Alternator excitement

[TernVI]

Try connecting the correct rating of bulb directly from the B+ of the alternator to the WL terminal.
If it works wired directly but not through the spaghetti, then the spaghetti is the problem.
If it doesn't work directly then your alternator is sick, it's unlikely to get better, quite likely to get worse or die.

Adding resistors which were not originally needed and should not be needed now is a sticking plaster for a terminal illness.
But it can help diagnosis. And it can allow the use of LED bulbs.

Better to measure the current going into the WL terminal (engine not running) and see if it's fairly consistent with the bulb rating.
The WL terminal will be a bit above 0V, so a nominal 2W bulb might only give 150mA or something, but a good alternator should excite with a margin for low battery.

But first check that with the light on, it has 12V on it and that the body or -ve terminal of the alternator is 0V relative to the battery.
Bad earths mess up diagnosis!

 

[in irons]

Well done! Progress indeed. It would appear that there is an unacceptable impedance in the wiring between the start switch and alternator’s field connection. Measuring voltages along the route doesn’t always tell the true story as to be meaningful the test should be carried out under dynamic conditions i.e with a current flowing. However, are you able to measure the resistance the resistance in the wiring between the start switch and the field terminal (with the terminal disconnected from the alternator? it should not be significantly more than the resistance of the bulb.
Mike

From the terminal on the ignition switch to terminal 4 on the distributor plate:
2.3 ohms

From terminal 8 on the distributor plate to terminal 61 on the alternator:
2.7 ohm

The warning lamp itself ( from connectors on back 56 ohms

 

[179580]

Seem to remember in past days that some alternators need to see battery voltage at the B+. One of the diode split charge manufacturers had a fix using a relay to provide a squirt of wrigglies when start motor was actuated. Looks like that is exactly what is happening here.

 

[TernVI]

From the terminal on the ignition switch to terminal 4 on the distributor plate:
2.3 ohms

From terminal 8 on the distributor plate to terminal 61 on the alternator:
2.7 ohm

The warning lamp itself ( from connectors on back 56 ohms

I reserve the right not to trust your ohmeter, but a cold light bulb should have a low resistance.
Values like 2 ohms can be meter error. Or not!
Because resistances are often not cnstant in these kinds of problem, it is often better to measure current and voltage.

 

[in irons]

Try connecting the correct rating of bulb directly from the B+ of the alternator to the WL terminal.
If it works wired directly but not through the spaghetti, then the spaghetti is the problem.
If it doesn't work directly then your alternator is sick, it's unlikely to get better, quite likely to get worse or die.

Adding resistors which were not originally needed and should not be needed now is a sticking plaster for a terminal illness.
But it can help diagnosis. And it can allow the use of LED bulbs.

Better to measure the current going into the WL terminal (engine not running) and see if it's fairly consistent with the bulb rating.
The WL terminal will be a bit above 0V, so a nominal 2W bulb might only give 150mA or something, but a good alternator should excite with a margin for low battery.

But first check that with the light on, it has 12V on it and that the body or -ve terminal of the alternator is 0V relative to the battery.
Bad earths mess up diagnosis!

Is the WL terminal the one on the alternator for the Excitor?
I am finding the alternator terminology a bit confusing. There seem to be numerous terms for the same thing.

The alternator I have is brand new.
Connections made are:
B+ connected to battery positive via the supply to solenoid.
B- connected to ground on the engine
D+ connected to terminal 8 on the distributor board on control panel.

From what you say I imagine you mean to, in my case connect bulb between B+ and D+ in order to cut out the distributor panel and wiring.

I take your point about the addition of resistors.
The 5w lamp I brought to test is providing excitement. I tried it with the original indicator lamp connected as well and all then functions correctly.
The board has a 68ohm 3w resistor between the points I have bridged with the lamp and I am now wondering if I have a dry joint on that as I believe these were added by Bukh in order to ensure sufficient current for excitement.
I am wondering if the aftermarket alternator I have requires a little more current to excite it and the 5w lamp seems to be doing the trick.
It goes out on starting indicating that the alternator is charging and that is confirmed by voltage measured at battery.

I have just taken following readings.
D+ to ground ignition off 0v
D+ to ground ignition on 3.41v
B- to ground ignition on 0v

From what I have gleaned that all looks good to me, but I am all ears.

 

[in irons]

I reserve the right not to trust your ohmeter, but a cold light bulb should have a low resistance.
Values like 2 ohms can be meter error. Or not!
Because resistances are often not cnstant in these kinds of problem, it is often better to measure current and voltage.

I could well be getting thrown off here as the multimeter I keep on the boat is a cheapy. I have checked some resistors with it though and the readings seem ok.
The other thing is with old connectors I could be picking up resistance from those in readings.

 

[TernVI]

Yes, alternator connection terminology varies.
'WL' is 'warning light' in my post. sometimes known as D+, legacy from Dynamo.

'W' is a 'winding', an AC output used for rev counters.

Most cars only have 2.2W alternator light bulb. 5W is a tail light.

 

[in irons]

Yes, alternator connection terminology varies.
'WL' is 'warning light' in my post. sometimes known as D+, legacy from Dynamo.

'W' is a 'winding', an AC output used for rev counters.

Most cars only have 2.2W alternator light bulb. 5W is a tail light.

It is indeed a tail light bulb that I am using.
I did try previously with a 3W bulb in place of the warning light but that did not provide excitation.
As it is for now I can leave the 5W bulb connected and at least everything is functioning until I get to the bottom of it.

 

[Boater Sam]

With a resistor across battery to D+, is the original warning lamp an LED?

It excites with a 5w bulb, suggests that either the wiring is poor or the alternator is not great. But if it works, you could stay with it if you are happy with such a large bulb.

 

[in irons]

With a resistor across battery to D+, is the original warning lamp an LED?

It excites with a 5w bulb, suggests that either the wiring is poor or the alternator is not great. But if it works, you could stay with it if you are happy with such a large bulb.

The original warning lamp is the original Bukh one.
I am at home now and referring to the Bukh Workshop Manual I see on page 231 they state that:
"Further the distributor plate is fitted with two pieces . of 270 ohm resistances ( Bukh pt No 552A2027) out of consideration for the exitation of the alternator which demands two watts. In previous executions of operating panels this demand was satisfied by using a charging lamp of 2W , wheras the type now used is 0.5W and thus makes the insertion of resistances necessary."

My plate had a 68 ohm 3 watt resistor, so it seems this has changed over time.

Since Bukh used a resistor in this way I am thinking of trying to determine what resistance I need to replace the one fitted with so that I can get rid of the additional 5W lamp.

Although I used contact cleaner and a small wire brush to clean the contacts I suspect the job could be much improved on.
It could also be that the old wiring has suffered from 'black wire' due to damp conditions over the years and has developed resistance so it may be that I will need to replace some of that.
At least I have it working in a way now and can resolve matters properly step by step.

 

[Boater Sam]

It would suggest that the original bulb is either an led or a grain of wheat bulb hence the resistor is required. 68 ohms would pass a current sufficient to equate to a bulb of about 2 watts.
Is the resistor still 68 ohms or has it gone high in value? If it has that's why it doesn't work.
I would try a 3 w bulb, if it works all well and good.

 

[in irons]

The 68 ohm resistor checks out when I tested it across the leads with my (cheapo) multimeter.
I am wondering if it could be a dry joint where it is fitted to the plate or possibly a bad connection on the plate terminals preventing it from coming into play.

 

[Boater Sam]

Too many probabilities, you need to investigate and whittle them down. In the end I bet you find that the alternator is not up to snuff.

 

[in irons]

I hope that is not the case as the alternator itself was fitted brand new on the journey back when I picked up the boat.
I tend to think the symptoms have been caused by degradation of the wiring and connectors.
Time, and patient investigation will tell.
At least it has made me familiarise myself with the Engines wiring!

 

[PaulRainbow]

I hope that is not the case as the alternator itself was fitted brand new on the journey back when I picked up the boat.
I tend to think the symptoms have been caused by degradation of the wiring and connectors.
Time, and patient investigation will tell.
At least it has made me familiarise myself with the Engines wiring!

Bukh fitted resistors because the system needed them, so ignore all the tales of not fitting resistors.

Try fitting a 5w 50ohm resistor in place of your test lamp.

 

[in irons]

Bukh fitted resistors because the system needed them, so ignore all the tales of not fitting resistors.

Try fitting a 5w 50ohm resistor in place of your test lamp.

Bukh fitted resistors because the system needed them, so ignore all the tales of not fitting resistors.

Try fitting a 5w 50ohm resistor in place of your test lamp.

I will order some and give that a go.
Thank you to all for your help.

 

[oldmanofthehills]

I wonder if anyone with an understanding of electrics could shed light on the issue I have?
My boat has a Bukh DV20 fitted.
I have replaced the alternator but when I start the engine there is no alternator output and the charge light on the control panel stays on.
If once the engine is running I briefly make a connection between terminals 4 & 8 on the control panel distributor plate ( the ones linked by the 68ohm 3watt resistor) the alternator starts to produce output.
I have replaced the diodes and large resistor on the distributor plate and cleaned the terminals on the plate and the harness connector. But still have the same symptoms.
When checking I noted that with ignition switch in on position from terminal 61 on the alternator (exitor) to ground I got a reading of just over 2 volts. When I disconnected the wire from terminal 61 and measured the voltage from the wire to it to ground I got 12.6v.
I have been puzzling over the circuit diagram but am none the wiser.
I am wondering if it would be ok to put a push switch wired between terminals 4&8 on the distributor plate.
Or would that be likely to do harm?
Would the regulator in the alternator be able to sense voltage correctly or could it overcharge?
I did have to manually excite it when I picked the boat up and brought her home but was hoping to resolve the situation before launching again.

The Bukh standard alternator is not self exciting so the output must be connected to a charged or part charged battery so has a small voltage on output to power the field coil to get it going. A bit of a nuisance if you go via diode splitter or anything that isolates it, then you need temp connector across one of the diode or resistor just at or after starting. ( I used a switch others use fancy relay)

Some Bukh alternators had a built in trickle feature bolted on to do the job, but when I changed alternator or engine I discovered not all had it.

Ex owner of Bukh20 and Bukh24

 

[TernVI]

It would suggest that the original bulb is either an led or a grain of wheat bulb hence the resistor is required. 68 ohms would pass a current sufficient to equate to a bulb of about 2 watts.
Is the resistor still 68 ohms or has it gone high in value? If it has that's why it doesn't work.
I would try a 3 w bulb, if it works all well and good.

There is some subtlety, the resistance of the 2W bulb drops as it cools, when the alternator is in the process of waking up, it may look like a quite small resistance that would dissipate a lot of heat if it ever saw the full 12V.

You often find a 'slow to start' alternator will spring into life given a few more RPM?
I've heard of (but not personally seen) brand new alternators taking more RPM to get going, because they have no residual magnetism in the rotor. A bit like old British motorbike dynamos which had no warning light.
Your new alternator is likely internally different from the one it replaced. Regulator modules evolve. So maybe it needs a different bulb than the old one?
Sometimes things which are 'equivalent' only work the same in a certain installation. Maybe the alt supplier can advise?
Is the pulley the same old one?
Some boat engines really could do with spinning the alternator faster.
Maybe Bukh had to add the resistor because they were operating on the edge of the original alt's performance?

If it works OK with a 5W bulb and nothing else, it should work with a bulb in parallel with the original to bring it up to (roughly) 5W.
If not, the wiring is prime suspect.

 

[oldmanofthehills]

There is some subtlety, the resistance of the 2W bulb drops as it cools, when the alternator is in the process of waking up, it may look like a quite small resistance that would dissipate a lot of heat if it ever saw the full 12V.

You often find a 'slow to start' alternator will spring into life given a few more RPM?
I've heard of (but not personally seen) brand new alternators taking more RPM to get going, because they have no residual magnetism in the rotor. A bit like old British motorbike dynamos which had no warning light.
Your new alternator is likely internally different from the one it replaced. Regulator modules evolve. So maybe it needs a different bulb than the old one?
Sometimes things which are 'equivalent' only work the same in a certain installation. Maybe the alt supplier can advise?
Is the pulley the same old one?
Some boat engines really could do with spinning the alternator faster.
Maybe Bukh had to add the resistor because they were operating on the edge of the original alt's performance?

If it works OK with a 5W bulb and nothing else, it should work with a bulb in parallel with the original to bring it up to (roughly) 5W.
If not, the wiring is prime suspect.

I would support that. Nothing magic in resistor values and boat wont sink or blow up if you get it wrong. I used 100 ohm resistor which is about 1.5W, then went for direct connect pushbutton as that resistor needed rev up when I didnt want it. 5W bulb about 30 Ohm (W=Vsq/R).

 

[in irons]

Thank you for the above information.
I was wondering about a pushbutton as in effect that was what I had to do to excite the alternator once I replaced the failed one on the trip home.
Only I had two wires on the connections that we would briefly touch together once the engine started.
I was unsure whether it could do damage though.
It seems not.