Odd alternator problem

[Peter]

Hope someone can help me with this alternator problem, symptoms are. When the ignition is turned on the alternator gets hot, without starting the engine. With ignition on/off there resistance to rotation on the alternator, which causes the engine to stop when idling and making it difficult to start. Remove the plug from the alternator this resistance goes and the alternator rotates freely. Everything seems to be wired up correctly, good voltages on the alternator +ve terminal and battery voltage to the exciter (I think that the correct term) when the ignition is turned on, good earth, general engine wiring in good condition, although I have some doubts about the way the engine panel has been wired up, although I don’t think this is the problem. This is a new (reconditioned) alternator only run for approx ½ hr. The first alternator was changed for a similar problem.

Might be an odd one, I have not heard of it before, hopefully some answers please

Peter



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[halcyon]

Sounds a bit like a semi short somewere that is putting a high out load on the alternator, thus giving the heat and resistance to turning.
Could be the engine wiring /panel, as you have had the fault before. Try doing a temperary alternator wireing, i.e. a seperate feed via a bulb to WL, a feed from from the pos to battery pos, and to safe a neg.If this corrects it, it's the existing wireing, if it stays the same get the alternator checked.

Brian

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[pampas]

The only way current can flow through a stationary alterator is if the main diodes are faulty, before proceeding any further, check that the Alt + goes to a positive on the battery, and that the + of the starter does the same. You do not say what make of alt, assume if you disconnect one plug only that its Lucas? double check all the connection that I have mentioned maybe somebody has removed the starter at some time and connected them incorrectly. Please let us know when you find the fault.

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[Alex_Blackwood]

Sounds as though you have one of two things. Either reversed polarity somewhere or faulty diodes.
If you have replaced the alternator because of a similar problem then I think I would look at the reversed polarity.
I would suggest that the heating and resistance to turning are being caused by the alternator windings getting a D.C. feed from the battery and or battery charger.
If it only happens when the ignition is turned on then the polarity of the warning light connection could be reversed allowing the stator to become energised.
If you remove the bulb from the warning lamp and the problem clears then definately look in that area.
I should try and check the connections and polarity without running the alternator as there is a chance that you could damage the regulator if my theory is correct.
If you are not sure then try and get hold of a marine electrician of good reputation!
Sorry cannot be of greater help but these faultfinding exersices can be difficult at long range and out of site. As per previous post let us know the outcome.

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[silverseal]

blocking diodes, blocking diodes. See if you can get a replacement brush and diode pack for the alternator. Alternatively you may have brush problem, causing differential current drain... I would still blame the diodes if pressed

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[William_H]

I saw on an aeroplane once the aux terminal of the alternator had been connected to earth. The replacement alternator had this terminal unlike the original and there was a spare earth wire. When the field was activated the alternator nearly haemoraged with a short on output that actually wrenched the coil section around until the internal wiring broke. This in your case would explain the stalling of the engine. The overheating and stalling may also be caused by a failure of the regulator ie transistor shorted giving full field current sending the alternator into overdrive. just a few thoughts will

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[VicS]

Your alternator problem is indeed a bit of a puzzle.

It is very unlikely that a recon unit would have the same fault as the original but what does seem likely is that whatever was the underlying cause of the original failure has now wrecked the replacement. I would, therefore, get that professionally checked as that may then give a clue as to the basic problem.

Getting the polarity reversed has been suggested as a possible cause. Presumably that means connecting the battery the wrong way round but if it has the traditional round posts that should not be easy to do as they are different diameters, but it is not impossible especially if something has been cobled together in the past. If the battery were connected the wrong way round the diodes in the alternator would be destoyed and if they failed to a "short circuit" rather than an open circuit you would get a large current though the stator coils which would cause the heating you observe but the ignition swich should not be in the circuit between the output terminal and the battery so being on or off would make no difference. Also if the battery were wrongly connected then other electronics on board would not work nor would any fluorescent lights, and you should have noticed the reversed polarity indication on your voltmeter. However, one of the disadvantages of a digital meter is that the reversed polarity indication is easily missed. (Easy to spot the needle on an analogue meter trying to go backwards against the stop though).

You don't say if you have had everything operating correctly in the past or what work as been done on the electrics which may have caused to problem. You say you have some concerns about the engine panel. Also you don't say whether you have any fancy alternator control system or split charging system.

I think I would check out the way everything is connected. Checking that there is no poarity mix up is obviously one thing to do but then look at the way the alternator output is routed (electrically that is) and also the connection of the warning light and just what is being fed via the ignition switch.

The output from the alternator should go directly to the battery via only the main isolator the ammeter and any split charging system. Although in terms of the physical layout of the wiring that may mean via the main connection on the starter motor solenoid. It should not go via the ignition switch!

The ignition switch should feed engine related circuits; the ignition system if we are talking petrol engine or the stop solenoid, and glowplug control relay if we are talking diesel engine, the oil pressure warning light, gauge and buzzer, the temperature gauge and any overheating or coolant flow monitoring systems, the rev counter and finally, in connection with the alternator, the alternator field regulator via the warning light. (And of course when tuned to the 'start' position the supply to the starter solenoid).

In the event of the alternator being battery sensed then a sensing wire will also run to the battery or at least as far as the isolator or the split charging system.

I just wonder if your problems have been cause by disconnecting the battery from the alternator while it was running. Unintentionally, as the result of some shortcoming of the way things are connected up or as the result of a dodgy connection, a faulty isolator, or a dodgy split charging system

Before you start poking around too much please disconnect the battery to avoid the possible consequences of shorting out the whole lot.

You may also find the <A target="_blank" HREF=http://www.reading-college.ac.uk/marine/>Reading college site</A> useful in understanding how things should be.

I hope something here may help

Vic

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[huldah]

Hi Peter

Hi,
I had a very similar problem. Alternator running hot when engine was stopped, and later failing. Alternators require a small trigger voltage to start them. This should be provided via the warning lamp circuit to the diode triod low current output, marked 61,IND,L,D+. In my case this was directly connected to the battery, giving too high a current flow. I have fitted a 4 watt warning lamp, but this reduces the trigger current a bit too much, so that charging starts a bit late. I suspect a resister will be required, fitted in parellel with the indicator bulb.

Excellent details at:
Http://www.vtr.org/maintain/alternator-overview.html

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[VicS]

For your information the waning light bulb in my car is only 1.2 watts. From that I would say that your 4 watt bulb is still allowing far too much current.
Fitting a resistor in parallel with it would increase the current even further, a resistor in series would reduce it but it would also reduce the brilliance of the bulb and fitting a resistor large enough to reduce the current to the 0.1 amp that a 1.2 watt bulb passes would make it useless as a warning light. The simple answer is, therfore, to fit a lower power bulb.

You may however have hit on the cause, or at least part of the cause, of Peter's problems although, since the field current is passes throuh the rotor, it will take a while before the rise in temperature is felt. Also it does not explain the resistance to turning when the "ignition" switch is off.



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[huldah]

I have today fitted a 100 Ohm resister across the ignition lamp, as suggested by the Reading article. This has fixed the problem. It appears that a 4 watt bulb does not pass enough current to trigger the fields. So, unless your car alternator is of a different design, I can only suggest that it may have a separate hidden resister? It's not worth doing, but it would be interesting to measure the field trigger current to see it equates with a 1.2 watt bulb. Thanks for your suggestions,

I think Peter's problems must include an unusually high field current, indicating perhaps a wiring fault or voltage regulator failure. Could be both. In my case the ignition switch was bypassed.

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[VicS]

No resistor as far as I can ascertain but what I had forgotten was that several of the other warning lights that come on initially are in effect connected in parallel, via diodes, with the no charge warning light.

The primary function of the 100 ohm resistor is to give an initial excitation current in the event of the warning light failing but if it's given you just that little bit of extra initial excitation you needed that's a worthwhile bonus.

It strikes me that two or more warning lights in parallel might be a good idea as that would overcome the problem caused by a single bulb failing but maintain the no charge warning function.

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[Alex_Blackwood]

Hi, Just reviseted this thread. I must admit to having to revise my original thoughts a bit, mainly due to the fact that I had misread the original. I thought the fault was only there when the ingition switch was on! I was not looking at a polarity reversal of the whole system, only a fault in the engine control panel causing a possible, unlikely, but possible reversal in the warning light circuit causing current to flow in the stator windings.
I would like to know what the plug is that Peter removes from the Alt. Does it, I assume contain only the auxiliary wiring? It may be a good idea to remove that plug before switching on the battery power and then finding out what wire(s) have power on them.
It would also be interesting to find out when the problem first occured. Was it after the *** was rewired/fitted etc.
I am sure that the fault is external to the alt. although it may have caused damage to the alternator.
I hope Peter lets us know the result. I don't think we shall find it by "distance fault finding" I think it is a systematic search with a meter job.


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