Alternator charging problem

[grailton]

Hi, Just put on the water a Virgo Voyager after 20+ years ashore. There is a problem with the charging system, the alternator light comes on and will not go off until the two wires to it are shorted, screwdriver to the pair of them, then the light goes out and the batteries charge. Alternator was a recon unit and has been sent back for testing but is OK, any ideas, please.

 

[lenseman]

I just wonder if the correct alternator warning lamp is fitted for the 'now new' alternator, compared to the unit which was originally fitted?

Do you know what the wattage is and hence the current drawn through the warning lamp?

Sounds like the system jumps into life when you give it a kick with a few amps therefore it might be requiring a lamp of a higher wattage?

Just a suggestion.

 

[halcyon]

Hi, Just put on the water a Virgo Voyager after 20+ years ashore. There is a problem with the charging system, the alternator light comes on and will not go off until the two wires to it are shorted, screwdriver to the pair of them, then the light goes out and the batteries charge. Alternator was a recon unit and has been sent back for testing but is OK, any ideas, please.

Bulb housing corroded, bulb to small, you appear to have a supply fault to the alternator via the bulb. By shorting out the bulb, you by-pass the fault, so start looking there.

Brian

 

[lenseman]

Bulb housing corroded, bulb to small, you appear to have a supply fault to the alternator via the bulb. By shorting out the bulb, you by-pass the fault, so start looking there.

Brian

Hiya Brian - But he said that the bulb comes on OK? and goes out OK once shorted? Hence my suggestion.

 

[William_H]

Alternator problem

An alternator system comes on line in a kind of avalanche the current through the lamp provides some power to the field coil via the regulator and brushes. This power enables the alternator to generate some powert which then provides current to the field coil and regulator (not via the lamp) this additional power then gets the alternator up and charging.
Some alternators will mot commence charging at diesel idle revs and need a little kick of revs to start charging or sometimes just a little more time. A somewhat lower voltage battery may also delay or stop the start of charging.

My point is that the alternator may be on the brink of coming on line and the short across the lamp giving more current to the field circuit could be just enough to start the process. Even if the lamp is the correct size. A larger wattage lamp may just be enough to get it to reliably start charging.

Will the alternator come on line ie lamp go out with more revs? Presumably that was the first thing you tried. does the lamp when glowing appear to be enough brilliance? It won't of course have full 12v across it, You should check all the wiring and the contacts of the ignition switch (engine power) as well as the lamp contacts. If all seems ok fit a larger wattage globe. good luck olewill

 

[grailton]

Alternator

The lamp will not go out with more revs, however it will come on again as the revs drop. The bulb was just a cheapie from the car parts shop so I think I'll change that first.

 

[William_H]

Alternator problem

The fact that the alternator light comes on again at low revs indicates that there is a problem with the alternator. Is the alternator pulley the same size as the previous alternator. If it is bigger then it is running too slow. Once charging the alternator should provide its own excitation (hence light goes out as no current needed). Usually only a very large drop in revs would cause the light to come on again.
IMHO needs further investigation and a new bulb may not help. good luck olewill

 

[davey]

Alternator Problems Answer.

Hi, Just put on the water a Virgo Voyager after 20+ years ashore. There is a problem with the charging system, the alternator light comes on and will not go off until the two wires to it are shorted, screwdriver to the pair of them, then the light goes out and the batteries charge. Alternator was a recon unit and has been sent back for testing but is OK, any ideas, please.

What's the engine and what kind of alternator? If you should perchance have a Petter Mini-6 with the venerable Lucas 15, 16, or 17 ACR beware! This is because these alternators can be had as machine sensed or battery sensed. The voltage regulators and the rectifier assemblies are the parts that differ. In general if the two large pins are joined together on the rectifier, the alternator is machine sensed. Cars usually have machine sensed alternators as the wiring to the battery is usually quite short. Boats can be either. Note also that some marine alternators have insulated return (unusual on engines under 10HP). Insulated return is actually a complete farce unless the starter motor also has insulated return.

The alternator warning light goes between the ignition switch and the alternator terminal often known as "F" (field). On most alternators this point is also fed inside the alternator by the "Trio" (three small diodes that also supply current to the voltage regulator once the alternator starts charging). Once the Trio starts making 12 volts the alternator warning light goes out because both ends of the bulb filament are at the same voltage. Machine sensing AKA internal sensing is usually more reliable than battery sensing as there is less chance of the alternator trying to "charge the world" when bad connections occur.

Good luck and tread carefully. Oh I've just remembered that years ago some cars, Vauxhalls I think it was, wouldn't charge unless the ignition warning light bulb was replaced with one with a slightly higher wattage. Some makers now wire a resistor in parallel with the bulb to ensure charging even if the bulb fails.

 

[grailton]

Alternator Problem

Yes it is a Petter mini 6! I changed the bulb to a higher wattage one and this sort of solved the problem. You really have to rev it then the light goes out however as soon as it goes back to tick-over it comes on again. Alternator charges OK with light out. You mention putting a resistor in parallel with the bulb, would this help and if so what size?