Alternator issues on a small inboard.

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welkman

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Hi, I have a SHE 27 which is fitted with a Petter mini 6 inboard diesel engine. The engine runs really well after a partial overhaul this winter. On testing the output from the alternator it was discovered that no real charge was reaching the batteries. I am seeing readings of about 12.5 volts, this is the same at the back of the alternator.

I have tried:

Cleaning all earths

Continuity testing all wiring

Rigging up a warning light

None of which resolved the problem. The alternator is brand new and I took it to the shop yesterday where it was confirmed to be fine. If you load the electrical system (Turn on lights, radio and auto helm) the voltage drops. The only thing I can think is that the engine cannot spin the alternator quick enough, which is a bugger because I’m using the smallest pulley lucas make!

Any suggestions for resolving my charging issues greatly appreciated, I have been on the boat every night this week trying to sort it out!


Regards,

James
 
You should be able to check that the alt is spinning fast enough by finding its datasheet and working out the ratios of the pulleys.
It needs a warning light.
Is it charging at all? i.e low charge rate is a different problem from none.
Do the volts rise at all at higher rpm?
Some engines have large 'overpulleys' to spin alts/dynamos faster.
In the past I have tested alts at home using a battery, warning light, ammeter and power drill to drive it.
Is it a modern model of alternator? Some older models don't do much at low rpm, modern car ones can make a lot of current at tickover, although the alt may be spinning 5x as fast as the engine.
 
Shouldn`t it have a charging light to excite the coil? or something similar. I seem to remember a conversion from an "old
seperate box regulator system to a combined alternator" had something like that to get it to start charging, I am a bit out of my depth here but others on this site will be able to help I`m sure.
 
12.5V means it is definately not charging at all. Is the warning light rigged up correctly and is it on or off when you turn the "ignition" and then on or off when the engine is running? Do you have a splitting diode between the alt and the battery? If yes, try to bridge it with a screw driver or spanner briefly. That might "kickstart" the alt.
 
I think you guys are on the right track here, an alternator needs an "exciting" current from the battery initially and probably isn't getting it. You should have a circuit that lights a lamp when the ignition is switched on and goes off when the battery is charging. Without this, or if you have a break in the circuit to this you won't get a charge.
 
Any alternator needs an 'excitement' voltage to the windings (which for an automotive setup is usually supplied via your warning lamp).

If all is wired correctly once the alternator is generating it supplies part of its output back up the exciter wire, so you end up with +ve voltage either side of the lamp and it goes out.

Don't want to teach you to suck eggs, but if you disconnected any electrics (to or from the alternator) during the re-build are you sure it went back as before?
 
To check the speed of the alternator simply measure the crankshaft pulley and then the alternator pulley.

Example - 6 inch crank, 3 inch alternator would be a 2:1 pulley ratio. Eg at 1000RPM, alternator would spin at 2000RPM

Most alternators need approx 1500 RPM to start charging.

If the batteries are flat, you may only see 12.5V for a while but it should, over time start to increase.

Not all alternators have a warning lamp, not all have a separate D+ excitation terminal.

Are you sure the batteries aren't just very flat and taking a while for the voltage to rise?

You may have a dynamo rather than an alternator. If you have the former, you may want to consider replacement. If you have an alternator, the regulator may be simply set to low - an alternator shop should be able to replace this with a higher output voltage regulator.

Cheers - James
 
Sorry, you are right James I wrongly said 'Any' . IMHO excitation wires are fairly common on automotive type alternators.

Going back to the original post - you say that the alternator is new - what terminals are found on the new unit ?
- and what type of unit were you replacing when you did the overhaul?
 
From memory, automotive alternators have only two needed connections, a warning lamp, small terminal, through bulb to earth via ignition switch, and large terminal - output to battery possibly via ammeter.

Someone tell me my memory is wrong here?
 
It may be your charge warning light is too small a wattage- try shorting out/ bridging the bulb contacts with the engine running at speed, and see if it starts charging. If it does, look for a higher wattage bulb, or add another of what you have in parallel with the original.
 
Thanks for the responce.

It is possible that the batteries are a little flat so I will try that. The warning light cannot go to the ignition as it only has power to it to start the engine, after which the key springs back to the off position. Due to this I have wired the light straight to a battery. It has a warning light and a positive output on the back of the engine and is about 2:1 as far as I can see. The warning light I purchased originally was too low in terms of wattage to kick start the altenator, so I have wired it through a cabin light to test.

Just going down to the boat with the idea of wiring the altenator straight to a battey, thereby bypassing all the exsisting wiring to see if that makes a difference. Altenator voltage increases by about 0.8 volts from tick over to full revs ~3000, but still never passes 12.8 volts.
 
This engine uses an oil pressure switch to connect alternator to batteries when above tickover to take load off alternator for starting.check this switch.
 
just a thought- are you using a cheap digital multimeter? some of the really cheap ones are notoriously inaccurate.
 
Yep - its a cheap multi meter but it does 'seem' accurate will check against another just incase. Not sure how to check the oil pressure sensor, any ideas??

Tried putting two bulbs in parallel but still not enough to 'kick' the altenator in!
 
On most small Peters the alt feed via a warning light also goes through the lube oil pressure switch,where the contact is open until the oil/engine is up to speed. If your engine does not have this system then it may have been modified at some time.
 
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