• put a meter on your battery as its running, if it above 12.6 Volts, its charging less the 12 volts, its not.
• Check your altenator belt,
• Check on your connections. (Battery Posts, Altenator)
• As far as checking to see if the altenator actually works, you can remove your altenator and take it into a garage, and have them check it, or you can also do it yourself. This is the most direct way. If you did the rest already, there is a good chance you altenator is bad.

Thanks pheonix 911,

I know that but like I've said what goes wrong, I was told years ago that most alternators fail because of an electronic component fails actually in the alternator & because I didn't pay attention missed what he said, Like most mechanics it was a case of if it stops working, replace it, well if you're in the back of nowhere you can't just get a new one.

Bazzer

I just replaced mine last fall, and no way was I going to buy new.

I bought a rebuild one at a automotive store that also has a warranty, at a fraction of the price. Or, you can buy it off the internet and have it delivered.

As far as repairing it yourself, that you'll have to decide. What I have done, is look on the website at some automotive repair blogs. I have done this, some more useful with pictures and diagrams in steead of just posts.

I was told years ago that most alternators fail because of an electronic component fails actually in the alternator & because I didn't pay attention missed what he said,
The rectifier diodes are the most common failure point. These are sometimes very hard to find replacements for in modern cars. The regulator is often built into the alternator as well, and can sometimes fail. The regulator is generally not repairable -- you just pop a new one in.
(Not long ago, I had a car for which the diode plates were a dealer-only item. A rebuilt alternator was about half the price of the diodes alone from the dealer.) In cars from about 2000 or so onward, the charging is often under control of the ECU, making things a little more complicated.

Alternators can get tricky and very complex depending on what generation you want to troubleshoot. depending on who the manufacturer is and how old it is make a big difference in if it is even possible to attempt a repair.

thee are books out on how to repair some types (mostly older)

do a search for "12v alternator doctor" or "12v doctor" & you should get a couple of leads on some books at least.

Thanks Sparkie Ill try that now.

Bazzer

Dead diodes would be my guess.

I must say that I'm a little confused about the wye-wound alternator. That's the first diagram for an automotive alternator that I've seen that hasn't been a delta wound stator with a 6-pulse rectifier.

I'm still racking my brain to find an advantage for this configuration. Maybe Sir Bedevere would know, he is so wise in the ways of science.

I have seen that design used a number of times on industrial alternators for big trucks and heavy construction machinery. They like to use Wye connected stators for many of the high output 24 volt units and its not uncommon to find one with 8 diodes opposed to 6.

Somebody seems to think there is an advantage to the design.

(Or else its how they justify a 50 amp 24 volt alternator costing over $600 - $800 when a common one of equal or higher amp rating can be bought and refitted in for 1/4 or less of that.)

Higher voltage was the only thing I could think of, but why not just wind a delta for 24 volt elex? Oh, well.

Depends on what the manufacture wanted to build for that specific model.

I have a number of used Leece Neville alternators at home I have modified for other uses and some are 24 VDC delta wound and others are 24 VDC Wye wound. Some use double and even triple winding sets in parallel while others just use one large gage set.

The 24 volt 185 amp triple winding delta ones are great for reconfiguring into applications where I want much higher output voltages. Just by changing them over to a Wye configuration and putting all three winding sets in series for each phase they can easily put out 350 VDC at 10 - 15 amps!

The other nice thing about their designs is that so far I have not found one model whether it was a 12, 24, or 60 volt unit that had diodes that did not pass the 1000 volt megger test for reverse breakdown voltage testing.

Now Motorola, Delco, and most other standard automotive type alternators they rarely make it past the 150 volt megger test range which is probably whey so many have diode failure issues.

Speaking of not paying attention in class. The Leese Neville instructor said that one of the configurations would have a steeper charging curve than the other.

No, I do not remember which one.

Other than electronics (regulator/rectifier type stuff), you might also check the brushes which you may or may not consider electronics, but is a part that can wear out.

Have you seen an alternator that uses brushes? Generators yes, alternators?

Standard issue method of getting power to the spinning rotor coil.

Oops! Of Course. I was thinking of PM alternators...

Brushes are used on the rotating field armatures of alternators and do fail.

Essentially all automotive alternators have brushes for the field coil, which rotates. The three phase charging coils are the stator.

The only exceptions are a couple kai cars that have engines developed from motorcycle engines. In one or two of those, the field is stationary, the charging coils are stationary, and the rotor is a reluctor.

i agree that it's an internal electronic problem. probably the rectifier as others suggested. unforunatly it's not a back yard fix. you'll need to take it to a shop that specializes in rebuilding alternators/ generators.

Well for the obvious but always over looked simple question.

What brand and model is it?

Hi tcmtech, I should have said It's a Bosch 12v 110amp hr alternator, I have large quantities of electronic components & have no doubt that once the faulty part is identified I can fix it.

Bazzer.

Apart from the diodes there are no discrete components. Sometimes even the diodes are pressed into a proprietary frame and have to be changed as a lot.

The regulator is a potted block.

The brush arrangement can vary from very serviceable to integrated with the regulator.

You'll have to do something else with your swag of components.

Not much help.

Hi tcmtech, I should have said It's a Bosch 12v 110amp hr alternator,

No, you should have said it's a Bosch 110 Amp alternator. Batteries are measured in amp-hours (a measure of energy); alternators are rated in amps, a measure of maximum current.

When one diode goes, it is not uncommon for several or all 6 of them to go. You may not be able to find new ones that will press into the holder/heatsink. But you can make your own heat sinks if need be: it is essential that the diodes are a press fit for good heat conduction. If you mount the heat sinks outside the alternator, then they need to be several times as large as the built-in one, unless you provide fan cooling.
Once you have disassembled the alternator, diodes are easy to check with an ohmmeter.

Worn brushes can contribute to diode failure. The intermittent connection to the field can cause large voltage spikes that damage the diodes.

A regulator can often be bypassed, but you need to check the schematic, to be sure that you are not trying to (for example) ground a hot lead. (Some Bosch alternators had a tiny hole that you can stick something into to bypass the regulator, if I recall. This grounds one end of the field winding.) If bypassing the regulator allows the alternator to output 14 volts (at 10 - 50 amps depending upon battery state of charge, etc.) then the diodes are fine.

Have fun.

...and to follow on with TCM's appropriate questions, what symptoms lead you to believe it's broken? A red light only tells you that the charging circuit isn't working properly, not what part, so that assuming it's the alternator is a leap, a slipping belt will give exactly the same red light.

A slipping belt is a good possibility, but I have fixed more electrical problems by tightening a loose ground - everybody checks the +12V.

Ground is very good answer.

The screw on my battery post was stripped, but was unnotice, because there was just enough thread to make contact and catch.

but every month or so it kept loosening. When I actualling looked close at the terminal bolt, Replaced it. and did not have problems since.

A slipping belt will give clues like screaming and a smell of burning rubber so no it's not the belts, Maybe I should say I know my way around vehicle engines , I've checked belts, relays & fuses,the thing I don't know is when an alternator fails What fails?

Bazzer

Might be repeating some of what others have said, but here goes.

First I'd check the belt is tight enough. It doesn't necessarily give a screaming noise or other symptoms if it's too loose. It needs to be pretty tight (compared with a dynamo say) but not ridiculous.

Then brushes. These can wear. How many miles has it done? I used to re-con Lucas alternators in the 80s and they had face-type sliprings, usually the outer brush wore after ~ 80000 miles and you could swap them. Bosch will have conventional sliprings which last longer but worth checking. Cheap and easy to replace.

How old is it? Is it connected to the ECU? (as K Fry suggests in #9) I don't know much about that type, but the diagram in #6 still has a voltage regulator within the alternator.

The alternators I'm familiar with have 3 additional diodes for field supply (so as not to drain the battery through the field when not running) but the details don't affect the tests below. The common output of these diodes is fed to one brush, the other brush going to earth via the voltage regulator, which puts a variable resistance in to control alternator output. You can test the regulator by connecting the earth-side brush direct to earth with a wire. If the regulator is faulty the charge warning light will go out, and if the engine is doing a good fast idle you'll hear it labour (too much rev swamps it).

Maybe some models have the regulator in the power side of the field, the other side solidly earthed. If so you can test by putting 12 volts on the brush after the regulator. If you examine the machine you can tell which it is. Neither of the above tests risks damaging anything, alternator is self-limiting on current, but make sure you don't short the live side to earth.

If none of that works, it's likely the rectifier, and you may have to disconnect it (probably un-soldering) to test the diodes. Fairly big job, and it's worth making sure you can buy a replacement before starting, or you're wasting your time. But I did fix a Femsa alternator once where one diode was physically broken, and I got one with long tails from an electronics shop and soldered it in remotely, but at the same place electrically. It worked fine.

Hope some of that helps. Good luck, let us know how you get on.

You might have a 110Ah battery, that's not the alternator capacity.

Automotive alternators are normally, 55A, 80A or 100A rating.

Red light? Slipping belt, rooted diode(s), stuffed regulator, O/C brushes, burnt windings, disconnected connector.

These are easy to repair (apart from rewind) in a backyard if you have the tools and parts. Even rewinds are performed in backyard conditions here.

What model alternator is it?

I just realized you are a Brit. My earlier response mentioned tightening a loose ground; I believe you Brits refer to it as earthing. It's a common fault and will produce your symptoms. It's cheap and easy to fix - I always check for proper earthing FIRST.

That's alright, we walk on the earth you walk on the ground same difference.

Bazzer

is this for a car? I fixed one by dousing w tuner cleaner and gum out once. coils were really dirty. other than that all there is, is a rectifier and brush. does it output anything?

I'm going to have it apart over the weekend and have a look see, looking at all the advice everyone has given I should be able to find the fault even if I can't fix it.

Bazzer

easy way to see if alternator is working. start motor and disconnect battery. motor should still turn. if it dies the alternator not outputing enough. measure voltage on cable w battery disconnected.

Depending on the vehicle, unhooking the battery while the engine is running may fry the electronics. You'll get a huge power surge from the alternator.

That was OK in the "old days".

Agreed, I wouldn't risk that. What's the point anyway, if the warning lamp is on it's a safe bet there's no charge. Safer to check voltage when the engine is running, as somebody must have suggested. Though obviously you need a meter for that.

It's a diesel.

Bazzer

...which means the fuel solenoid will shut off....unless of course your vehicle has a shutdown lever that you have to pull to cut the fuel.

I'll be honest and say that I can't risk disconnecting as the gearbox is controlled by a computer and I have various problems with it & if I disconnect the battery and then reconnect and the computer doesn't reboot I'm in deep do do's because it seems even fords don't know how to fix these, I have got to know all the problems and how to avoid them but I can't risk causing another fault.

Bazzer

I agree you don't want to disconnect the battery while the engine is running (risk to the alternator) but do you mean you daren't disconnect the battery at all? You'll need to do something nifty if you need to change the battery sometime! Though I'm sure you and some lengths of wire are equal to that.

I run a 2004 Mondeo, manual but with a ECU. The electronics gave some trouble, not really its fault and been fine since I realised what the problem was (it's a long story) but there's no problem disconnecting the battery. Have to reprogram the electric windows after but that's about all.

You didn't tell us the age/mileage of the alternator, that might help diagnosis.

Good call mate.

Motorola external regulated alternators of the 70s needed a voltage signal from the battery to tell the alternator to limit charging voltage. Not uncommon to hit 70 volts at an engine idle. DO NOT DO IT.