Alternator exciter circuit lamp parallel resistor, what size?

[DHV90]

With the engine not running, the WL terminal should be low voltage, the light should be on.

First thing to check is the brushes.
Then check the bulb/holder/wiring, you've really already done this by seeing battery volts at the WL terminal.

From what I can see on the web, the field coil is connected to the diode pack and is energised by pulling current out of the F terminal on the alternator. So a low voltage here suggests the alternator is the problem not the regulator.

Amazing thanks, this is really helpful sorry for taking up so much of your time.

Does this mean that I, in theory, should see a voltage at the F terminal on the alternator when the field cable is disconnected?

So the power to excite the coils probably comes from the alternator diode pack, through the regulator and out to the F terminal on the alt? Getting switched by the WL circuit but not directly linked?

To check the brushes is resistance between F and alternator body/earth the correct test?

Thanks again all this help is much appreciated

Edit:
Just realised that the brush box is it's own plastic unit with terminals on the top. Should one of these terminals be led to earth? The way the PO installed this has the armature wire from the regulator going to one of the terminals on the brush box, with continuity between A and F. Could this be wrong?
I couldn't see any other terminals on the alternator that A should connect to but the access is too tight to see if there are any markings on the brush box for which terminal is which

 

[TernVI]

Amazing thanks, this is really helpful sorry for taking up so much of your time.

Does this mean that I, in theory, should see a voltage at the F terminal on the alternator when the field cable is disconnected?

So the power to excite the coils probably comes from the alternator diode pack, through the regulator and out to the F terminal on the alt? Getting switched by the WL circuit but not directly linked?

To check the brushes is resistance between F and alternator body/earth the correct test?

Thanks again all this help is much appreciated

Edit:
Just realised that the brush box is it's own plastic unit with terminals on the top. Should one of these terminals be led to earth? The way the PO installed this has the armature wire from the regulator going to one of the terminals on the brush box, with continuity between A and F. Could this be wrong?
I couldn't see any other terminals on the alternator that A should connect to but the access is too tight to see if there are any markings on the brush box for which terminal is which

When the alternator is running, the field current comes from the diode pack to the field coil, out of the F terminal and is modulated by the rightmost transistor, to ground via the - terminal

To start up, the field current goes through the 'gen' lamp into the WL terminal, out of the A terminal of the reg, into the A terminal of the alt, through the field coil etc.
Neither brush is earthed. The brushes are not shown in the diagram above, they are at either end of the Field coil.

A in this context is the 'Armature' connection.
Confusing if you are familiar with dynamo regulator/ cutout boxes which have the terminals FADE for Field, Ammeter, Dynamo and Earth

I would assume the H/M/L terminal actually wants to be on the switched side of the 'ign switch' otherwise the reg will slowly flatten the battery when the engine isn't running?

 

[DHV90]

View attachment 102667
When the alternator is running, the field current comes from the diode pack to the field coil, out of the F terminal and is modulated by the rightmost transistor, to ground via the - terminal

To start up, the field current goes through the 'gen' lamp into the WL terminal, out of the A terminal of the reg, into the A terminal of the alt, through the field coil etc.
Neither brush is earthed. The brushes are not shown in the diagram above, they are at either end of the Field coil.

A in this context is the 'Armature' connection.
Confusing if you are familiar with dynamo regulator/ cutout boxes which have the terminals FADE for Field, Ammeter, Dynamo and Earth

I would assume the H/M/L terminal actually wants to be on the switched side of the 'ign switch' otherwise the reg will slowly flatten the battery when the engine isn't running?

Awesome, I had the direction of the F cable confused in my head, think its falling into place so I'll go back with the multimeter and do some more digging

Thanks for that, again, its really helped!

 

[DHV90]

So, at the regulator end of the F wire with the ignition on I have full battery voltage, showing there is full power going through the Armature wire, through the slip rings and back through the field wire with correct resistance. There is also switched live with good power at the H/M/L terminal wire and a good ground along with the working charge light and wiring.
Looking like an alternator fault then? Any opinions on whether to rebuild, replace like for like or go to a simpler modern alternator with an integrated reg?

More digging on the 12v side (generic modern 2 wire alternator, B+, D+ and body earth)
D+ to earth is showing 0.00 ohms, shouldn't there be some resistance from the field windings? possibly a short in the alternator?

 

[vas]

DHV90, I'm trying (not v.hard I have to admit!) to follow all that. Maybe now is the time to remove the alternator and take it to a car electrician shop to have a look? When I fitted my Sterling external regulator to the house charging bank engine, I had to fit two cables (well one actually but didn't know which one!) Took it to him expecting he'd just solder the cables, ended up paying the massive amount of 20euro for a full service including replacing the one of the two bearings which was working alright but you could feel it crunching a bit (engines are approx 20yo with 800h). So always worth taking it to an electrician for a service/check. Then you know you're good to go and only have to deal with possible problems to the wiring (not that it's easy mind...)

cheers

V.

 

[DHV90]

DHV90, I'm trying (not v.hard I have to admit!) to follow all that. Maybe now is the time to remove the alternator and take it to a car electrician shop to have a look? When I fitted my Sterling external regulator to the house charging bank engine, I had to fit two cables (well one actually but didn't know which one!) Took it to him expecting he'd just solder the cables, ended up paying the massive amount of 20euro for a full service including replacing the one of the two bearings which was working alright but you could feel it crunching a bit (engines are approx 20yo with 800h). So always worth taking it to an electrician for a service/check. Then you know you're good to go and only have to deal with possible problems to the wiring (not that it's easy mind...)

cheers

V.

That was my plan originally, i had the alternators off and rang round all the 4 or 5 local auto electricians, none of which have test rigs any more. I was told by all of them that with modern stuff if the alternator light comes on the dash they bin it and fit a new one, no point testing and rebuilding anymore. Couldn't find a single shop able to test them. The only one that used to have a test rig got rid of it years ago.
The local marine diesel/electrical specialists weren't any help either unfortunately. I have tried though!

To be fair, this boat isnt meant to be a reliable appliance, not yet at least, currently its more a learning experience/project type thing so Im enjoying finding my way around bits of equipment I haven't worked on before and the understanding that comes with it.

 

[TernVI]

I bench tested my last alternator by wiring it up to ammeter, voltmeter and battery, then spinning it with a power drill.
You'd need a big motor to drive it at full current, but it enabled me to test it off the boat.
I could see some amps going into a battery and regulation at the right volts.

 

[TernVI]

So, at the regulator end of the F wire with the ignition on I have full battery voltage, showing there is full power going through the Armature wire, through the slip rings and back through the field wire with correct resistance. There is also switched live with good power at the H/M/L terminal wire and a good ground along with the working charge light and wiring.
Looking like an alternator fault then? Any opinions on whether to rebuild, replace like for like or go to a simpler modern alternator with an integrated reg?
....

I'm confused now.
The 'no charge light' is not coming on, but there is current flowing through the field?

I'm aware that these diagnoses by internet run a high risk of talking at cross purposes!

 

[TernVI]

....

More digging on the 12v side (generic modern 2 wire alternator, B+, D+ and body earth)
D+ to earth is showing 0.00 ohms, shouldn't there be some resistance from the field windings? possibly a short in the alternator?

The field (rotor) can be quite low resistance, I don't trust cheap multimeters, particularly at low ohms. The rotor could be shorted.
To measure such a low resistance, you could put it in series with a 5W 12V bulb and see if you can measure a voltage.
In the trade, they would perhaps measure it with AC.

 

[DHV90]

I'm confused now.
The 'no charge light' is not coming on, but there is current flowing through the field?

I'm aware that these diagnoses by internet run a high risk of talking at cross purposes!

Both no charge warning lights illuminate with the ignition but dont go off when the alternators are spinning. 24v light dims a little some times but never goes out, 12v stays on all the time

The 12v exciter wire shows 1.2v when connected to the alternator but when disconnected it shows full battery voltage, so the exciter wiring and warning light wiring is all fine as well!

I went down this morning to take the alternators off and funnily enough managed to find a starter motor shop with a test rig who have spun both up and confirmed they are in perfect order and charging well from low rpms. now idea how i didnt come across them before.

Well confused now! ill think on it for a while and figure out what else could be wrong.

 

[PaulRainbow]

Both no charge warning lights illuminate with the ignition but dont go off when the alternators are spinning. 24v light dims a little some times but never goes out, 12v stays on all the time

The 12v exciter wire shows 1.2v when connected to the alternator but when disconnected it shows full battery voltage, so the exciter wiring and warning light wiring is all fine as well!

I went down this morning to take the alternators off and funnily enough managed to find a starter motor shop with a test rig who have spun both up and confirmed they are in perfect order and charging well from low rpms. now idea how i didnt come across them before.

Well confused now! ill think on it for a while and figure out what else could be wrong.

You can very simply rule out the exciter wiring, just briefly apply a 12v supply directly to the alternator terminal. Leave the lamp connected while you do this and you'll see straight away if it's worked.

Failing that, check the positive and negative connections on the alternator. Of the alternator has a negative connection to the block (rather than making the connection through the case), remove the cable from the block and clean it. Note that visual checks of cable connections cannot be relied upon, you need to physically remover the terminals, clean and replace them.

 

[DHV90]

Another small update.
All battery and alternator terminals cleaned and reassembled, as well as checking resistance.
Ran engine up, still no charge, substituted the indicator lamp/resistor combination for a much bigger bulb (21w indicator bulb I had lying around) on one alternator exciter circuit then the other. This got both alternators charging, and they remained charging after I disconnected it (engine still running) and reconnected the original charge indicator lamps.
Alternators were turning around 1000rpm at this point (alternator RPM not engine RPM)

14.2v on the 12v side, around 25-26 on the 24v side at idle but both increased with revs

Is there any possible harm from using a bigger lamp, or an even bigger resistor which would replicate the 21w bulb, from now on, or is that a recipe for damage to the alternators? thanks again!

 

[TernVI]

A bigger wattage bulb won't help much, it may make a marginal improvement in the speed at which the alt kicks into life.
In the limit, too big a bulb may affect the regulation adversely.

 

[VicS]

Another small update.
All battery and alternator terminals cleaned and reassembled, as well as checking resistance.
Ran engine up, still no charge, substituted the indicator lamp/resistor combination for a much bigger bulb (21w indicator bulb I had lying around) on one alternator exciter circuit then the other. This got both alternators charging, and they remained charging after I disconnected it (engine still running) and reconnected the original charge indicator lamps.
Alternators were turning around 1000rpm at this point (alternator RPM not engine RPM)

14.2v on the 12v side, around 25-26 on the 24v side at idle but both increased with revs

Is there any possible harm from using a bigger lamp, or an even bigger resistor which would replicate the 21w bulb, from now on, or is that a recipe for damage to the alternators? thanks again!

They should not need bulbs of this power, or the equivalent value resistor . Its too much of a bodge to be a sensible work around IMO.
A 12 volt 21 watt bulb will be passing 1.75 amps. Previous calculations suggest it needs only a much smaller current. a fraction of 1 amp , to excite an alternator.

My feeling is that something, somehow is by-passing most of the current around the alternator. The correct solution I am sure is to find the reason for it and correct this.

I am wondering if the output from the field diodes is also being partly by passed to earth. if so there may be enough current flow to make something rather hot.

Not got any relays connected to the warning light circuits by any chance ?

 

[frankv]

Hi all,
I'm new here and trying to solve a problem. I've replaced my ALT light with an LED on my '97 Subaru EJ20, and now have problems with the engine stopping intermittently. This seems to happen at revs above 1700 or so, not below it. So I suspect it was caused by changing the light to an LED.

I've Googled, read many pages, read through the entire thread here, and am still (more?) confused. I do have a simple grasp of simple electronics. Possibly that's clashing with a lifetime of "knowing" something that's actually untrue. So if someone could explain in simple terms if I've got the right idea, I'd be really grateful!

Let's start with the original system with the lamp. My understanding is that this is how it works:
1. The alternator needs current through the exciter coil to make more current on the main coil.
2. To get started, current passes from the battery +12V through the ALT light, through the exciter coil, to ground. (It seems to be this way round on the Subaru at least).
3. Once the engine is started, this exciter current comes from the alternator itself, so the alternator disconnects the ALT light wire from the coil.

Have I got all of that right? So, the questions:
1. How much current does the exciter coil need? Since we're talking about 50 ohms, I assume it's 2.4A?
2. So, when the ignition is on but the motor is off, there's 2.4A illuminating the bulb? Maybe less because the bulb's resistance is higher when it's hot? And this current has to also run through the exciter coil. But it's giving up some energy there too, so I guess there's less than 12V drop across the lamp? So maybe this 3W lamp is actually dropping only a 1.2V, and 80% of the energy is used by the coil?
3. Turn the ignition key, the engine starts, it's idling at (say) 1000RPM, the ALT light goes out. Does that mean that the alternator is now producing current for the exciter coil so has disconnected the ALT light? So, unlike my belief, the ALT light on isn't telling you that your battery isn't charging... it's telling you that there's no current coming from the main coil, which then means that your battery isn't charging.
4. I believe that at idle, the alternator is not producing enough current to charge the battery. Is that right? But there's enough current to run the exciter coil, so the ALT light stays off? At some higher revs (I guess less than 2,000) the alternator is putting out enough current to run the engine (ignition, computer, injectors, etc) and all the other stuff (lights, wipers, radio, etc) and what's left over goes to charging the battery.

OK, on to the questions about the LED...
5. I guess an LED, even a nice bright one, only uses about 20mA. So a "12V bulb replacement" LED has (if I calculated right) a 600 ohm resistor in series to limit the current to 20mA. Which is nowhere near enough current for the exciter coil.
6. So I need a resistor to allow 2.4A - 20mA to go through the exciter coil, which is where the 50 ohm resistor comes into it? How accurate does that need to be? I expect a 47 ohm resistor would be OK, what about a 56 ohm? What are the limits of this resistance range? I see someone halved the resistance to make it work.

And, some wrap-up questions:
7. I guess I do need the 50 ohm resistor. Or do I? How is it that my alternator is working at all? My battery isn't going flat, despite many starts and short runs. I can run my lights and stuff. Or is it trickling along on a 20mA current in the exciter somehow?
8. How does this affect the intermittent engine stops above 1700RPM? Could the missing resistor cause that? Or am I looking for some other fault as well?

I know this is a huge post, and I apologise for rehashing a thread that's died some time ago. But I am really in need of some understanding here.

Frank

 

[VicS]

Hi all,
I'm new here and trying to solve a problem. I've replaced my ALT light with an LED on my '97 Subaru EJ20, and now have problems with the engine stopping intermittently. This seems to happen at revs above 1700 or so, not below it. So I suspect it was caused by changing the light to an LED.

I've Googled, read many pages, read through the entire thread here, and am still (more?) confused. I do have a simple grasp of simple electronics. Possibly that's clashing with a lifetime of "knowing" something that's actually untrue. So if someone could explain in simple terms if I've got the right idea, I'd be really grateful!

Let's start with the original system with the lamp. My understanding is that this is how it works:
1. The alternator needs current through the exciter coil to make more current on the main coil.
2. To get started, current passes from the battery +12V through the ALT light, through the exciter coil, to ground. (It seems to be this way round on the Subaru at least).
3. Once the engine is started, this exciter current comes from the alternator itself, so the alternator disconnects the ALT light wire from the coil.

Have I got all of that right? So, the questions:
1. How much current does the exciter coil need? Since we're talking about 50 ohms, I assume it's 2.4A?
2. So, when the ignition is on but the motor is off, there's 2.4A illuminating the bulb? Maybe less because the bulb's resistance is higher when it's hot? And this current has to also run through the exciter coil. But it's giving up some energy there too, so I guess there's less than 12V drop across the lamp? So maybe this 3W lamp is actually dropping only a 1.2V, and 80% of the energy is used by the coil?
3. Turn the ignition key, the engine starts, it's idling at (say) 1000RPM, the ALT light goes out. Does that mean that the alternator is now producing current for the exciter coil so has disconnected the ALT light? So, unlike my belief, the ALT light on isn't telling you that your battery isn't charging... it's telling you that there's no current coming from the main coil, which then means that your battery isn't charging.
4. I believe that at idle, the alternator is not producing enough current to charge the battery. Is that right? But there's enough current to run the exciter coil, so the ALT light stays off? At some higher revs (I guess less than 2,000) the alternator is putting out enough current to run the engine (ignition, computer, injectors, etc) and all the other stuff (lights, wipers, radio, etc) and what's left over goes to charging the battery.

OK, on to the questions about the LED...
5. I guess an LED, even a nice bright one, only uses about 20mA. So a "12V bulb replacement" LED has (if I calculated right) a 600 ohm resistor in series to limit the current to 20mA. Which is nowhere near enough current for the exciter coil.
6. So I need a resistor to allow 2.4A - 20mA to go through the exciter coil, which is where the 50 ohm resistor comes into it? How accurate does that need to be? I expect a 47 ohm resistor would be OK, what about a 56 ohm? What are the limits of this resistance range? I see someone halved the resistance to make it work.

And, some wrap-up questions:
7. I guess I do need the 50 ohm resistor. Or do I? How is it that my alternator is working at all? My battery isn't going flat, despite many starts and short runs. I can run my lights and stuff. Or is it trickling along on a 20mA current in the exciter somehow?
8. How does this affect the intermittent engine stops above 1700RPM? Could the missing resistor cause that? Or am I looking for some other fault as well?

I know this is a huge post, and I apologise for rehashing a thread that's died some time ago. But I am really in need of some understanding here.

Frank

I have not re-read the whole of this old thread although I see I contributed to it previously and I have read your post rather quickly.

But a couple or 3 points

1. The warning light is not actually disconnected . It goes out because the volts on both terminals become equal when the alternator is running and generating

2. The excitation resistor used in more recent Volvo Penta engines is, IIRC , 30 ohms
Hitachi alternators used in some engines of far eastern origin have two connections where others have just one . One is for the warning light the other for a direct feed from the ignition switch. This second connection is to an internal 50 ohm resistance.

3. I don't see how the alternator excitation circuit can cause the engine to stop intermittently.

.

 

[Refueler]

I have not re-read the whole of this old thread although I see I contributed to it previously and I have read your post rather quickly.

But a couple or 3 points

1. The warning light is not actually disconnected . It goes out because the volts on both terminals become equal when the alternator is running and generating

2. The excitation resistor used in more recent Volvo Penta engines is, IIRC , 30 ohms
Hitachi alternators used in some engines of far eastern origin have two connections where others have just one . One is for the warning light the other for a direct feed from the ignition switch. This second connection is to an internal 50 ohm resistance.

3. I don't see how the alternator excitation circuit can cause the engine to stop intermittently.

.

I find the thread interesting and I agree with your points here ...

What I do find interesting is the need for resistors ... I never even thought about it ... I took the exciter wire from alternator - up to a switch so I can isolate when not using engine ... the excitation power is then via suitable wires. My Light when lit to indicate alternator excite live and then alternator actually outputting by it extinguishing ... is connected in parallel, so it does not actually carry the load to excite the alternator .. it just lights up - goes out as power changes on that 'circuit'. No resistors needed at all .. no heat to get rid off ... just had to match the voltage ...

Just posting .. nothing more.

 

[VicS]

I find the thread interesting and I agree with your points here ...

What I do find interesting is the need for resistors ... I never even thought about it ... I took the exciter wire from alternator - up to a switch so I can isolate when not using engine ... the excitation power is then via suitable wires. My Light when lit to indicate alternator excite live and then alternator actually outputting by it extinguishing ... is connected in parallel, so it does not actually carry the load to excite the alternator .. it just lights up - goes out as power changes on that 'circuit'. No resistors needed at all .. no heat to get rid off ... just had to match the voltage ...

Just posting .. nothing more.

2 observations:

1. Maybe I dont follow your description but a direct connection in place of the excitation resistors I describe above will short out the warning light ........ therefore rendering it ineffective in its primary role as a "no charge" warning

2. the resistor will limit the current .......... maybe necessary maybe not. I dont know but I would follow the practice I see alternator and engine manufacturers use.
.

 

[Alex_Blackwood]

2 observations:

1. Maybe I dont follow your description but a direct connection in place of the excitation resistors I describe above will short out the warning light ........ therefore rendering it ineffective in its primary role as a "no charge" warning

2. the resistor will limit the current .......... maybe necessary maybe not. I dont know but I would follow the practice I see alternator and engine manufacturers use.
.

Agree with that. The resistor limits the current through the field windings (and regulator) stops you frying things.

 

[Refueler]

2 observations:

1. Maybe I dont follow your description but a direct connection in place of the excitation resistors I describe above will short out the warning light ........ therefore rendering it ineffective in its primary role as a "no charge" warning

2. the resistor will limit the current .......... maybe necessary maybe not. I dont know but I would follow the practice I see alternator and engine manufacturers use.
.

Don't know about that - but it works ....

I power on via my 1 - 2 - off old fashioned switch. I flick on the 'exciter' switch and RED (12V) LED comes on ..... I start engine ... once revs pas about 650 - 700 rpm ... alternator is starting to charge - RED LED goes out.

As far as I recall - its about 15 - 18yrs ago that I wired it - that's how its wired ...

How would it short out the light ? As you said yourself and I know ... 12v voltage is one way and light is on ... alternator starts charging and that voltage in the excite lead goes to zero based on now alternator is charging ... and any excess voltage as in its 14.8v or whatever over the 12V - the LED sorts anyway

Anyway - I was just commenting - its no use to OP.