Engine alternator issue: dim light when engine running

[Tasemu]

Hi all, have another issue with a close friends boat and was hoping to get some advice from you. We have a new alternator and have installed it as per the manual. We are running into an issue where, when starting the engine the battery charging indicator light is on, however when we increase RPM with the intention of starting the charge cycle, instead of turning off the indicator light is dimming. As we increase the RPM the light dims further (to almost off) when RPM is exceedingly high. However when lowing the RPM, the light increases in brightness. 

 

Also we have noticed that the battery voltage is increasing roughly in sync with the RPM of the alternator from 13.5 at idle, up to 14.9 at full RPM.

 

We have also tried to diagnose a dirty ground by placing a extra ground cable from the alternator bracket directly to the negative of the battery, only for diagnostic purposes, however we have no change in results.

 

We have been told one of the field wining phases has been damaged, however upon inspection the windings appear to be in good condition, visually at least.

 

Thanks in advance for any and all advice!

[Tony Brooks]

 More likely, one of the field or aux diodes has is faulty BUT if there is a heavy load on the ignition switch such as you might have on a car with wipers and heater fan turned on (unlikely on a boat) it can be worn contacts in the ignition switch or resistance between the battery and switch. This is because the field diodes are supplying some of the load.

 

I doubt that will be the problem, but check all multi-plugs in the main loom and if there is an "ignition" fuse make sure that is clean and tight. Also bridge the master switches to see if one has gone resistive.

[Tasemu]

2 minutes ago, Tony Brooks said:

 More likely, one of the field or aux diodes has is faulty BUT if there is a heavy load on the ignition switch such as you might have on a car with wipers and heater fan turned on (unlikely on a boat) it can be worn contacts in the ignition switch or resistance between the battery and switch. This is because the field diodes are supplying some of the load.

 

I doubt that will be the problem, but check all multi-plugs in the main loom and if there is an "ignition" fuse make sure that is clean and tight. Also bridge the master switches to see if one has gone resistive.

 

Could you please elaborate on what a master switch is? As we haven't seen any.

[Tony Brooks]

The battery isolator switch(es), often with a big red plastic "key". If you don't have any on inland waterways it is a BSS fail.

[Tasemu]

2 minutes ago, Tony Brooks said:

The battery isolator switch(es), often with a big red plastic "key". If you don't have any on inland waterways it is a BSS fail.

 

We do have one, can't remember if we tried to bypass it weeks ago. But definitely worth a shot. Cheers!

[Tony Brooks]

If you have just one, then it is probably in the negative. If so, that is not likely to be the cause. If you only have one battery bank then the swich might be in the positive.

[matty40s]

It's also the most likely point of failure in a boat system if you are using the crap ones the Swindlers sell 

[nicknorman]

It may be a blown field diode. If you have a multimeter, check the voltage between the alternator case (or B-) and the D+ terminal, with engine running at moderate rpm such that you would expect the warning light to be out. You should get much the same voltage as when you move the positive meter lead to B+ (the fat cable from alternator to the battery). If there is a big difference, it points to a blown field diode in the alternator.

Edited February 28, 2023 by nicknorman

[Quattrodave]

2 hours ago, matty40s said:

It's also the most likely point of failure in a boat system if you are using the crap ones the Swindlers sell 

 

 

Indeed, replace it with a decent one before it fails... I've seen them fail both on and off I.e. you either can't turn it on or can't turn it off!!

[casper ghost]

10 hours ago, Quattrodave said:

 

Indeed, replace it with a decent one before it fails... I've seen them fail both on and off I.e. you either can't turn it on or can't turn it off!!

What would be considered a decent one as our boats will almost certainly have the cheapest that were available at the time.

[Tracy D'arth]

55 minutes ago, casper ghost said:

What would be considered a decent one as our boats will almost certainly have the cheapest that were available at the time.

Browse https://webshop.durite.co.uk/s/c/switches-and-indicators/battery-isolators  for the current required, I would suggest nothing under 200A

[Tony Brooks]

Further thoughts.  We don't know the make and model of the alternator, so although the answers above are probably correct for most alternators if the one is question is a six diode machine then it is just possible it is an internal fault with the warning lamp switching or if it is a Lucas 10/11AC (an antique system) it could be a faulty warning lamp control. Note - All unlikely.

[Tasemu]

37 minutes ago, Tony Brooks said:

Further thoughts.  We don't know the make and model of the alternator, so although the answers above are probably correct for most alternators if the one is question is a six diode machine then it is just possible it is an internal fault with the warning lamp switching or if it is a Lucas 10/11AC (an antique system) it could be a faulty warning lamp control. Note - All unlikely.

 

Prestolite Electric is the manufacturer. A127 is the generic model

[Tony Brooks]

7 minutes ago, Tasemu said:

 

Prestolite Electric is the manufacturer. A127 is the generic model

 

In that case, all original suggestions are correct. Nick's test is a good one, or put a voltmeter between B+ and D+ and rev the engine. There should be less than about 0.5V showing. If more, a field diode has failed. Basically it is the same test as Nick's but all in one set of connecting.

[Tasemu]

We connected d+ and b+ using a multimeter. Upon starting the engine we were getting a voltage drop of 10.5v, when increasing rpm to high speed the voltage drop went down to 0.3v. when returning throttle to idle, the voltage drop settled at 3.5v. also the indicator light dimmed and brightened accordingly.

 

We used little alligator clips, cause its so damn easy to short against the casing by accident!

 

EDIT: noticed we didn't supply a reading at moderate rpm, so here as an amendment to include it: 2.2v  on moderate revs. 0.3v on very high revs. 3.5v on returning to idle

Edited March 1, 2023 by Tasemu

[nicknorman]

Blown field diode.

[Tasemu]

1 minute ago, nicknorman said:

Blown field diode.

 

Could you explain your reasoning to me? I'd be really keen to understand and learn

[Tony Brooks]

If that is one meter between B+ & D+ then that 2.2 and 3.5V certainly suggests a blown field diode.

 10.5 before the alternator has energise is probably about right because the warning lamp and rotor form a voltage divider with most voltage dropped across the bulb.

 

To answer the question you asked Nick.

 

The alternator output is split between the field diodes and the main diodes. apart from the fact they are different sorts of diode so may cause a slightly different volts drop across themselves, once the engine is running both should be outputting the same (charging) voltage. The main diodes to charge the batteries and supply the engine electrics and the field diodes to power the alternator rotor, via the regulator, AND to try to push the charging voltage backward through the warning lamp. As the main diodes are putting the same voltage on the other side of the warning lamp, no current can flow, so the bulb goes out

 

The alternator has three generating coils, each with a positive field diode, a positive main diode and a negative main diode, so if a diode fails you only get the output from two coils. (The circuit from both field and main positive diodes return to the alternator via the same negative diode).

 

So, with a failed field diode the voltage pushing "backwards" through the warning lamp is less than that coming via the ignition switch so current can flow. That flow is usually enough to illuminate the warning lamp dimly.

 

 

[Tasemu]

13 minutes ago, Tony Brooks said:

If that is one meter between B+ & D+ then that 2.2 and 3.5V certainly suggests a blown field diode.

 10.5 before the alternator has energise is probably about right because the warning lamp and rotor form a voltage divider with most voltage dropped across the bulb.

 

To answer the question you asked Nick.

 

The alternator output is split between the field diodes and the main diodes. apart from the fact they are different sorts of diode so may cause a slightly different volts drop across themselves, once the engine is running both should be outputting the same (charging) voltage. The main diodes to charge the batteries and supply the engine electrics and the field diodes to power the alternator rotor, via the regulator, AND to try to push the charging voltage backward through the warning lamp. As the main diodes are putting the same voltage on the other side of the warning lamp, no current can flow, so the bulb goes out

 

The alternator has three generating coils, each with a positive field diode, a positive main diode and a negative main diode, so if a diode fails you only get the output from two coils. (The circuit from both field and main positive diodes return to the alternator via the same negative diode).

 

So, with a failed field diode the voltage pushing "backwards" through the warning lamp is less than that coming via the ignition switch so current can flow. That flow is usually enough to illuminate the warning lamp dimly.

 

 

 

I see... So am I right in my understanding that this doesn't actually cause any issues with the charging, but just with the warning light? or will the fact that only 2/3 of the field windings are powering the rotor result in a 1/3 decrease in current to the batteries (but still a good voltage)?

[Tony Brooks]

Just now, Tasemu said:

 

I see... So am I right in my understanding that this doesn't actually cause any issues with the charging, but just with the warning light? or will the fact that only 2/3 of the field windings are powering the rotor result in a 1/3 decrease in current to the batteries (but still a good voltage)?

 

Paradoxically, it may well cause an excess charging voltage because most alternators are machine sensed as opposed to battery sensed. That means they measure the voltage at the machine. and usually use measure the output of the field diodes, not the main diodes. So if you have a low voltage from the field diodes, the regulator sees that low voltage and boosts the charging voltage until it sees the correct voltage from the field diode. That means the voltage from the main diodes will be too high. This will not become apparent until some time into the charging cycle when the regulator is properly controlling the voltage, so not in the first stage and later in the second. that is not in "bulk" when the alternator is running flat out and only later in "absorption" to liken it to battery chargers, although with basic alternators it all runs into one.

 

The batteries are charged via the MAIN diodes, so there is no loss of charging voltage. The rotor only needs 3 to 4 amps to deliver maximum output, so the remaining field diodes can happily supply that.

 

If a main diode fails, then you do get a reduced charging current AND charging voltage, but the reduction in voltage is not as much as most expect and you still get a degree of charging. This confuses many.

[Tasemu]

1 hour ago, Tony Brooks said:

 

Paradoxically, it may well cause an excess charging voltage because most alternators are machine sensed as opposed to battery sensed. That means they measure the voltage at the machine. and usually use measure the output of the field diodes, not the main diodes. So if you have a low voltage from the field diodes, the regulator sees that low voltage and boosts the charging voltage until it sees the correct voltage from the field diode. That means the voltage from the main diodes will be too high. This will not become apparent until some time into the charging cycle when the regulator is properly controlling the voltage, so not in the first stage and later in the second. that is not in "bulk" when the alternator is running flat out and only later in "absorption" to liken it to battery chargers, although with basic alternators it all runs into one.

 

The batteries are charged via the MAIN diodes, so there is no loss of charging voltage. The rotor only needs 3 to 4 amps to deliver maximum output, so the remaining field diodes can happily supply that.

 

If a main diode fails, then you do get a reduced charging current AND charging voltage, but the reduction in voltage is not as much as most expect and you still get a degree of charging. This confuses many.

 

Excellent, this clicks for me. Thanks!

[nicknorman]

In addition to Tony’s post I would say that a blown field diode reduces maximum alternator output in correlation to the voltage split between B+ and D+ that you have observed. Ie not much at max rpm, but quite a lot at normal cruising rpm and even more at idle. At high rpm when that split isn’t much, and with low batteries so the alternator is trying to produce max output, you are getting 2 field diodes to do the work of 3 so they will get hot and will have a shorter life.

[Quattrodave]

13 hours ago, casper ghost said:

What would be considered a decent one as our boats will almost certainly have the cheapest that were available at the time.

 

Used these a few times :

 

https://www.autoelectricsupplies.co.uk/mobile/product/1812

 

Or these :

 

https://www.autoelectricsupplies.co.uk/mobile/product/994

Edited March 1, 2023 by Quattrodave