engine batt not charging,alternator/electrics testing advice needed

Hi, I am hoping there are some tests that I can do to narrow down what the hell is going on with my charging system!

I have 3 banks (inverter,house,engine) and 2 alternators. The engine battery isn't charging and only has 3 wires attached: an earth to the engine, an earth to the other batt banks, and a positive to the starter (presumably to the solenoid). There wasn't a problem with the engine battery charging previously, so I assume that this wiring is sufficient. The other two banks are charging at 14 point something.

However, I dont understand how the engine batt can be charged, so I dont know where to begin with testing things!! Does the charge come through the starter motor? In one of my other boats, I have a charging wire and a starting wire (positive) to the battery.

I have measured the output of the top alt and its 15.3V. the bottom alt has a "plug in" connection, so hte only way to measure this is to pull it off and run the engine with this off.
Is this safe/good idea to do?

One of the alternators has a belt that slips sometimes (can hear it, dont know which one) and when I hear it slipping, the rev counter stops working and the engine batt starts charging. This usually happens when I have recently started engine (lister 4 cylinder on narrowboat) and stops when things have presumably heated up.

I have a Sterling 4 step Alternator Booster. I believe this may provide the alarm I hear. On this box a light shows Low Voltage Warning. On the engine panel, the batt light lights up (so could be producing the alarm noise, however, I think it is coming from the sterling box.)

The volume of the noise and the strength of the engine panel light increase together. If I increase the revs (last time to 2000) the noise and light go out: however, the engine battery still ISN'T charging. Its all weird to me.

The set up also has a split charge diode (that's what it looks like anyway) and a box called Control Relay Box - 2 are to do with heaterplugs, cant remember what the third says.

Anyone got any ideas - I dont even understand what all the wires coming out of the alternators are for.

thanks, sarah

most engine alternator's have their output wire going to the BAT terminal at the starter's solenoid. being as I can't see what is where, wiring-wise, you'll have to check that. Some also install inline high capacity fuses in that line as well. Best thing to do is trace the wire fromm the alternator's output lug.

I wouldn't suggest unplugging an alternator on a running engine unless you know exactly what connects where. If the output is 15.3VDC, It may be ok depending upon where the regulator is sensing.

Slipping belts should be tightened or replaced; not good on anything.

Split charge diode - over here we'd call that an isolator. That could also be the source of the charge for the starting battery. Again, you'll have to trace it out to know.

Based on the control panel description, sounds like something isn't working. May be best to figure out which alternator connects to where and take it one step at a time.

For a battery to charge, the source needs to provide current at the proper voltage. Lots of ways of making the combination but if the product isn't right, the battery won't charge.

Thanks for advice. Now know that the engine charging alternator is measuring the same volts as the engine battery. When I hear the screeching of the belt (upon start up and once or twice when running) volts at this alternator goes up to 14.07V briefly and the battery charges at 13.2 or 13 point something.

If I rev the engine at other times I get slightly more volts at the alternator but not at the battery: 10k revs = 12.60 (or whatever the battery has got to as its voltage sinks back down from a brief charge) 15k-20k revs = 12.7 - 13.22 and this voltage constantly changes between those two measurements. The warning light/klaxon stop at high revs, however the battery stays at 12.5 or wherever it has got too with its sinking back down to its resting voltage ie not charging.

The route appears to run between the alternator and the box of relays, with 3 red wires poking out and attaching at the solinoid where the fat one from the battery attaches. The box of relays has 1for heater plugs, 1 fuel control solenoid relay, 1 starter motor relay,an aux circuit breaker,fuel control solenoid breaker. I assume that some of the wires go up to the relays and return back to the different things ie glow plugs/starter. If any of these are having a problem, I cannot see it affecting the charging - can you?

The glow plugs appeared all blown (which I thought strange) and I have replaced them. The engine starts in cold weather now.

I believe that the split charger is for the other alternator (putting out 15.4V and charging both house banks fine) and My guess is that the alternator in question is only working very occassionally, and when it works the extra drag makes the belt slip. However, I am appreciative of any other theories and help.

thanks

The issue could be as you described. I'd be inclined to look for a bad connection somewhere in the charging path. Usually easiest to see by looking for an excess voltage drop somewhere in the circuit. I'd label any voltage drop over 0.2vDC as suspect. Don't forget the 'earth side' of the loop as well. Many OEMs install a dedicated ground lead (#10 AWG) between the alternator frame and the engine block to ensure a good ground path.

1st, I would tighten / replace the alternator belt. If it's "screaching" in the slightest, it's slipping and too loose and you won't get the full 13.8 to 14.5 VDC charge at full output of alternator.

Once that is corrected, then start your troubleshooting.

Thanks for replies, it was the alternator that was faulty: been replaced now. this thread can be closed.