Battery charging/alternator problems

[Ca Jon]

Hi guys.

5 new batteries one starter. The red light on ignition has been staying on for a while but the alternator has been giving charge. In the last week the alternator seems to be giving 14.4 v but only the starter battery is charging. I have a simple durite relay. Could that be the problem?

[nicknorman]

Hi guys.

5 new batteries one starter. The red light on ignition has been staying on for a while but the alternator has been giving charge. In the last week the alternator seems to be giving 14.4 v but only the starter battery is charging. I have a simple durite relay. Could that be the problem?

 

You're giving us pretty scant information Mr Jon, however the circuit that operates the red warning light is probably the same one that operates the relay. All I can suggest is to check out the wiring between the warning light, relay and alternator. Normally the relay will be connected between the alternator terminal and ground, and the light is connected between the same alternator terminal and +12v (when the ignition is on).

 

The power to operate the relay comes from the alternator field circuit via some diodes and so I suppose it could be that one or more of the field diodes is faulty and thus not enough oomph to operate the relay. This would reduce the alternator's charging capability, but not stop it altogether.

 

If you can, try to measure the voltage on the relay coil connection (the one that goes to the alternator) with the engine running.

 

Oh and exactly how does the red light behave? On bright when you turn the ignition on, and still on bright after you start and rev the engine? Or on dimmer after start? When it goes out does it do so suddenly or gradually get dimmer over time?

 

So many questions!

[Ca Jon]

Thanks. I will start the old lister tomorrow and put a meter across the alternator. The red light used to go out with revs, for a while it has been on and gets dimmer as revs fall. Now it is constantly on.

[Arthur Brown]

So you have one alternator and two battery banks?? Only the starter battery is charging? So the linking circuit has failed.First as a bodge join the Starter positive to the house positive see whether the house battery starts to charge. This season you should have no need to charge off the engine if there is even a moderate solar panel.

 

Then, in a hurry you need to draw the circuit of all the parts in the charging circuit, with part numbers and possibly post it here for advice. Then you may understand the electrics enough to solve your problem and all the ones to come!

[nicknorman]

Thanks. I will start the old lister tomorrow and put a meter across the alternator. The red light used to go out with revs, for a while it has been on and gets dimmer as revs fall. Now it is constantly on.

Sounds a bit like an internal alternator fault then (diodes) but checking the voltage on both the main alternator output, and the warning light connection, might give more clues. If possible manually connect the engine and domestic batteries and run the engine like this to check the voltage. A decent starter battery won't have a dent taken out of it to start an engine so the alternator voltage might seem reasonable if connected just to that one, simply because it is barely required to produce any current to top up the battery. Having the domestics in circuit too will give it something to think about.

 

Presumably you have checked the obvious ie belt tightness?

[Tony Brooks]

Sir N said once put a voltmeter between B+ and D+ (main output and w/l terminal) starts and rev the engine. If the meter reads more than about 1 volt the field diodes are in trouble

[nicknorman]

Sir N said once put a voltmeter between B+ and D+ (main output and w/l terminal) starts and rev the engine. If the meter reads more than about 1 volt the field diodes are in trouble

Yes that is a much better way of putting it, rather than measuring the two voltages and then subtracting.

[Tony Brooks]

Should have said - or main positive diodes. If a main negative one had failed then it may not show up with that test but the charging voltage and current would be low.

 

 

 

 

However both Nick dan I are assuming its a 9 diode alternator and not one of those A127A (I think) things SirN told us about within the last year where the electronics in the regulator just "throws a switch" and turns the warning lamp off. Note the A suffix. If an alternator change or a split charge diode rewire/replace has also taken place then check the alternator designation in case its open of these.

 

Also best to check its not yet another split charge relay wiring bodge where the main alternator output has been left on the engine batteries rather than being connected to the domestic bank so the relay contacts are now burned

 

Edited August 16, 2016 by Tony Brooks

[WotEver]

Also best to check its not yet another split charge relay wiring bodge where the main alternator output has been left on the engine batteries rather than being connected to the domestic bank so the relay contacts are now burned

 

It certainly sounds to me like that's exactly what he has. Plus a suspect alternator of course.

 

Tony

[Sir Nibble]

It does sound like one field diode went phut and now another has followed suit. I would imagine it's only getting the voltage it is because the start battery, as always is brim full charged. Replace the alternator, they're not worth opening. And sort out the wiring!

[Sir Nibble]

If you already have a repaired one then it's a no brainer chum. Fit it, after sorting out the wiring.