but that's how I start).
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Post by JohnV on Aug 14, 2017 18:41:40 GMT
sounds to me as if the alternator has lost part (most) of it's diode pack. ..... could be many things but my money is there.
The standard alternator fitted to BMC engines was one of the ACR series by Lucas. If there is an automotive starter/alternator rebuilder near (there are lots around try google or yellow pages) a rebuilt replacement should be quite cheap.
edit to add ...... a test meter at the terminals of an alternator (basic types) should read between 13.9 volts and 14.2 a couple of decimal points higher is ok but lower is not
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Post by faffer on Aug 14, 2017 18:45:15 GMT
The alternator should put put as you say 14 volt.
If not giving out the correct volts as John says, whip it off and get it rebuilt, cheap aswell, i get them done for less than £20 around here.
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Post by Telemachus on Aug 14, 2017 18:53:27 GMT
The thing is, the voltage at the alternator should be the same as the battery it's connected to's voltage. Is the battery down to 7v? If not there is a connection problem between the alternator and the battery. An intermittent connection 'twixt alternator and battery is a good way to blow the alternator. So check and if necessary fix that first.
Also check that the battery -ve is properly connected to alternator -ve. Many boats have alternators whose casing is the -ve, but some have isolated casing with a seperate fat -ve wire going to the battery.
Is the alternator being excited? Most alternators need excitation current provided by the alternator warning light bulb. Is the warning light illuminating when the ignition is switched on before starting the engine? If not, check the bulb hasn't blown, and the circuit from the ignition switch, via the warning bulb, to the alternator's D+ connection, and check the voltage at the D+ connection with ignition switched on / engine not running. It should be a volt or two.
If the D+ connection has reasonable voltage on it and the alternator isn't charging when revved, only then does it point to a blown alternator.
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