Most common alternator issues in the field

[01kiwijohn]

Had one failure, after installing a remote regulator. The insulation on the output wire appeared melted, for about 4 inches so I assumed the higher, longer charge rate was causing this. So, I replaced the alternator output wire with one that was two gauges thicker; no problems since, over the past 3 years, and I cruise about 4 to 5 months per year.
Hope that helps.

[Thumbs Up]

Some alternators use a light bulb to energize the field windings. I recently troubleshooted two separate boats where this was the only problem. If you have a charge light on your panel that doesn't come on before the engine is started then the first thing that you should check is the bulb and connections to it.

[ZULU40]

Originally Posted by Wotname

I think you mean rotor rather than stator.

The slip rings / brushes makes the connection to the rotor windings (i.e. field winding). The stator windings as their name suggests are stationary.

you know if you didnt mention it Id still be calling it the stator tomorrow, so thanks for that.

[waterman46]

Originally Posted by sailm8

The only problem I have ever had with several boats was broken/ slipping belts.

Good advice. I never have broken a belt but often had slippage issues. Make sure that you have plenty of room left on the bracket to tighten the belt as it wears. Last cruise I reached the end of the adjustment slot and all 3 spare belts became worn down to that point. I had been buying same belt size as used by the PO of my boat.

When I got to civilization I bought 3 new belts that were two cm shorter than the old belts. No more problems with slipping belts.

[GordMay]

Prestolite / Leece-Neville’s website offers many excellent technical bulletins & troubleshooting guides etc.

Here ➥ http://www.prestolite.com/Prestolite - Leece Neville

To contact their ecellent customer service group:
Phone: (800)354-0560
E-mail: webmail@prestolite.com

Including:

Troubleshooting ➥ http://www.prestolite.com/Prestolite - Leece Neville

Installation Instructions ➥ http://www.prestolite.com/Prestolite - Leece Neville

An Overview of the Charging System ➥ http://www.prestolite.com/Prestolite - Leece Neville

... My Westerbeke has a Motorola Prestolite alternator for which I need a regulator, but I cant ID the model, makes it hard to find the part

Zulu: Is it the alternator model or regulator you cannot identify?

[fschaefer4]

@Thumbs Up- I can see a love hate relationship with POs. On one hand, they sell you a boat and tell you everything about it. On the other hand, how much of what they tell you is flawed.

Regarding the indicator light: I understand that the field windings need low current for the alternator to initially work. How does that bulb control that current? Is it wired in series to the field windings?

@Main Sail- Excellent list! I wondered how you had such a definitive list and looked up your website. That will be a goto resource for me.

[becrux]

Originally Posted by fschaefer4

Regarding the indicator light: I understand that the field windings need low current for the alternator to initially work. How does that bulb control that current? Is it wired in series to the field windings?

Exactly. This is typically done done with internally regulated negative field control. The light gets its 'plus' supply from the battery via the ignition switch with the 'minus' side tied to the 'plus' side of the field and the internal regulator, which is also tied to the output of the stator bridge, or there may be a dedicated bridge just to supply the reg. Field current flows as soon as the ignition switch comes on. (Note that the bulb also limits the current and a failed bulb will render the alt inoperative.)
As the voltage rises on the output of the stator it eventually matches the battery voltage and the light goes out because there is no voltage difference across the filament.

[wsmurdoch]

Sometimes there is a resistor in parallel with the bulb to prevent a bulb failure from becoming an alternator failure.

[ZULU40]

Originally Posted by GordMay

Zulu: Is it the alternator model or regulator you cannot identify?

both, all I know is it came off a Westerbeke 42B4 and its 50AH
its factory red, but there is no script or stampings on it at all
it has a built in regulator and seems in the pattern of Prestolite 8MR

its a bit odd because the diodes pass testing individually reading on the multimeter 1 and about 540, so within the right range. But reassembled neg to the B post reads 1340

also voltage is low at 12.7v at 2000rpm so I think the regulators failed, which is common because its internal and the heat apparently gets them

I need to solve the problem of the diodes, like does it really have a problem, and I need to get a regulator or cut and shut another to fit

[Wotname]

Originally Posted by ZULU40

........

its a bit odd because the diodes pass testing individually reading on the multimeter 1 and about 540, so within the right range. But reassembled neg to the B post reads 1340

also voltage is low at 12.7v at 2000rpm so I think the regulators failed, which is common because its internal and the heat apparently gets them

I need to solve the problem of the diodes, like does it really have a problem, and I need to get a regulator or cut and shut another to fit

OK, there are a couple of issues here and I am not sure if it is the way you wrote it or your testing technique or the type of meter you are using or if there is a fault with the diode pack.

However the simplest way to consider a diode is that it should have continuity in one direction and no continuity in the other. Any regular diode that does not behave this way is faulty.

I am going to assume you are using a regular multimeter that has a diode check function. If so, the numbers you posted (i.e. 1 and about 540) suggests the diode is faulty. A serviceable diode would have numbers like OL and 500 ~ 600. In your instance the number 1 indicates there is short circuit (i.e. continuity) in that direction and 540 indicates normal diode continuity in the other direction.

As an aside, the numbers are actually telling you the voltage drop across the diode junction in millivolts so in the forward bias direction, there should be about 500 to 600 millivolts dropped across the conducting junction. When measuring the reverse bias or blocking direction of the junction, there should be no current flow and the meter will read OL (over limit) which is in essence, open circuit. Your reading of 1 in that direction tells us the diode is conducting in that direction as well.

Again I'm not sure of your measuring technique but you have to be aware of how the diodes are connected together in the diode pack. The measurement of 1340 between E and B seems right to me for the conducting direction as you are measuring two diodes in series i.e. it is the voltage drop across two diode junctions connected in series. Of course, the reverse direction should remain at OL (i.e. open circuit or non-conducting).

Perhaps this circuit of a normal alternator diode pack will make it clearer.

The six diodes in the lower half are the main rectifying diodes for the stator output (U,V & W)and the three at the top are to drive the rotor (field) via the regulator and the charge light (L).

[Wotname]

Originally Posted by fschaefer4

..........

Regarding the indicator light: I understand that the field windings need low current for the alternator to initially work. How does that bulb control that current? Is it wired in series to the field windings?

.............

Originally Posted by becrux

Exactly. This is typically done done with internally regulated negative field control. The light gets its 'plus' supply from the battery via the ignition switch with the 'minus' side tied to the 'plus' side of the field and the internal regulator, which is also tied to the output of the stator bridge, or there may be a dedicated bridge just to supply the reg. Field current flows as soon as the ignition switch comes on. (Note that the bulb also limits the current and a failed bulb will render the alt inoperative.)
As the voltage rises on the output of the stator it eventually matches the battery voltage and the light goes out because there is no voltage difference across the filament.

Originally Posted by wsmurdoch

Sometimes there is a resistor in parallel with the bulb to prevent a bulb failure from becoming an alternator failure.

fschaefer4, you have received two correct answers above where the charge light may affect the regulator so let me add another typical alternator circuit where the charge light does not affect the regulation of the alternator should the charge light fail. I believe the circuit is self explanatory but if not, please ask for guidance.

[ZULU40]

Originally Posted by Wotname

OK, there are a couple of issues here and I am not sure if it is the way you wrote it or your testing technique or the type of meter you are using or if there is a fault with the diode pack.

However the simplest way to consider a diode is that it should have continuity in one direction and no continuity in the other. Any regular diode that does not behave this way is faulty.

I am going to assume you are using a regular multimeter that has a diode check function. If so, the numbers you posted (i.e. 1 and about 540) suggests the diode is faulty. A serviceable diode would have numbers like OL and 500 ~ 600. In your instance the number 1 indicates there is short circuit (i.e. continuity) in that direction and 540 indicates normal diode continuity in the other direction.

These particular pattern of multimeter are quite common at the very bottom of the multimeter price scale, At the diode check it doesnt read OL for out of range it just gives you a value of 1

Originally Posted by Wotname

As an aside, the numbers are actually telling you the voltage drop across the diode junction in millivolts so in the forward bias direction, there should be about 500 to 600 millivolts dropped across the conducting junction. When measuring the reverse bias or blocking direction of the junction, there should be no current flow and the meter will read OL (over limit) which is in essence, open circuit. Your reading of 1 in that direction tells us the diode is conducting in that direction as well.

Again I'm not sure of your measuring technique but you have to be aware of how the diodes are connected together in the diode pack. The measurement of 1340 between E and B seems right to me for the conducting direction as you are measuring two diodes in series i.e. it is the voltage drop across two diode junctions connected in series. Of course, the reverse direction should remain at OL (i.e. open circuit or non-conducting).

ok thanks for this answer, Im actually a lot more confident that the diodes are working properly. I also fiddled with continuity both ways and it is only passing current one way, it was the difference in values that had me wondering (540 - 1340), but the series connection thing makes complete sense to me. Also as is indicated below, that one side of the rectifier is servicing the stator and the other the field.

Originally Posted by Wotname

Perhaps this circuit of a normal alternator diode pack will make it clearer.

The six diodes in the lower half are the main rectifying diodes for the stator output (U,V & W)and the three at the top are to drive the rotor (field) via the regulator and the charge light (L).

as an example of my multimeter (it looks exactly like this) and what appears to be happening see

https://youtu.be/xgikeXt91vM?t=79

[Wotname]

Originally Posted by ZULU40

These particular pattern of multimeter are quite common at the very bottom of the multimeter price scale, At the diode check it doesnt read OL for out of range it just gives you a value of 1

............
as an example of my multimeter (it looks exactly like this) and what appears to be happening see

OK, thanks for that and yes you are correct when using this meter. I gotta say, I haven't used one of these very bottom end meter's for years, even my $20 multimeter is more upmarket .

Pedantic Mode On ()
Technically speaking the meter isn't reading 1 as the numeral has been placed at the most significant end of the scale (LHS) with blanks towards the RHS; if a reading of a value of 1, this would be placed at the RHS (least significant digit). Being placed at the LHS with no other numerals towards the LHS is indicating over limit.
Pedantic Mode Off.

Hope you get it sorted and get back if we can help in any way!

[fschaefer4]

@wotname- this makes sense. I have to say, in class we've moved on, but I keep coming up with diagrams and questions about alternators for the instructor. I think he's a little taken back. He knows why I'm taking the class so I can work on sailboats, so he's into it.

[belizesailor]

Originally Posted by billknny

Very few people will carry a full set of diodes and regulator parts for their alternators. When an alternator craps out, most people take it to a rebuild shop. It is easy and inexpensive.

Smart sailors have either spare alternators, or other ways of charging their batteries. Even for those with the knowledge and skills to rebuild an alternator onboard, a spare unit takes away the time pressure to get the job done while on a crossing.

I agree alternator shops are certainly the easiest answer if you are in a populated area. They are nearly ubiquitous even in less developed areas around the world. But, if you are in more remote areas...it sure is good to have some spares, a decent multimeter, soldering iron, and know how to use them.

Most common failures like regulators & diodes are easy to fix. I carry spare diodes and basic spare regulator that will get the job done.

Taking it a step further...with twin engines you get built in redundancy. Ive also set mine up w independent external regulators for each alternator. So, any single failure and Ive still got a way to charge at least...just at reduced capacity...until I can repair or limp into an alternator shop.

When I had a single engine boat I did carry a full spare alternator...quickest and easiest solution in that case.