How do I test my alternator ?

[eebygum]

Some dumb electrical questions here......

When coming back last week noticed #1 battery (startor) was showing low battery on the echo/log which was odd as it had been on recharge and we were under power....switched to #2. When got in found #1 almost dead; jumped to the conclusion that the Lucas alternator on the Yanmar 1GM had packed up and put in a replacement and trickle charged the #1 battery back home which I got back to 12.3V....Putr battery back in, and started engine with new alternator fitted..read that I should be getting 14.5V across the batteries when charging....did on one battery; but not the other then the first one stopped showing 14.5V (bit vague as my partner did this testing)..

Question
a)\How do I test my alternator is working OK; can I measure directly off the alternator rather than across the battery terminals and how do I do this ?
b) If I have a short, what is the best way to track this down ?
c) Any other suggestions ?


All help gratefully received
...thanks Andrew

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Age is a question of mind over matter. If you don't mind, it doesn't matter.

 

[LadyInBed]

If you are getting 14.5v on one battery and low on the other, you shouldn’t let it drop below 12.4v, then I would say you have a stuffed battery. Take the battery home and stick it on a decent, three stage, mains charger and get a hydrometer.

See the chart on this site www.solent.net/~hamwic/notes/battery charge.htm

<A target="_blank" HREF=http://www.vtr.org/maintain/alternator-overview.html> and here for Alternator info </A>

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[cliff]

Unless you have some way of providing a current drain equal or greater then the max rated output of the alternator it is not possible to fully test the alternator. Voltage readings alone are not sufficient. There are some basic tests thought that you can perform.

Before starting engine check battery voltage (~12.5v)
On the back of the alternator you will se a number of cables, you are looking for the heaviest (thickest) cable which is the charging cable which is where you need to connect a voltmeter (+) the other lead from the volt meter going to ground.

Start the engine and allow to idle - ing warning light should be on or glowing,Check voltage (<11~12v).

Slowly increase engine revs until ign warning light goes out, check voltage (13.5~14.8v)

If you have a windlass hooked up to main battery you may be able to do a load test.

For that you want to be able to switch on as many loads as possible at the one time upto or exceeding the alternator max rated output.

Basically, with the engine running you switch on lights, heater, fans, inverter (with load) and or the windlass depending on what gear you have or can connect to the battery being charged. once all is on then measure the output amps (clamp on ammeter otherwise known as a tong tester) and check amperes output.

Ideally, if you can get your hands on a "Wilkinson King tester" or similiar the job will be quite straight forward.

Failing the above remove the alternator and take it back to "Lucas" for test along with the battery for a discharge test.

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[VicS]

My earlier reply seems to have disappeared into the ether.

Fistly charge the batteries with a decent charger until the volts have exceeded 14.4 for at least a couple of hours. Check the volts after they have been standing idle for 24 hours. If they are in good nick and "fully" charged you should get a reading of about 12.7. 12.3 is far to low, infact only about 1/2 charged. If you have the time you should find that it will take about 3weeks for the volts to drop by 0.1. In fact it is possible to cautiously allow the charging voltage to rise even higher perhaps even to 16v in which case the voltage after 24 hours standing should be well over 12.8 and still be between 12.7 and 12.8 after 10 days.

Next put one battery in sevice start the engine and monitor the voltage with the engine at normal running revs. The battery volts should quickly rise and steady out at 13.8 to 14.4, depending on alternator spec, in 5 to 15 minutes depending on how readily the engine started. (I think the high figure of 14.8 Cliff suggests is too high)

If you want to check the volts at the alternator then do so between the heaviest connection and the body as Cliff sugests. It would then be interesting to make sure the volts at the battery is not much less. I.M.O is is safer to just check at the battery which is presumably well away from drive belts and rotating pulleys.

Cliff has suggested ways of checking the current output but if you have any doubts about the alternator, or the batteries, then go to good automobile electrician.

Nigel B suggests getting a hydrometer to check the batteries but most of those you can buy are pretty much rubbish and if you get duff readings it will only confirm what you already know from voltage readings. It really isn't very helpful to know which cell out of six is the dud one.

If you think the batteries are discharging when installed then check for current flow with everything switched off by connecting an ammeter in series with the battery +ve lead. Do it with the ammeter on its highest current range.(10 or 20amps) ( I have just blown the fuse in my multimeter by trying just such a check on the car with the meter on the 200mA range) you will expect to find a small flow backwards through the alternator and to any thing that has an "always on" supply to maintain a memory like a car type radio. If you find a current tracking it down is just a tedious process of elimination I'm afraid.

As you have replaced the alternator I would start by assuming that that is not the problem but check the connections, not forgetting the earth. If there isn't an earth wire and you are relying on the mounting bracket make sure the relevant faces are clean and free of rust. It is not impossible for a battery to suddenly give up but your initial observation of low volts while under power does rather point the finger of suspicion in the direction of the alternator.

You have not told us what the switching arrangements are between the batteries or whether you have any split charging systems.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[cliff]

"(I think the high figure of 14.8 Cliff suggests is too high)"

Well spotted young man, should have read - 13.8 ~ 14.4. Twas too, too early in the morning for me - hadn't had my caffeine fix yet.

A good reference can be found <A target="_blank" HREF=http://www.boten.nl/motoren/motoren_pdf/alternatorhandbook.pdf> <<HERE>> </A> although heavy reading in the beginning for the non techie minded, further on there is a good explanation of testing methods. Give it time to load as the file is BIG. The link does work but loading might be slow, have patience. (needs Adobe reader)

Off for my caffeine fix before I drop any more! /forums/images/icons/wink.gif

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[William_H]

The key question is about the switching arrangements or set up to charge 2 batteries. An alternator needs to have a battery with some charge connected to it or specifically to the line to the regulator via the charge light. It needs some outside power to feed the field coil before it can start to charge. Usually once it has started it is able to provide its own power for the field coil.So it is conceivable that your new alternator worked ok on the good battery but if switched to a bad battery it may not charge.I don't know how likely this is but certainly in a one battery set up where you hand start a diesel you may not get any charge. It is doubtful the replacement alternator is bad. As suggested more likely a battery or wiring corroded or disconnected. good luck will

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[silverseal]

I have looked at the replies, and they have all forgotten one thing. Blocking diodes!
Alternators have to be excited by an existing current, whilst dynamos dont. Hence one or more blocking diodes are fitted to stop the back flow of current from the charged batteries. I had a Ford Granada, which gave perfect charge from its alternator and then would suddenly have a flat battery. Result duff blocking diode.
Best to stop the engine, and connect a meter in series with the "ignition switch", when it is turned off. There should be a nil reading. Make absolutely sure that everything else is switched off, and then disconnect the positive cable to the battery, and then connect the meter between the battery post and the cable. No current should flow... if it is, you have a blocking diode problem.
Most "automotive electrical distributors", sell replacement "brush kits" and often include a diode pack. Take the alternator off the boat adn take it to them. They will be able to test it but do not let them flog you a service exchange unit for megabucks, when you can get the brush kit for a few quid. In my Granada case an exchange alternator was £180, and the brush kit £4.50 plus vat! It took several minutes to replace the unit, by undoing a couple of set screws

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