Over charging alternator

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Ron from Big D

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I lost my alternators back in October from an over charge rate.
Re-built just one of the units and it charged correctly until
yesterday (1400 miles since re-build) and went to 16 volts and 100
amps output again. The other alternator was not connected and has not
been re-built. I found out that this can be caused by loosing the
circuit,(Open circuit) such as, losing the connection of the battery
cable on the alternator. Anyone know of a similar case that would aid
me in troubleshooting this problem.

Since the alternator is the front unit under the bedroom floor on
my 92WB40, I do not intend to remove the floor till I get home. I've
disconnected the alternator and now running my generator and charging
from the battery chargers. Hate to put that many hours on the genset,
but don't want to waste time during this trip trying to find what's
causing this problem.

Give me any thoughts you might have.

R.E. (Ron) Marabito, Dallas, TX 92WB40
Currently in Tucson, AZ and will be attending the Lone Star Birds
Rally in Kinder, LA the 16th of Feb.

 

Tom

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Ron,

I've seen numerous warning labels adjacent to battery cutoff switches saying that the alternator could be damaged if the switch is turned off while the engine is running. I've never really thought about the reason or, if I did, I've forgotten the answer. But, if that's true, it would suggest you need to check all your battery and alternator cables and connections. Also check for a possible bad (intermittent) battery switch.
 

Ron

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In some circuits a faulty transfer switch can cause the same condition as a open cutoff switch like Tom suggested.  I also concur with Tom's suggestion.  Have a safe trip.
 

Tom

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I vaguely recall seeing some battery switches (on boats) that have an additional pair of low current terminals which, I believe, opens the field on the alternator (thereby preventing generation)  in the even the switch is turned to OFF.
 

Karl

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Ron,
An alternator typically varies its' output based on the voltage it senses at the battery. If it senses a low (or no) voltage, it could very well step up the output considerably. Check all cables for corrosion and tightness. Most modern alternators have the voltage regulator integrated in them. If you rebuilt the alternator without replacing the v/r, that may be part of the problem. Bad diodes are unlikely to be to problem. If they open or short, your voltage and current would most likely drop; not increase.
 

Tom

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That's correct Karl and the low current field gets opened by the additional low current contacts when the switch I mentioned is turned off. Without the field, no (or very little) generation. The residual magnetism in the rotor generates only sufficient current to excite the field. But, it's a long time since I rewound motors, generators and alternators for a living.
 

Ron from Big D

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I'll check all the connections carefully.  Sure hope it isn't the transfer switch, because it is a bear to get into.  Did it once and really don't want to do it again.  The only thing I have noticed is, some corrosion on the last terminal of the start batteries.

 

Tom

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Ron,

Maybe they hid the switch and you just haven't found it yet  ;D
 

Lou Schneider

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Look for a loose connection between the alternator and it's remote sensing point - which is most likely NOT the battery itself.  Lifting a battery lead should not make the alternator take off - for this you need to break the loop between the alternator and the sensing lead.

Do you have a house / starting battery isolator?  Usually the voltage sense is on the starting battery side of the isolator to compensate for the diode voltage drop.  If the isolator or it's connections are intermittant this can keep the sensing lead from seeing the alternator voltage.
 

Ron from Big D

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Please define sensing point.  There are 3 connections to the alternator.  Main battery lead, small exciter lead and ground.  Nothing wrong with battery lead, but until I remove the floor, can't tell about the other two.  I might be able to see where the exciter lead ends, but not the ground.

 

Karl

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Sorry, but I have to disagree with Lou. The regulator senses the voltage at the battery through a second, smaller wire. This in turn, signals the regulator to either increase or decrease its' output voltage to maintain the charging voltage at roughly 13.6V. This second wire sometimes comes directly fromt the main positive battery terminal and has a fusible link to prevent excessive current flow. Other arrangements will pick off the sense voltage at a terminal/fuse box and will be similarly fused, usually at about 30A. 
 

John From Detroit

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I  can only add that there are a bunch of different alternator configurations.

Some, as Karl says, sense via a lead hooked to the battery

Some, sense via a lead hooked elsewhere

Some sense internally

Some only have one wire hooked to them.. That's right  JUST ONE (They sense internally)

Bottom line... Anything is possible.
 

Lou Schneider

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The regulator senses the voltage at the battery through a second, smaller wire. This in turn, signals the regulator to either increase or decrease its' output voltage to maintain the charging voltage at roughly 13.6V. This second wire sometimes comes directly fromt the main positive battery terminal and has a fusible link to prevent excessive current flow. Other arrangements will pick off the sense voltage at a terminal/fuse box and will be similarly fused, usually at about 30A.

That's what I meant to say, Karl.  The sensing lead may not necessarily be at the battery - in any case it's not connected to the battery terminal, but on the connector side.  Lifting the battery will do nothing to the voltage at the connector, so the alternator output will stay constant.

If there is an interruption (a loose or corroded connection, or a bad isolator) between the alternator output and the point where the sensing wire senses the voltage, the alternator output will rise because it thinks the voltage is too low.
 

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