alternator problem

[robbio c]

Hi to all and thanks for replies to last problem ,any who this time its my alternator giving problems.On starting engine i use to have to rev engine a little once before alternator would kick in and red light go out,and for my ammeter and voltmeter and tacho to respond which i believe is normal.Now when engine starts no red light is on and all relative guages are not working even with a rev.have measured volts and only getting 12v, out of alternator ,all connections are ok on alt. basically it looks like alt has just stopped working all together, any ideas what may be the problem,....new alternator ?? is there a part in alt that can go ?? thanks Robbio

[Robbo]

Is the Alt. bulb lit when ignition is on but engine not started?

[by'eck]

Sounds like there is no battery feed from ignition switch when its turned on. This will provide power to instruments as well as alternator D+ terminal via warning light. The 12 volts you are seeing at the alternator output (B+) is probably just the battery voltage.

 

Check ignition switch and wiring.

 

ETA: As I read your post the alternator warning light doesn't come on at all now, but neither do the instruments which is why I dismissed a bulb failure.

Edited September 10, 2014 by by'eck

[robbio c]

alt bulb is lit with switch turned on robbo ,will check ignition switch and wiring tomorrow

[Robbo]

As your getting a alternator light when on ignition, the ignition switch is probably okay, as the negatives from the gauges are different I would start there. Doesn't explain only 12volts tho!

[Tony Brooks]

I thought he said NO gauges and NO light. As they all are fed from the ignition switch it sounds like a either faulty switch or more likely no supply to they switch. I would be checking for a multi-way plug on the main engine harness or an in-line fuse, again alongside the main harness somewhere.

 

If it's a Vetus then the fuse is often in a little holder fixed to a bracket behind the rocker cover. They can alternatively have a circuit breaker on the same bracket.

 

Remember no feed form the warning lamp to the alternator usually results in the alternator refusing to energise. A quick check is to bridge the B+ and D+ terminals with the engine revving. If the alternator energises you know its OK so the fault is somewhere else as described above.

[Robbo]

I thought he said NO gauges and NO light.

 

Check post #4 http://www.canalworld.net/forums/index.php?showtopic=69915#entry1393187

[Sir Nibble]

can you photograph the alternator please?

[Arthur Brown]

When did you last replace the fan belt? If the light comes on with the (ign) switch and then goes off something must be working, if the belt was worn or slack it could generate a little power but not enough to charge much therefore sit at 12v rather than 13 - 14

 

Where is your nearest automotive electrician? Can you get a replacement alternator easily?

Edited September 10, 2014 by Arthur Brown

[Sir Nibble]

Let's not throw too much guesswork at this eh. The crucial facts as I see them is that the light comes on and goes off as should. However, the tacho is not working so therefore there is no ac. We need to identify the alternator so as to know what the internal circuit is before speculating.

[robbio c]

Fan belt was changed last year and has had little use,will take a photo of alt over the weekend ,engine is ford xld 416.

[Sir Nibble]

It's not the belt, if there's output enough to put the light out and slip the belt there's power enough to run the tacho.

[Robbo]

Let's not throw too much guesswork at this eh. The crucial facts as I see them is that the light comes on and goes off as should. However, the tacho is not working so therefore there is no ac. We need to identify the alternator so as to know what the internal circuit is before speculating.

It's all? the gauges not just the tacho, and the tacho may not be alternator driven, mine isn't. So can't say no AC.

[Robbo]

Robbio, where did you measure the voltage? Was it at the alternator on the B+ terminal or at the battery?

[Sir Nibble]

It's all? the gauges not just the tacho, and the tacho may not be alternator driven, mine isn't. So can't say no AC.

The only other gauges mentioned are voltmeter and ammeter. Are you sure the voltmeter is reading zero? Though it shares a supply with a functioning warning light? What sort of fault will shut down charge and instruments but not warning light?

[Robbo]

The only other gauges mentioned are voltmeter and ammeter. Are you sure the voltmeter is reading zero? Though it shares a supply with a functioning warning light? What sort of fault will shut down charge and instruments but not warning light?

The alternator warning light will use the same positive source (ignition), but connects to the alternator which when not working is 0 volts, when alternator is working this produces volts and when the difference is 0 volts you have no light.

 

So the warning light doesn't share the same negative as other gauges

Edited September 10, 2014 by Robbo

[Sir Nibble]

The alternator warning light will use the same positive source (ignition), but connects to the alternator which when not working is 0 volts, when alternator is working this produces volts and when the difference is 0 volts you have no light.

 

So the warning light doesn't share the same negative as other gauges

Garbled but tru-ish as an explanation of how most warning lights work. Doesn't answer the question though does it?

Edited September 10, 2014 by Sir Nibble

[Robbo]

Garbled but tru-ish as an explanation of how most warning lights work. Doesn't answer the question though does it?

Other warning lights don't go out by been provided with voltage from both sides of the light tho, but by a "lack" of negative.

[Sir Nibble]

Other warning lights don't go out by been provided with voltage from both sides of the light tho, but by a "lack" of negative.

Usually yes quite right. Some alternator warning lights too, which is why I need to know the alternator type to determine if this is a switched lamp or earthed conventionally through the rotor. The thing is that if you lose the -ve from the voltmeter then quite right it will stop and the lamp will work, but so will the alternator, so that's not it is it. I suspect that what the OP means is the meter fails to show the voltage increase he is accustomed to. This is most likely a conventional 9 diode alternator with a brush problem but it is quite possible (for instance by trying to start the alternator by bridging B+ and D+) To demolish one type by using techniques applicable to the other.

[RLWP]

For reference, the diamond-in-the-rough Sir Nibble spent years dismantling, rebuilding and testing alternators of many types. He is undoubtedly the man for this, despite his natural sarkyness

 

Richard

[Sir Nibble]

You make me sound like a bench fitter!

[nicknorman]

... despite his natural sarkyness

Richard

Not showing through in this thread though. I think he is being very restrained! But yes, he is the forum's former momentary jet of liquid (aka expert) in alternators.

[RLWP]

You make me sound like a bench fitter!

You can fit benches too?

 

Richard

[Robbo]

Usually yes quite right. Some alternator warning lights too,

Is that alternator common? Reason I ask is that all split charge relays ive seen are wired from alternator to negative.

[Sir Nibble]

It's what is known as "3rd generation" alternators which are arriving now in the marine world whilst the automotive market is well into 4th generation! becoming more common but their relative scarcity means people neither know about nor expect them. I recall a few years ago a chap came on here complaining about his alternator needing a fistful of revs to cut in. He had an alternator of a type where the field was supplied directly from B+ and returned through a regulator which only barked up when it saw a slight ac signal to indicate rotation. A transistor circuit would switch off the warning light when output was detected. Immediate advice was to keep adding parallel resistors to the warning light which would of course have no effect until ultimately blowing the warning light transistor. This would of course happen instantly if B+ and 51 were bridged for test purposes. Problem was no supply at B+ due to blocking diode, fix was to move warning light -ve to alternator B+. If the forum had charged ahead with received wisdom of the old 9 diode 2nd generation A127 et al then it would have cost this guy his alternator.