Can an alternator behave like this?

[concentrik]

First, it's an intermittent problem - and it goes like this:

Bosch 12v 55A alternator with 'W' output to drive engine rpm tacho.

Works OK when engine first started but these symptoms can occur as soon as half an hour after starting.

The charge voltage (measured with a Fluke digital meter) initially at the batteries is 13.8v or 14.2v which I imagine depends on the state of charge of the batts at first.

THEN the following symptoms occur very briefly, over maybe 2-3 seconds - the engine note changes as the rpm drops slightly; the rpm tacho drops to zero or almost zero.

THEN normal operation resumes but the charge voltage (again measured as above) is down to 13.5 0r 13.6v. If I load it with nav lights, deck lights etc to take around 20A it drops still further, maybe to 13.4v. I can't load it much more without running the anchor windlass.

Once these symptoms have occurred the charge voltage stays low and drag-downable but the rpm indicator is good and the engine revs are normal.... then it'll happen again - we notice an engine note change and the tacho flutters down but recovers almost right away.

The fanbelt is correctly tensioned and not slipping. The high current leads are good and secure all round. The alt has been tested locally by a Bosch dealer on his test bench- he showed me that it was giving 14.2v but I don't know if it was loaded and it was a short 30 sec test.

What could be happening here? I can't get a replacement alt locally (Spain) for under 250 euros (shiiped in).

Suggestions for further testing or possible causes are welcome! Or a cheaper diesel car type alt to fit?

 

[nigelmercier]

Sounds like you have a smart controller, and the symptoms occur when it changes modes.

 

[concentrik]

Sounds like you have a smart controller, and the symptoms occur when it changes modes.

No, just a plain old Bosch alt.....

 

[VicS]

Dodgy regulator would be my first guess, although not sure I could explain the taco behaviour.

 

[concentrik]

Dodgy regulator would be my first guess, although not sure I could explain the taco behaviour.

I've looked at a few schematics for internal alt wiring and the tacho 'W' signal just seems to come off one of the nodes on the delta winding 'triplet'. I wonder if this would disappear if the field current collapsed?

 

[charles_reed]

Dodgy regulator would be my first guess, although not sure I could explain the taco behaviour.

Suggest windings faulty. Get it out and have it properly bench tested by a competent auto-electrician. Should be done under load and over about a 30' period - worth paying to have it done properly.
Aftermarket replacement between €69 - €150, no need to fly one out from UK.

Suggest a 55 amp alternator is a bit small for Med use. It's probably been knackered by trying to charge close to 50 amps too long. 14.2v is close to the bottom end of desirable output, especially if you're charging through a splitting diode.

 

[lw395]

Measuring the voltage at the batteries, there is a lot of scope for a fraction of a volt drop in cables, earth of the alternator etc.
If it was dead, you would not get over 12.5V when drawing 20A.
The regulator may be a bit sick, but it is basically charging.

 

[charles_reed]

Measuring the voltage at the batteries, there is a lot of scope for a fraction of a volt drop in cables, earth of the alternator etc.
If it was dead, you would not get over 12.5V when drawing 20A.
The regulator may be a bit sick, but it is basically charging.

Trouble is - it's an intermittent fault. It might be working OK when you do the test, but what's it doing when your back is turned?
Hence the suggestion for putting it under load and over an extended period.
I, personally, wouldn't think it's a regulator problem.

 

[halcyon]

What could be happening here? I can't get a replacement alt locally (Spain) for under 250 euros (shiiped in).

Suggestions for further testing or possible causes are welcome! Or a cheaper diesel car type alt to fit?

Have you checked brushes, or for sticking brushes, if it starts playing up try hitting it with a lump of wood.

Brian

 

[concentrik]

Hence the suggestion for putting it under load and over an extended period.
I, personally, wouldn't think it's a regulator problem.

In effect that's what I've been doing I suppose, with it fitted to the boat! A crucial aspect to the problem seems to be the disappearance of the tacho signal - I don't think the regulator/brushes unit could be responsible for this as the tacho signal comes from a winding node directly. The apparent sudden load on the engine seems relevant too. I'm reluctant to force-fit explanations but what if there was an internal short? Could this cause a sudden mechanical load (engine revs drop) and loss of tacho output?

 

[charles_reed]

In effect that's what I've been doing I suppose, with it fitted to the boat! A crucial aspect to the problem seems to be the disappearance of the tacho signal - I don't think the regulator/brushes unit could be responsible for this as the tacho signal comes from a winding node directly. The apparent sudden load on the engine seems relevant too. I'm reluctant to force-fit explanations but what if there was an internal short? Could this cause a sudden mechanical load (engine revs drop) and loss of tacho output?

Why invite guesses, when the only real way is to bench-test it.

When I last had a tacho-problem (with charge) it took about another 200 hrs of running for the whole alternator to burn out and stop charging.

 

[Leonidas]

No, just a plain old Bosch alt.....

I have noted similar behaviour i.e the rpm fluctuates between zero and 700. with engine idling. Then steadies at 700 rpm when a load (e.g the fridge) is applied. The problem also disappears when the solar panel is disconnected. Any ideas what is causing this??? Dodgy regulator?, full batteries causing cut out? Dodgy solar panel regulator?? New 90A alternator was fitted 100 engine hours ago.

 

[halcyon]

In effect that's what I've been doing I suppose, with it fitted to the boat! A crucial aspect to the problem seems to be the disappearance of the tacho signal - I don't think the regulator/brushes unit could be responsible for this as the tacho signal comes from a winding node directly. The apparent sudden load on the engine seems relevant too. I'm reluctant to force-fit explanations but what if there was an internal short? Could this cause a sudden mechanical load (engine revs drop) and loss of tacho output?

The brushes feed the rotor, the rotor produces the magnetic field that produces the current in the stator, that is rectified and feeds the tacho.

No brushes no output.

Brian

 

[concentrik]

The brushes feed the rotor, the rotor produces the magnetic field that produces the current in the stator, that is rectified and feeds the tacho.

No brushes no output.

Brian

Thaks Brian but the brushes are in A1 nick....

 

[William_H]

If you ask a shop to bench test the alternator you would need to tell them that the problem is intermittent. They may still not see the fault.
I would suggest you check the brushes and the condition of the slip rings. Worn or sticking brushes will cuase burning of the slip rings so making the connection more inclined to be intermittent. If his is the case it will get worse. Usually a cover over the brushes often actually the regulator with 2 screws. Remove the cover or regulator and the brushes are often attached. You can see and clean the slip rings and check the brushes. good luck olewill

 

[lw395]

Trouble is - it's an intermittent fault. It might be working OK when you do the test, but what's it doing when your back is turned?
Hence the suggestion for putting it under load and over an extended period.
I, personally, wouldn't think it's a regulator problem.

Once you stop trusting it, it's time to think about replacing it.
A trip to a breakers yard will find you something that fits, gives more charge at low rpm and the right output voltage.
If you know any vehicles where the alternator is a mechanical fit, you may get one cheaper on eBay.