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Alternator Wiring & Exciter

7K views 19 replies 6 participants last post by  br3nt 
#1 ·
Let me start off by saying that boat projects are so much never-ending fun!

I decided to toss the barely working instrument panel on my Pearson 323 with a Volvo Penta MD11C and install some nice looking VDO gauges with a TinyTach. The unfortunate part is the wiring diagram doesn’t match the panel I had - oh yay, a mystery! I then figured out how the fuel shutoff solenoid valve was somehow connected to the post on the voltmeter and I thought I was on my way (it’s now connected to the ignition pos bus bar). After installing a lot of new and labelled wiring I fired up the motor. Well, I’m getting low readings from the voltmeter. Ugh. I did some investigating and it appears the alternator has an exciter wire that I finally traced today. (It’s the little things, right?) And it’s a black wire that I had errantly thought was another ground. The wire is now disconnected and as far as I can tell I’ve done no damage yet. Yet.

Now the question is where to connect this wire. I’ve read a lot about charger warning lights with diodes and voltage balance. I unfortunately do not have one and really don’t want one. Do I connect this wire as a ground to the voltmeter? Or do I just connect it to the positive ignition bus bar?

Thanks in advance for any help!

-Brent
s/v Indigo
 
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#2 ·
Not knowing how your boat was originally wired, I'd try to find a wiring diagram from a sistership to see how that particular alternator was supposed to be installed. Or, contact the alternator maker and ask them--they're usually pretty good about it--how that exciter wire was supposed to be connected.

Meanwhile, order a Brother PTouch labelmaker online, and get in the habit of sticking labels on wiring.(G) It won't take long before you have one of those days when you say "Wow, I should have bought one of these years ago."
 
#3 ·
Disclaimer: This may not be your problem, but here goes.

I recently replaced a faulty alternator on a power boat. I think it wasn't a very sophisticated one, Hitachi I think with an integrated voltage regulator (I know you can do better than this). This particular style of alternator had an idiot light that was connected to it, and like you I had changed the control panel, and in this case went to Faria gauges. I figured, hey, I got a ammeter and voltage gauge on my new panel, and I'm smart enough to live without the idiot light. Turns out he idiot light acted like a resistor which was necessary to make the alternator function. I got it to work, without using a light, by putting a resistor in it's place. I cannot remember if the resistor (light) was wired to + or -....and I'm away from the boat right now.

Again, might not be your problem in this case, but sounds suspiciously familiar. Do you know what alternator you got?
 
#4 ·
Thanks for the ideas. The alternator says Marine Alternator and the model number is 79 something. It's worn off. No other markings I can see without taking it off. The wiring diagram is the same on other Pearson 323s but doesn't match what was installed.

I have a very nice Brother P-Touch that prints labels from my computer and every wire I've touched has a label with clear heat shrink tubing (thanks Mainesail). My mistake here was assuming the black wire was a ground and the wiring diagram was accurate.
 
#6 ·
"Turns out he idiot light acted like a resistor which was necessary to make the alternator function. "
Actually, that's standard automotive installation, been that way for 50+? years AFAIK. The light bulb costs a little more than a resistor, but "tells" a little bit more, and at those reduced power levels "instrumentation" type bulbs seem to last forever.
 
#7 ·
The "Field" wire goes, one way or another, to the Battery Positive Terminal. The Alternator typically needs a couple or so Amps in the Field to produce output Amps. Any voltage that you did see is probably just stray stuff.
Does your Alternator have an External Regulator? (Mine does- Bosch Alternator, an external adjustable French regulator, and there are two "#12" Main Switches, Positive and Negative, so that the Batter(ies) can be made to "Float".) That's one variation of one way or another.
Anyway, on Page 35 of this PDF is Volvo's _recommended_ wiring diagram:

http://www.bluemoment.com/manuals/VolvoMD11C_D_17C_D.pdf

If you make it like this, with the Field hot lead going through the "Charge Warning Light", to Terminal "#75" of the "#5" Key Switch, it should work. Grounding the Field hot lead shouldn't cause any harm. The Regulator is built into the Alternator.

The Problem is, what if Pearson _did_ decide to do things differently?
Draw out what goes where now, carefully compare that to your Pearson Wiring Diagram, and then to the Volvo Wiring Diagram.
Have never-ending Fun!

¬Erindipity
 
#8 ·
As others have said it is automotive ...if I understand your question correctly you want to know where to connect the wire.....it should go to the +12v coming from your "ignition" switch here is how it works.

This wire puts +12 on one side of the lamp, the other side (of the lamp) is grounded through the alternator winding while the engine is stopped. When you "switch on" the lamp therefore glows. After you start the engine the alternator will generate (because the field is being energised by current flowing through the lamp to the field coil) and it's output voltage will rise, the output will then be regulated (internally) and will now be equal on both sides of the lamp which therefore goes out.

if you have a fault and are generating a lower voltage than your battery you would still see a glimmer...automotive style...or full light with broken belt and no generation.....should be obvious how this works now from above explanation.

I have used +12v for the explanation, of course it will be more than this depending upon the set point within the particular alternator.
 
#9 ·
Thanks for all the helpful responses. I've completely traced all the alternator wires. The only wiring diagram is from Volvo and says to connect the wire to the warning light which was never on this boat.

I guess my question is, should I just connect it directly to the +12v ignition circuit or does it need a light or resister in the path? If it does need a resistor, anyone have any direction as to what I should use? That's beyond my knowledge.

I really appreciate all the help.

-Brent
 
#10 ·
sorry, i misread / confused your op with one of the responses, i thought you already had an indicator light .....now i understand you do not.

the simplest thing is to introduce an indicator light, wiring this new light between the +12 v switched engine power (the "ignition" circuit) at the engine on / off switch (or somewhere else which is convenient on the engine) and the "loose" wire you currently have at your alternator. a small wattage lamp, like say 5w, will be fine but whatever you have will work. this should be easy to do and will prove this is in fact the issue here. once you have it working you can decide what to do as a permanent fix.
 
#12 ·
if you dont want to add a light, and you are going to radio shack for a resistor, then i would add a diode, not a resistor.....although both will work.
i am reasonably sure there will have been a diode in your old wiring if you still have it, typically in the harness close to the alternator.

the diode is the real fix, the light was the easy fix....and also a check to be sure this is the problem before you go any further...the light will tell you what is going on as explained earlier and fixi things if we are on the right lines. cant be sure with an internet post !!
 
#13 ·
Yea, second that all this advice is based on incomplete information. Proceed with caution. I didn't know a diode would work, and was somewhat concerned about the amount of current passing exceeding some limit when the circuit is is "lit" mode since the diode will drop say .6 volts then look pretty much like a short....but again we cannot see what you have, so caution is the word.

So the disclaimer is, what worked for me with a specific situation might not apply to your case.
 
#14 ·
I have written recommendations based on op.... "the alternator has an exciter wire" ......if you are sure its an exciter wire....and if you don't have an external regulator set up (i am assuming you dont as you would have mentioned it) then you have a "plain vanilla" automotive style alternator. adding the light will replicate how the alternator is wired in a vehicle and by using a small lamp you will soon see what is going on and should not cause any damage if we have this wrong !!

as capecodda is saying it is always a little difficult "remotely", I have done this before, but if you are worried here is a way to tackle it slowly slowly.

temporary extend the exciter wire to the voltage gauge position, I guess this is your control panel, hang it somewhere handy where the tail will not touch anything, strip a small piece of the tail

get a small 12v lamp in a lampholder with some say 6 ft long tails, hang it where you can see the voltage gauge and where it will reach the exciter wire

strip the lampholder tail ends

connect one lampholder tail to the switched run output of the engine stop / run / start switch ("ignition switch") at a convenient place

turn engine switch to run position, do not start engine yet, touch free tail of lamp to ground, lamp will light, this has tested that you have the lamp working, lift tail off ground.

touch the tail that was on ground to the extended exciter wire ...lamp should light again, this has shown that it is going to ground somehow through the exciter wire, it should do this, it should not get hot, lift tail off again.

now start engine

observe voltmeter, nothing happening I guess unless it has fixed itself ....repeat the last step, touching the lampholder free tail to the extended exciter tail, hold together, again it should not get hot

when you touch the wires together you should see the voltage pick up and the light go out

then you can decide how you want to complete the repair
 
#16 ·
Diodes...
There can be a Diode involved, but _only_ if it was wired in a particular manner, which I won't get into, because I think that what it is best to do, is to Wire as Volvo intended, and make it reliably work, and only then try to make it better.
(Diodes conduct in one direction with a relatively fixed voltage drop, and provide Zero diagnostic information, which Lamps do provide, if one is paying attention.)

Why there is no original Charging Lamp is a mystery. The few cents added in manufacturing cost on a Boat is invisible. Removing the Lamp does add in a bit of reliability; one less thing to fail. But bodkins-all for indicating what may be going wrong otherwise.
(These Pearsons supposedly came stock with an Atomic 4, but with Boats, anything is possible. One possible reason for wiring confusion is that at some point, the Atomic 4 was replaced with the Volvo, with some questionable measure of competence.)

About Volvo's wiring:
Digging into historical Alternator innards reveals that the Lamp may not be just an Indicator, but that the Lamp resistance characteristics could be built right into the Field Regulation Circuitry. An Incandescent Lamp is _not_ a Diode, or a Resistor. At low temperatures and currents, it has much lower resistance than when hot and lit. It allows, for some milliseconds until warmed up, higher currents to go to the Field wiring, which can be a good thing. It can thus be an initial starting Regulator working in concert with the running Regulator. (To recap: the Alternator Field Current determines the Alternator Output Voltage, within limits.)

(On this concept, HP was born. In their first 200 Series Audio Oscillators, they put an Incandescent Bulb in the feedback loop as a Linearizing Element, and for this very reason- predictable non-Linearity. HP made "Pure" Oscillators, whose Output Voltage was reliably fixed, over a wide frequency range.)


Ten minutes in place, I could suss this all out with a DMM; but I'm not in place, so all that I can offer now is speculation...

¬Erindipity
 
#17 ·
I'm not sure that it matters, or is a reason for the standard automotive use of a lamp (rather than a diode or resistor) but a lamp is a THERMISTOR. It starts with low resistance near zero, and as it heats up and draws current, that resistance rises, so the lamp acts as a current limiter in the circuit as well.

We used to use 100W and 200W (120VAC) bulbs in homemade electric "drilling" mchines, because they would safely limit the power going into the electrode instead of short circuiting the mains power in the event of a direct contact.

Could be they perform the same role, limiting the power to the exciter.
 
#19 ·
Yes, that would work...
But how do you know that the Batter(ies) are charging?
I did a little searching deep into the dark ugly soul of Datasheets, and I found that Bosch seems to recommend two different Wattage Lamps- ~1.2W for smaller Alternators, and ~2W for the beefier ones. These are standard "Instrument" lamps, and for convenience sake, Manufacturers tend to stick with the same value lamp for all instruments. (Alternator, Oil Pressure... Seat Belts?)

For the first few milliseconds after "Ignition" "On", the 1.2W Lamp passes around 2 Amps into the Field Coil, effectively "Charging" it, and then the current quickly drops to around a tenth of that value as the Lamp heats up. After the Engine starts turning over, Voltage starts rising at the output of the Rectified Alternator, and at some point the Regulator kicks in, and the Lamp goes (Mostly!) out- the Regulator is now feeding the Field windings. (The Lamp is now seeing (Mostly!) the same Voltage on either side. In total Darkness, a Calibrated Eyeball can tell just what is actually going on second by second by paying far too much attention to just how dimly the lamp glows...)

So what about just wiring the Field windings directly to the Oil Pressure Switch, and maybe using an LED with a Differential Comparator to indicate charging?
It's a minor thing really, other than all of that extra circuitry to go wrong-
Something else can go wrong, (See next Post.)
(Why don't manufacturers just add enough Iron to the Field windings so that there is enough residual Field to not need any initial Current? Actually, some _do_, and why others don't probably has more to do with Corporate Inertia and Patents, rather than Engineering.)

FWIW, I added a Velleman Digital Panel Voltmeter, ($5 on sale at Fry's.), to my Auxiliary Electrical Panel so that I can monitor Boat Voltage with 10mV resolution:
FRYS.com*|*VELLEMAN

In addition, I have a 500A/50mV shunt in the common Battery ground that will be wired to the Velleman in the future for Charge or Discharge Current monitoring, with 1 Amp resolution.

The Velleman "floats" with regards to the Boat's electrical system, and can run for about a year on its own 9V Alkaline battery; it draws ~380 Microamps.

¬Erindipity

(As this Post has gone on far too long as it is, my next Post will deal with Inductance and Failure modes.)
 
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