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I’ve got a 1.5.BMC and it looks like there is no output from the alternator (lead to starter is siting at 12.6V) and charge light on. Firstly is there anything else I should check or is it a knackered alternator ? 
assuming it does need replacing I’m wondering what the best approach should be. 
I’ve got acorn kestrel 90 alternator controller which was clearly fitted when the boat was built in the early 80s and I’ve been told these are next to useless. 
I plan to get a couple of PV panels in the next 12 months and plan keep the boat all 12v (perhaps a small inverter for very occasional use). 
I could do without shelling out loads of cash right now to do the whole lot in one go but have you got any advice as to what I should do with the alternator ?

 

btw, I run the engine for 30-45 mins every day to heat the calorifier but I’m hoping that this along with the PVs should give be enough charge so considering whether a higher output alternator might be worth investing in. 

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1 minute ago, Strettonman said:

I’ve got a 1.5.BMC and it looks like there is no output from the alternator (lead to starter is siting at 12.6V) and charge light on. Firstly is there anything else I should check or is it a knackered alternator ? 
assuming it does need replacing I’m wondering what the best approach should be. 
I’ve got acorn kestrel 90 alternator controller which was clearly fitted when the boat was built in the early 80s and I’ve been told these are next to useless. 
I plan to get a couple of PV panels in the next 12 months and plan keep the boat all 12v (perhaps a small inverter for very occasional use). 
I could do without shelling out loads of cash right now to do the whole lot in one go but have you got any advice as to what I should do with the alternator ?

 

btw, I run the engine for 30-45 mins every day to heat the calorifier but I’m hoping that this along with the PVs should give be enough charge so considering whether a higher output alternator might be worth investing in. 

 

In their day Kestrels were not useless but often only covered up other faults and allowed too much scope for ruining batteries by re-triggering. I would rather have a 14.5V regulated alternator these days.

 

In the alternator belt in place and tight?

 

Pull the thin D+ cable from the alternator and see if the light stays on. If yes then you have a short on the warning lamp to alternator wire. If no the brushes in the alternator are probably OK so probably overhaul or new alternator time.

 

Best approach is look for a LUCAS A127 clone of 70 amp output and a regulated voltage of around 14.4 to 14.5 volts in my view.  Kill the Kestrel, but make sure the charging leads are fat enough and ensure that you are nor using a simple split charge diode.

 

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Hi tony,

I think I need a little education on alternators here. 
 

I disconnected the small spade connectors at the back of the alternator and the charge light stayed on and still the voltage at the +ve starter terminal (to which the main alternator lead is connected) was at 12.6V.

 

i then disconnected the lead on the split charge relay ( DURITE 0-727-18) that I am pretty sure is a T off the lead between the small spade connector and the warming light. The light went out. Does this suggest a fault in the relay ? I only had it replaced a few months ago. 
 

One area of education I need is when an alternator delivers its charge voltage. I thought that they pumped our 14+V all the time. When it works normally mine seems to put out 14.5V. 
 

if the short is in the ‘measuring/control’ circuit to the relay and warming light how does that prevent the alternator delivering 14+V . 
 

also what voltage levels should I see on the small spade connector on the alternator ? 
 

cheers

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24 minutes ago, Strettonman said:

Hi tony,

I think I need a little education on alternators here. 
 

I disconnected the small spade connectors at the back of the alternator and the charge light stayed on and still the voltage at the +ve starter terminal (to which the main alternator lead is connected) was at 12.6V.

 

i then disconnected the lead on the split charge relay ( DURITE 0-727-18) that I am pretty sure is a T off the lead between the small spade connector and the warming light. The light went out. Does this suggest a fault in the relay ? I only had it replaced a few months ago. 
 

One area of education I need is when an alternator delivers its charge voltage. I thought that they pumped our 14+V all the time. When it works normally mine seems to put out 14.5V. 
 

if the short is in the ‘measuring/control’ circuit to the relay and warming light how does that prevent the alternator delivering 14+V . 
 

also what voltage levels should I see on the small spade connector on the alternator ? 
 

cheers


The circuit is from battery + via the warning light, to both the alternator D+ and the split charge relay coil. So if either or both is connected and ignition on engine not running, one would expect the warning light to be on (maybe at different brightnesses). So having disconnected the alternator and then the relay, one would expect the light to go out after the second disconnection - this is normal behaviour for an alternator that isn’t rotating.

 

So I think it is necessary to reconnect the wire to D+ but leave the relay disconnected. With ignition on, is the light on or off? If off, there is a fault with the alternator rotor circuit, most likely the slip ring brushes. If on and the alternator is definitely spinning fast (belt not slipping, engine revved up a bit) then something is up probably with the field diodes. But let us know what happens with the D+ connected and the split charge disconnected and we can take it from there.

 

To answer your question about the purpose of the D+ connection / warning light circuit, this provides initial current to the rotor coil to generate magnetic field to “kick start” the process. Without it, the alternator won’t start working. With it, and once the alternator starts producing power, the voltage on the D+ terminal is internally raised to battery positive voltage (14.4v or whatever) and with both ends of the light at the same voltage, no current flows and the light goes out. So the light / D+ circuit is only needed to get things going. Once going, you could remove the bulb and nothing would happen. Until you tried to start it next time.

Edited by nicknorman
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Thanks chaps. 
 

an idiots guide to charging would be great. I’ll be very happy if it is just the relay. 
 

With both relay and D+ connected IGN on and engine NOT running light is on. When I disconnected the relay the light us still in but slightly dimmer. If I start the engine with some revs do the alternator is spinning quite fast the light is still on. 
 

does this all still point to the relay. I had some work done a few months back and a couple of days before the engineer arrived I had a charging problem. He diagnosed a relay and swapped it for one he had in the van. Are all relays essentially the same or do you need to be careful which one you fit ? 

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54 minutes ago, Strettonman said:

Hi tony,

I think I need a little education on alternators here. 
 

I disconnected the small spade connectors at the back of the alternator and the charge light stayed on and still the voltage at the +ve starter terminal (to which the main alternator lead is connected) was at 12.6V.

 

i then disconnected the lead on the split charge relay ( DURITE 0-727-18) that I am pretty sure is a T off the lead between the small spade connector and the warming light. The light went out. Does this suggest a fault in the relay ? I only had it replaced a few months ago. 
 

One area of education I need is when an alternator delivers its charge voltage. I thought that they pumped our 14+V all the time. When it works normally mine seems to put out 14.5V. 
 

if the short is in the ‘measuring/control’ circuit to the relay and warming light how does that prevent the alternator delivering 14+V . 
 

also what voltage levels should I see on the small spade connector on the alternator ? 
 

cheers

 

Now you identify a split charge relay your tests suggest to me that the warning lamp circuit is OK and that the alternator should energise even if you have to rev it a bit unless the relay coil has shorted out so do as Nicknorman says. Reconnect the D+ but not the relay and report what happens.

 

Small spade on  the alternator: assuming its D+ and not the W (rev counter) or radio suppressor terminal. Cable off and/or ignition off and engine stationary = all but zero volts. With cable back on, ignition on  and engine stationary = more than zero but less than 12. It depends upon the relative resistances of the warning lamp bulb & rotor. Engine running with energised alternator (cable on or off) = charging voltage.

 

How it works.

 

You have a voltage regulator that might be set to anything between about 13.8V on old alternators to about 14.7 on a few modern ones. In most cases that is built in.

 

Alternators are designed so that in theory they can not burn themselves out so as they produce more and more current the voltage they produce falls. At their rated output or a bit more the maximum voltage they will produce may be a slow as perhaps 12.5 volts but usually above 13 volts. The converse is also true, as the current falls the voltage they can produce rises and I have had it well above 100 volts under demo conditions.

 

At first start with discharged batteries the current will be high so the voltage produced will be low. It will be well below the regulator setting, so the regulator does nothing.

 

Check the voltage when you have well discharged batteries and then over the next few hours watch it rise.

 

As the batteries charge they  start reducing the current they will accept so the voltage starts to rise until its approaching the regulated voltage. At that point

regulator starts to work and stops the voltage rising further but as Nick says the regulator is a bit soft so its not a sudden regulation but you may as well think that it is.

 

Hence we can work out that your regulator has  a maximum setting of 1.5 volts so is probably relatively modern.

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17 minutes ago, Strettonman said:

Thanks chaps. 
 

an idiots guide to charging would be great. I’ll be very happy if it is just the relay. 
 

With both relay and D+ connected IGN on and engine NOT running light is on. When I disconnected the relay the light us still in but slightly dimmer. If I start the engine with some revs do the alternator is spinning quite fast the light is still on. 
 

does this all still point to the relay. I had some work done a few months back and a couple of days before the engineer arrived I had a charging problem. He diagnosed a relay and swapped it for one he had in the van. Are all relays essentially the same or do you need to be careful which one you fit ? 

 

The current through the bulb has the option of flowing to negative via the alternator rota and the relay coil. Both have resistance. The rotor 3 to 4 ohms but the relay will be higher. So with both connected the current can get to negative both ways so the lamp is bright. Pull the relay coil off and there is no alternative path to negative via the relay coil so the bulb will dim a little.

 

Now the crux of the matter. With D+ disconnected and the relay coil connected the high coil resistance should result in a dim or very dim bulb but it appears yours is bright. That suggest zero resistance through the coil (burned out) so not current wants to flow through the rotor thus the alternator can't energise.

 

All relays are pretty much the same, but their current capacity and coil resistance varies. overloading the relay contacts is very unlikely to burn out the coil but if its all mounted on plastic   the plastic may overheat and distort so something shorts out.

 

Red bit - try the electrical notes on tb-training.co.uk

Edited by Tony Brooks
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11 minutes ago, Strettonman said:

Thanks chaps. 
 

an idiots guide to charging would be great. I’ll be very happy if it is just the relay. 
 

With both relay and D+ connected IGN on and engine NOT running light is on. When I disconnected the relay the light us still in but slightly dimmer. If I start the engine with some revs do the alternator is spinning quite fast the light is still on. 
 

does this all still point to the relay. I had some work done a few months back and a couple of days before the engineer arrived I had a charging problem. He diagnosed a relay and swapped it for one he had in the van. Are all relays essentially the same or do you need to be careful which one you fit ? 

So just to be clear, with the warning light connected to D+, the relay coil disconnected, the engine running fast, the warning light is still on? If so, this points to an alternator fault. The fault is likely to be that the field diodes have blown. This could have happened due to a fault with the relay / relay wiring which has damaged the diodes.

 

But the first step is to get the alternator working - it will need to be repaired or replaced. Then have a look at the relay circuit to make sure it isn’t shorted to ground BEFORE you reconnect it! The relay coil will probably have a few 10s of ohms resistance - if you give precise details about the relay we can check the value, or it might even be marked on the relay. If the resistance is effectively zero ohms, then it is faulty and will damage the alternator again.

 

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When you get back to it if you want a quick and dirty test of the alternator.

 

Remove the D+ cable and get the engine revving. Take a length of cable connected to the engine battery positive or the alternator B+ (thick wire) terminal and slowly stroke the bare end of that cable  across the D+ connection. If the alternator works the voltage should jump up. TAKE CARE not to let that bare end touch any metal part of the boat and engine.

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1 hour ago, Strettonman said:

The missus has just fallen into the engine bay of our cruiser back with the deck up

 

You have a basic problem there. The boss shouldn't be anywhere near the engine 'ole when her servant is playing.

 

In my case, the Memsahib would have been knitting in the front saloon to the strains of Classic FM.

 

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4 hours ago, Strettonman said:

Chaps, 

 

sorry but got to halt my investigation. The missus has just fallen into the engine bay of our cruiser back with the deck up so I’ve got to divert my attention in order to save my marriage. 
I’ll try to get my head round your comments later. 
 

thanks. 

Oh dear, I hope she is OK.

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Back on the case. 
 

it all got a little nasty last night. She dropped right through the deck and was stopped when her stomach hit the deck support bar (now badly bruised) . She then proceeded to faint while I was trying to help her. On the upside it gave me a good excuse to slap her around the chops. There was no evidence of it but she must have bashed her head as for the first time every she admitted it was her fault entirely !!! Stepping across deck support bars in flip flops was never going to end well. 
 

back to the important subject of the alternator. 
 

nick - yes, with D+ connected , relay disconnected and engine running fast the warning light is on. 
 

Tony - I’ve then disconnected the D+, started the engine, put it on decent revs and brushed a lead connected to the battery over it. No jump up in voltage (sitting at about 12.5V). 
 

no signs of melting relay housing. 
 

I’ve attached a photo of the relay. 
 

It sounds like I need to get the alternator off for test and repair or replace. Is repair a cost effective option and any idea where to take it. I’m on GU Leicester arm near foxton. 

56F1DE08-DED7-45A7-9983-25E7A5078563.jpeg

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6 minutes ago, Strettonman said:

Back on the case. 
 

it all got a little nasty last night. She dropped right through the deck and was stopped when her stomach hit the deck support bar (now badly bruised) . She then proceeded to faint while I was trying to help her. On the upside it gave me a good excuse to slap her around the chops. There was no evidence of it but she must have bashed her head as for the first time every she admitted it was her fault entirely !!! Stepping across deck support bars in flip flops was never going to end well. 
 

back to the important subject of the alternator. 
 

nick - yes, with D+ connected , relay disconnected and engine running fast the warning light is on. 
 

Tony - I’ve then disconnected the D+, started the engine, put it on decent revs and brushed a lead connected to the battery over it. No jump up in voltage (sitting at about 12.5V). 
 

no signs of melting relay housing. 
 

I’ve attached a photo of the relay. 
 

It sounds like I need to get the alternator off for test and repair or replace. Is repair a cost effective option and any idea where to take it. I’m on GU Leicester arm near foxton. 

 

 

Oh dear, if that relay coil has shorted possibly the field diodes have failed so:

 

Check the resistance of the coils is, as Nick said, a few tens of ohms and if it is it probably OK but I would take the black cover off  for  good look inside.

 

Then its a new alternator or take yours for overhaul.

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I used lots of those relays and had very few problems with them. You can test it by tapping the D+ wire onto the B+ terminal on the alternator, you should hear and feel the relay actuating.

 

 

Prestolite have a mobile engineer, Mark Adams  MAdams@prestolite.com   07771978746  They also have stockists throughout the UK, I have found them very helpful.

Edited by Tracy D'arth
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Ok. Thanks. I’ll check the relay but it sounds like if the alternator is not worth repairing it’s a new one and possibly look at a 70 amp Lucas A127 clone and ditch the kestrel controller. 
 

regarding the cable size the current cable between the alternator and starter is roughly the same as the battery to starter so I guess that means it will be fine as an alternator surely will not output more current than is drawn when cranking. 

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