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Discussion Starter · #1 ·
I have a question about my alternator, yesterday while out for a drive I noticed my volt gauge was reading like 17 volts. Now I have never seen this before as I didn't have a volt gague or an idiot light hooked up. I'm running an 10si style alternator with internal regulator and a 3 wire system to it. The alternator is the original one that I put on the car so it is about 13 years old, is there a way I can check it to see if its bad or do I need to take it to an auto parts store to have them bench check it.

From doing some searching on this site it seems that it should only read about 14 volts when the engine is running, thanks for any help.
 

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zbad55, at 17 volts the alternator is running at peek vrms.
In other words flat out at peek voltage output.
This is generally caused by a fault in the regulator system within the alternator.
Possibly only a diode gone out.
This overcharge state will cook your battery if you run it to long at this range.
I would suggest checking all wires to the alt and volt meter for corrosion at a first point.
Then if no change, if you do not have the correct test equipment, take to your local repair center to have it corrected.
This high output is not good for your electrical system either and has a bad habit of letting the smoke out of the little tubes it runs in.
Regs David,
 

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Trifive Automotive Electrical Wiring Expert
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Check for a connection of the #2 terminal of the alternator small plug (right hand terminal looking at back of alt) to make sure it is connected either to the battery or the output terminal of the alt. This is the sense terminal and without it connected it can overcharge. It should read battery voltage (12+ volts) when the engine is off.
 
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A clarification, if I might, NOT a criticism at all.

"Possibly only a diode gone out."

What a diode does is allow voltage, to flow in one direction, and not in another. They are polarity sensitive, which means they need to be installed the correct direction for what you wish to do.

An ALTERNATOR produces ALTERNATING CURRENT (A/C volts), which has both positive and negative electrons moving both ways within a single wire. To "rectify" an alternated current, a pair of diodes, one polarity to positive, the other to negative are needed. These diodes, set up this way, take all the positive electrons and move them in ONE direction, instead of the A/C's both directions, the other diode handles negative in one direction, creating current flow in DIRECT CURRENT, positive one direction, negative the other.

Since our alternators are all '3 phase' A/C charging circuits there are 3 individual charging circuits for A/C production, and 3 each diodes, positive, and negative polarity. So, basically, we have 3 fully separate A/C charging circuits in each alternator, and 3 rectifier circuits, one for each charging circuit, to make the D/C our cars run on. This is done because A/C current is easier and more stable to make than D/C, which was proven way back in the early days of Thomas Alva Edison's D/C, and George Westnghouse's A/C.

Knowing all this, one fact is, diodes do not fail closed, they do not get to the point they allow current to pass both directions. When they fail, they fail OPEN, lose all continuity, which would stop all voltage passing through them, including the charge current for that directional flow.

So, when a diode fails in one of our alternators, the alternator won't over charge, it will under charge. For one diode to fail, charging loss will be one sixth of the output, or part of one phase. Two failed diodes will stop one third of the charge current will fail.

So, the statement "Possibly only a diode gone out." should apply to under charging, and not over charging, as is alluded to in the poster's text. NO big deal, no fault, just a clarification on diodes and how they work in our alternators.

Also as stated above, over charging is a function of the regulator, not the rectifier/diodes. I have a stock 1982 S10 with 2.8 V6, and 3 wire internal regulator alternator, and volt meter. One day decades ago, I was some distance from my home, and the voltmeter went to full 18 volts charge. I immediately pulled over, shut the thing down, and pulled the wires off the alternator, drove the rest of the way home total loss, in daytime, no lights needed. When I took the alternator apart the next day for repair, the regulator had fried closed, and a replacement made the alternator work golden again. Simple fix for a serious problem.

A brief explanation of the two different charging methods used in our alternators.

The original remote regulator alternators we have, use an on-off-on-off method to charge. The alternator is 'cold', no charge when he regulator is down, no charge, by NOT energizing the rotor windings into an electro-magnet, then, the regulator "turns on" the rotor into an electro-magnet to create charge A/C voltage. As the charge rate gets better, time the rotor is ON, is reduced, off, increased. The off-on-of-on is seen graphically on the fluctuation of headlights, low, high, low, high at idle in the dark. What you are seeing is the regulator turning the rotor in the alternator on-off-on-off.

There are now electronic regulators available, such as Wells VR715, that change the method of charge for early alternators from the method described above, to the method described below.

Internally regulated second series alternators are the internally regulated types. These types all have electronic regulators as stock parts. the rotor function is the same, become an electromagnet, and induce magnetism into the stator, to produce A/C current. However, this type regulator doesn't go on-off-on-off, it gets to a certain engagement capture rpm at first run, and energizes the rotor and holds it in ON mode, doesn't turn the rotor off. this makes the alternator go to full over charge mode, and if not regulated, will do what my S10 did, go to 18 plus volts. This type regulator then takes the excess voltage/current, and simply dumps the over charge to ground, regulating the charge at a sensible level, typically, 14.40 to 14.60 volts on average.

There are benefits to the always on rotors. The windings don't really like he start up currents when they are turned on-off-on-off, but have a lot better time when they are engaged, and held on. Voltage management is also better, in that the charge voltage/current is held steady, not changing up-down, because, in the electronic regulator, the excess voltage is dropped to ground, steady charge rate, idle to maximum RPM's of engine/alternator.

Just more info to consider about alternators, and, the way they prodcue voltage/current.
 

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A clarification, if I might, NOT a criticism at all.

"Possibly only a diode gone out."

What a diode does is allow voltage, to flow in one direction, and not in another. They are polarity sensitive, which means they need to be installed the correct direction for what you wish to do.

An ALTERNATOR produces ALTERNATING CURRENT (A/C volts), which has both positive and negative electrons moving both ways within a single wire. To "rectify" an alternated current, a pair of diodes, one polarity to positive, the other to negative are needed. These diodes, set up this way, take all the positive electrons and move them in ONE direction, instead of the A/C's both directions, the other diode handles negative in one direction, creating current flow in DIRECT CURRENT, positive one direction, negative the other.

Since our alternators are all '3 phase' A/C charging circuits there are 3 individual charging circuits for A/C production, and 3 each diodes, positive, and negative polarity. So, basically, we have 3 fully separate A/C charging circuits in each alternator, and 3 rectifier circuits, one for each charging circuit, to make the D/C our cars run on. This is done because A/C current is easier and more stable to make than D/C, which was proven way back in the early days of Thomas Alva Edison's D/C, and George Westnghouse's A/C.

Knowing all this, one fact is, diodes do not fail closed, they do not get to the point they allow current to pass both directions. When they fail, they fail OPEN, lose all continuity, which would stop all voltage passing through them, including the charge current for that directional flow.

So, when a diode fails in one of our alternators, the alternator won't over charge, it will under charge. For one diode to fail, charging loss will be one sixth of the output, or part of one phase. Two failed diodes will stop one third of the charge current will fail.

So, the statement "Possibly only a diode gone out." should apply to under charging, and not over charging, as is alluded to in the poster's text. NO big deal, no fault, just a clarification on diodes and how they work in our alternators.

Also as stated above, over charging is a function of the regulator, not the rectifier/diodes. I have a stock 1982 S10 with 2.8 V6, and 3 wire internal regulator alternator, and volt meter. One day decades ago, I was some distance from my home, and the voltmeter went to full 18 volts charge. I immediately pulled over, shut the thing down, and pulled the wires off the alternator, drove the rest of the way home total loss, in daytime, no lights needed. When I took the alternator apart the next day for repair, the regulator had fried closed, and a replacement made the alternator work golden again. Simple fix for a serious problem.

A brief explanation of the two different charging methods used in our alternators.

The original remote regulator alternators we have, use an on-off-on-off method to charge. The alternator is 'cold', no charge when he regulator is down, no charge, by NOT energizing the rotor windings into an electro-magnet, then, the regulator "turns on" the rotor into an electro-magnet to create charge A/C voltage. As the charge rate gets better, time the rotor is ON, is reduced, off, increased. The off-on-of-on is seen graphically on the fluctuation of headlights, low, high, low, high at idle in the dark. What you are seeing is the regulator turning the rotor in the alternator on-off-on-off.

There are now electronic regulators available, such as Wells VR715, that change the method of charge for early alternators from the method described above, to the method described below.

Internally regulated second series alternators are the internally regulated types. These types all have electronic regulators as stock parts. the rotor function is the same, become an electromagnet, and induce magnetism into the stator, to produce A/C current. However, this type regulator doesn't go on-off-on-off, it gets to a certain engagement capture rpm at first run, and energizes the rotor and holds it in ON mode, doesn't turn the rotor off. this makes the alternator go to full over charge mode, and if not regulated, will do what my S10 did, go to 18 plus volts. This type regulator then takes the excess voltage/current, and simply dumps the over charge to ground, regulating the charge at a sensible level, typically, 14.40 to 14.60 volts on average.

There are benefits to the always on rotors. The windings don't really like he start up currents when they are turned on-off-on-off, but have a lot better time when they are engaged, and held on. Voltage management is also better, in that the charge voltage/current is held steady, not changing up-down, because, in the electronic regulator, the excess voltage is dropped to ground, steady charge rate, idle to maximum RPM's of engine/alternator.

Just more info to consider about alternators, and, the way they prodcue voltage/current.

Thanks, great information. bowtie-trifive :gba:
 

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Discussion Starter · #6 ·
Thanks for the info guys good stuff, will get this checked out. I currently have one large red wire going to the back of the alt and then a two wire plug going to the plug on the side of the alt. I think they are a red and brown wire, not sure where they run to as they are part of the harness. I have an original harness with the wires added for a internal regulated alternator.
 

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You can get a rebuild kit for a 10SI from Scott at S and D repair on ebay. I have dealt with him many times and he is a great seller. I had one problem with a wrong bearing sent and he took car of it right away and did not want the wrong part back. He also took care of shipping charges. Sounds like your alternator needs a rebuild. Jim
 
G

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for the most part, the 10DN with remote regulator will have a two wire connector plug out the REAR of the alternator.

The 10SI and 12SI internally regulated alternators will have the two wire connector facing out the side of the alternator.

I am attaching a diagram of the 3 wire stock setup for the 12SI, and as you can see, the number 2 side terminal is directly connected to the large battery cable, some place up the alternator wiring loom.

The number 1 side terminal is the charge light wire connector.
 

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If your voltmeter reads 17 volts, fix it now. Otherwise you are going to blow all the light bulbs and all the electrical devices that don't have over voltage protection.

The problem is the voltage regulator.

When you have problems with the diode trio, each one is 1/3 of your alternator output. So if one is out, your 63 amp alternator just became a 42 amp alternator, or your 100 amp alternator just became a 66 amp.
 

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If your voltmeter reads 17 volts, fix it now. Otherwise you are going to blow all the light bulbs and all the electrical devices that don't have over voltage protection.

The problem is the voltage regulator.

When you have problems with the diode trio, each one is 1/3 of your alternator output. So if one is out, your 63 amp alternator just became a 42 amp alternator, or your 100 amp alternator just became a 66 amp.
Good explanation, Dave Ray thats a detailed overview :tu
 
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angs1957, thank you for the kind words, sir, they are greatly appreciated.

Yeah, I do that kind of stuff all day long, and build my own small-body HEI and MSD trigger conversions for a living, as well.

Just shocking, very shocking!

I completely agree, any GM version alternator, OEM and/or after market, should not go past 14.60 volts maximum. More volts, and components blow right up.

Here is a little info on failed light bulbs.

If the filament is not damaged, but broken off its holders inside the bulb, and flopping loose all over the place, it came off from vibration. The lens will still be clear.

If the lens is either black or silver, and you cannot see through it, the filament exploded from over voltage.

If the filament is broken from only one of its holders, and the broken end of that filament is mushroomed, but not black, there are grounding issues. The mushrooming of the filament is from increased heat created by the lack of grounding added resistance. This added hear isn't enough to blow the filament up, as in the over volt issues, just to melt the grounded end of the filament away from its holder, and mushroom shape it.

Just more info to contemplate.
 

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Discussion Starter · #12 ·
Just wanted to give an update, after checking all the wiring to the alternator and putting the plug back on everything seems to be working correctly, maybe the plug wasn't on very good. I did have the alternator checked and it was good. The gauge is reading about 14 volts now.

Thanks for all the help and I learned a lot from this thread, the knowledge on this forum never ceases to amaze me, thank you
 
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