Alternator not charging good

[Sniper82]

I got a alternator thats not charging good. It drops when there is a load. Is it likely the brushes, regulator or something else?

 

[Bartman39]

Most that I know of have a diode trio (rectifier assy) and one of the diodes fries so it only has 2/3`s of the power and will barely keep up or may not keep up on demand in other words weak...

 

[phucheneh]

Can't you check for a bad diode by testing the output with your multimeter set to AC voltage? I dunno how conclusive that is...AC voltage is always a bad thing, but I'm not sure if that'll show up in all cases of a bad part in the rectifier bridge.

OP, are you trying to repair the alternator yourself? How much is a reman?

 

[Sniper82]

a reman is around $130. I figured if it was something as simple as the two mentioned items in my post I'd just replace them.

 

[Bartman39]

Do not think you can check the AC voltage as its internal where the connections are made...? Also that is not the way to check a diode which is nothing more than a P/N junction which means current will flow one way and not the other... You might be able to take the alternator apart and check the diodes with a multimeter using either a diode check setting or depending on the diode break over potential just setting it to ohms might allow you to test each one...? Either way its much easier to just take it to a starter/alt shop and have it tested on a bench and get a price from them to repair it...

Just a little more info... (may be rusty on my terms)

The AC sine wave goes up above ground potential on the positive pulse and down below ground potential on the neg pulse and the diode in effect clips off the the down pulse (negative) and allows the positive pulse to go through and be regulated later as a DC signal... Put 3 of these sine waves 120 degrees apart and you get pulses that overlap close to each other which gives you fairly smooth DC power but lose one diode and you get the picture (2/3rds of the power)... In addition the negative part that is clipped off is energy and is dispersed as heat (might see a heatsink which is for both the diode trio and the voltage reg which in most alternators is combined together as one unit)...

 

[Raizinman]

Most auto parts stores will check your alternator for free. Most of their test equipment will diagnois many of the componant parts. Heck, you can check most all the components on an alternator with just a simple multi tester if you know how. If you don't, there are YouTube videos that will walk you thru it. If you are short of funds, depending on the make and model of your car, a used alternator at a salvage yard will likely cost you less than the time and parts necessary to do any repair.

 

[Mark R]

Can't you check for a bad diode by testing the output with your multimeter set to AC voltage? I dunno how conclusive that is...AC voltage is always a bad thing, but I'm not sure if that'll show up in all cases of a bad part in the rectifier bridge.

OP, are you trying to repair the alternator yourself? How much is a reman?

This may work depending on the multimeter. Some meters get confused when they are set to AC voltage but a DC voltage is present. A number of people I use a hand-held oscilloscope so they can see the AC/DC components unambiguously.

AC voltage should always show up in the event of a bad diode. The alternator is 3 phase, and when rectified with a 6-diode bridge, this produces a smooth DC voltage . Lose one or more of the diodes, and the DC gets noticeably less smooth (i.e. lumpy DC = DC + AC component).

 

[Bartman39]

This may work depending on the multimeter. Some meters get confused when they are set to AC voltage but a DC voltage is present. A number of people I use a hand-held oscilloscope so they can see the AC/DC components unambiguously.

AC voltage should always show up in the event of a bad diode. The alternator is 3 phase, and when rectified with a 6-diode bridge, this produces a smooth DC voltage . Lose one or more of the diodes, and the DC gets noticeably less smooth (i.e. lumpy DC = DC + AC component).

This is only if the diode has shorted otherwise it will be an open and not allow the AC voltage in the output... But a good point none the less and the AC voltage might also show in the sound from the radio depending on the quality of its filtering the input...

 

[Mark R]

This is only if the diode has shorted otherwise it will be an open and not allow the AC voltage in the output...

No. This is if the diode fails open.

I haven't come across a short diode; I would assume something would blow up pretty quickly if a diode was shorted (probably the diode or one of the other diodes) .

 

[Zivic]

I think your goodest option for an alt that is not charging good would be go to the goodest parts store, ask for the goodest replacement alt. then find the goodest mechanic to install it, if you cannot do it yourself

 

[Bartman39]

No. This is if the diode fails open.

I haven't come across a short diode; I would assume something would blow up pretty quickly if a diode was shorted (probably the diode or one of the other diodes) .

Uh might want to re-think that...? An open circuit will not pass anything so how could you expect to get AC into the output of the alternator...? If its shorted then yes it will pass the AC into the system... If you can explain that to me I might buy it but 30+ years of training is telling me other wise...?

Just to add I have never seen an AC induced voltage from the output of a an alternator but know its is possible just from the config of the circuit but highly unlikely since the diode (PN junction) just pops to the open condition when it fails hence no current flow at all... But a shorted diode is not out of the question but would be very rare as heat and over current tend to "open" circuits...

 

[BUTCH1]

Uh might want to re-think that...? An open circuit will not pass anything so how could you expect to get AC into the output of the alternator...? If its shorted then yes it will pass the AC into the system... If you can explain that to me I might buy it but 30+ years of training is telling me other wise...?

Just to add I have never seen an AC induced voltage from the output of a an alternator but know its is possible just from the config of the circuit but highly unlikely since the diode (PN junction) just pops to the open condition when it fails hence no current flow at all... But a shorted diode is not out of the question but would be very rare as heat and over current tend to "open" circuits...

Shorted diodes in power supplies for consumer electronics do happen but that's where the main fuse comes in and shuts the whole deal down. shorted output transistors were also common. Perhaps diodes designed for automotive (hostile environment) are made to just open when they fail, IE: the P/N junction just fries quickly in event of a shorted diode..

 

[LTC8K6]

Diodes most often fail short.

Heat and voltage spikes cause the shorted failure.

Overcurrent causes open failure.

Intermittent failures also occur rarely.

IIRC, it's about 75% short and 25% open for diode failures overall.

 

[Mark R]

Uh might want to re-think that...? An open circuit will not pass anything so how could you expect to get AC into the output of the alternator...? If its shorted then yes it will pass the AC into the system... If you can explain that to me I might buy it but 30+ years of training is telling me other wise...?

We're talking at somewhat cross purposes.

What I mean by AC voltage is an AC component (i.e. a time changing voltage) which may be present simultaneously that a DC voltage is present.

As a thought experiment, Let's just take a bare alternator, and lets assume that it is generating "12V" (let's ignore the fact that it needs a battery to excite, etc.). As a 3 phase device with 6-diode bridge, the voltage will actually vary between about 11V and 13V. When the voltage on one alternator phase drops below 11V, another will be increasing about 11V, so the diodes will switch the output over to that phase.

This fluctuating voltage could be thought of as 12V DC + a superimposed 1V AC. And it is this 1V AC that I am referring to (and some, but not all, meters will measure in AC mode).

Now, in a real car, you have a battery, that will serve to stabilise the voltage, so that the AC component or "ripple" is much reduced - often less than 0.2 V. If I set my decent multimeter to AC mode and test it on a running car, this is what it will read. My cheap ass DMM in AC mode will come up with a random number around 25.

So, what happens if a rectifier diode goes open. The result is that during the periods of time when that diode should be taking power from the alt, another phase with a lower voltage is having to take over. The result is that the "ripple" is much increased. Precisely how much depends on battery quality, and precise design of the alt's regulator.

In the event of a diode going short, it will effectively short out one of the alt windings and two other diodes - resulting in something frying, very quickly (usually one or more of the other diodes). In the event that something doesn't fry, it will drain the battery in double-quick time once the engine is stopped.

 

[Bartman39]

That does make sense to a certain point and wont disagree with the "ripple effect" but as you point out the battery is the "buffer" stage and this is also the reason most electrical systems on cars/trucks even equipment has a charging wire direct from the alternator to the battery and not a direct power feed to the system... A battery produces whats know as "quiescent state" DC which of course is a true smooth DC voltage and will absorb any of these ripples so the rest of the system wont...

Back to diodes in automotive alternators they do go to an open condition when they fail more often than not and could do so by design...? I have taken to many apart and checked to be sure of the issue and found them open... Just to be honest I have never found a shorted diode in an alternator in either the 12V or for sure the 24V systems I have worked on... When I have found an alternator draining a battery its always been the regulator has the issue and is causing the drain... Also on the 24V alternators we use the "R" terminal which comes off the regulator is the terminal that sends a signal to the anti-restart relay on the starter... If the voltage if over 2.5 volts it wont allow the start to engauge... What happens is when the regulator dies or shorts it sends the full battery voltage to the anti-restart relay but at the same time back feeds the main power ground disconnect relay and just drains the batteries... Engine wont start and the electrical wont cut off... Ok was off in a different area...:\

Back to the ripple effect... You may see this on a multimeter and interpret this as AC voltage but its nothing more than a dip in voltage caused by that phase not being added to the full circuit... There would not be a current flow just a drop in voltage, you have to have the current flow to make it a true AC voltage this is what I am getting at... An open circuit means no current flow so back to what I'm saying I have found with diodes in alternators (fail open)...


BTW good discussion to all, different opinions do not make anyone wrong it sheds light on how things work... (BTW fella's I'm not always right but I'm always learning)

 

[Squisher]

While the two Mr. Wizards battle it out heart: Bartman39 and Mark R), I agree with this post:

Most auto parts stores will check your alternator for free. Most of their test equipment will diagnois many of the componant parts. Heck, you can check most all the components on an alternator with just a simple multi tester if you know how. If you don't, there are YouTube videos that will walk you thru it. If you are short of funds, depending on the make and model of your car, a used alternator at a salvage yard will likely cost you less than the time and parts necessary to do any repair.