Alternator Testing - Idiots' Guide Please

[blackfish]

Alternator Testing - Idiots\' Guide Please

Can anyone help this particular idiot please? I would like to test the output of my alternator with my multimeter. The alternator is a Lucas Type A127 and the business end is as in the picture attached (but not quite so shiny!). What range should the multimeter be set at and where do I put the prongs (polite answers only please)?

 

[blackfish]

Re: Alternator Testing - Idiots\' Guide Please

I'm having trouble uploading image but you can see it here. alternator picture

 

[ShipsWoofy]

Re: Alternator Testing - Idiots\' Guide Please

Are you looking to see if it is working, or working properly... two slightly different things.

The first is simple, put your multimeter onto DC volts 20v range, put the red and black onto the corresponding + - battery terminals and take a reading. With the engine running and the alternator charging you should see something like 14.4 volts. Without the engine running and a good battery you should have 12.7 ish volts.

When starting if you are measuring and the battery voltage drops below say 10v then the battery is probably shot, I know you did not ask that, but as you are concerned about charging it is worth noting your batteries do play a part.

While measuring the alternator voltage it is important to give the engine a bit of a kick, at tick over it may not be charging at all, mine don't really start up until around 1100 rpm, many will be different. The idea of this is so that the engine does not stall when the alternator is charging at low revs, an alternator puts a substantial load onto the engine... A post a short while back discussed this and showed how large an alternator would swamp my little engines, I wish I could find it now.

The other tests is to work out how much current your alternator is actually putting into your battery, although if the first test is working then the second is only a tweak test, i.e. if you are getting 14.4v then you will in 99.9% of times be getting full charge, if not it might be looking for why your alternator is only kicking out 52A instead of the stated 55A if you see what I mean. But if you wish to measure the charge current then I would suggest one of the meters from halfords which strap around the cable, rather than breaking the circuit.

FWIW, you may like to read some alternator workshop pages I scannned, although a different model, it will show you how you can run some simple bench tests if the unit is failing. There are two chapters below, well worth a read.

yanmar_35A_Alternator.PDF

yanmar_55A_Alternator.PDF

 

[VicS]

Re: Alternator Testing - Idiots\' Guide Please

Dogwatch says you should see 14.4 volts but unless you have a fancy alternator regulator that is about the highest you should see. I say expect around 14 to 14.2 but the lowest you should see with the engine revving is about 13.8.

Although it is possible to service an alternator yourself some are easier than others. I have done it but really the best bet, unless you are really knowing what you are up to and handy with asoldering iron, is to take it, if you suspect any problem, to an automobile electrician who can test it thoroughly and replace defective components.

As it's a Lucas a Lucas service depot is the obvious place to go.

 

[Bodach na mara]

Re: Alternator Testing - Idiots\' Guide Please

VicS suggests that Lucas depot may be the best, but did they not go belly-up last week? If not, I could have saved myself a long trek to get oil and fuel filters as Lucas were the last place that I knew that carried the ones I needed.

(By the way if anyone else in the Glasgow area is wondering where to get obsolete filters, try Andrew Watt in Orr St Bridgeston, or Rattray Motor Spares off Paisley Road West near the toll.)

 

[pande]

Re: Alternator Testing - Idiots\' Guide Please

Lucas were bought and a week later put in to receivership by their new owners. The locks were changed in every branch one sunday leaving lots of people turning up for work on the monday morning to find that they had no jobs. /forums/images/graemlins/frown.gif

 

[VicS]

Re: Alternator Testing - Idiots\' Guide Please

I'd forgotten about that! Actually it is LSUK (& all those local service depots) that has gone. Lucas Electrical is unaffected.

 

[oldharry]

Re: Alternator Testing - Idiots\' Guide Please

It sounds as though you were planning to run the engine up with the alternator disconnected, and try to get a reading off the terminals. DONT! 1: you risk blowing the diodes, then it will definitely need expensive repair, 2: Unless its a self exciting alternator, there will be no power to start it generating, and 3: if you didnt damage the diodes the voltage regulator would detect a 'no load' situation, and the reading would be meaningless anyway.

Either take it to any Auto Electric repair shop who will test it for you - usually free - on a properly set up test rig, or as suggested in other posts, check the battery voltage is between 13.8 and 14.4 volts on charge. A further test is to connect a 60w car headlamp across the fully charged battery for about 10 minutes with the engine off, then run the engine up. Voltage at the battery terminals (with the bulb disconnected) should return to around 14V within about 5 minutes if the alt and battery are good.

 

[VicS]

Re: Alternator Testing - Idiots\' Guide Please

[ QUOTE ]
up with the alternator disconnected, and try to get a reading off the terminals. DONT! 1: you risk blowing the diodes,

[/ QUOTE ] Sorry thats not quite right. You can safely run an alternator up with it's output disconnected BUT you will more than likely blow the diodes if you actually disconnect it while it is running. That can create a surge in the voltage which is what does the damage.

 

[matelot]

Re: Alternator Testing - Idiots\' Guide Please

to add further to your confusion, beware reading too much into amp output figures. All batteries have an internal resistance, there is resistance also in the wiring, and as the battery charges upo it developes ever higher back emf to resist current flow even more. So a 50 amp alternator charging at 10 amps might be nothing at all to do with the alternator and might well not even be a fault. In fact its likely not to be a fault. Could simply be a full battery.

Voltages to read off your output terminals are somewhere around 14.4 when charging though my Volvo goes up to 15 quite correctly according to volvo. More than 15 or so means alternator problems which will quickly lead to battery problems. Once charged the alternator should go to something around 13.5 v as a float voltage (ie just maintaining the battery fully charged despite internal losses) and when the engine is off the battery should eventually mdecline down to 12.7 or so. Takes some time.

As for discharging - 12.5 still alive, 12.2 much to do.

 

[oldharry]

Re: Alternator Testing - Idiots\' Guide Please

Exactly, VicS: yes you can run it off load, but poking around terminals of an alt running off load with a testmeter - which was what Blackfish seemed to be suggesting - could easily cause enough of a spike to blow the diodes.

 

[VicS]

Re: Alternator Testing - Idiots\' Guide Please

Sorry I did not put that interpretation on Blackfish's post and still don't for that matter. I don't think using a high impedance voltmeter will casue any problems. I could not quantify it but it'll will only be when disconnecting a significant load that you will get a voltage surge large enough to do any damage.

Really need to check the volts output over a range of loads. Partial failure of the rectifier could lead to a normal volts reading at low loads but a low reading at high loads.

 

[Refueler]

Re: Alternator Testing - Idiots\' Guide Please

Simple way .... forget all the bumf about multi-meters and figures. Originally being the cheapskate I am - I bought a LED Battery / Alternator tester from Halfords. Made by Draper and £9.99.
All you do is clip negative croc to -ve battery terminal and press +ve probe to alternator output or +ve battery terminal. With no engine running - you get the lower 3 LED's light corresponding to battery charge level. With engine running and about 1500 - 2000rpm - you get those + the upper 3 LED's light up corresponding to alternator output.
Reason for checking at +ve out of alternator and then at +ve terminal of battery - tells you if any losses via cable / connections to battery.

Purists / Experts will of course poo-poo the whole idea, but I can say straight up - that £9.99 item instantly diagnosed my alternator regulator fault. £20 later alternator was back on boat and charging like a good-un. In fact the charging was so good that I certainly did not need to up the power level of alternator. Sort of advice I had prior was :

a) Multi-meters and all sorts of probing / science.
b) Advised to change and increase alternator power from standard size.
c) Get electrics guy on board ....

just as examples. Lot more expensive and hard work than Drapers magic tester ! (You can get cheaper online ... I've seen them at less than a fiver .....)

 

[LadyInBed]

Re: Alternator Testing - Idiots\' Guide Please

Poo-Poo Aldi £3.99

 

[VicS]

Re: Alternator Testing - Idiots\' Guide Please

[ QUOTE ]
Purists / Experts will of course poo-poo the whole idea

[/ QUOTE ] Absolutely. Whats the point of a device like that when for about £7 from Maplin you can get the precision and greater capabilities of a digital multimeter. /forums/images/graemlins/grin.gif

Sadly I don't think the Draper device is now available and I cant say I have seen anything similar recently in Halfords (I would probably buy one if I did). I do in fact have an even simpler and EVEN CHEAPER gadget that plugs into the cigar lighter socket. It is good enough to tell me what I already knew, namely that my car alternator is not working properly but I knew that because one of my "special offer two for £5 Maplin multimeters" told me so.

There used to be an LED based monitor called VoltWatch but that is no longer available. By and large boat owners have shed loads of cash and are looking for fancy hi-tech gear to fritter it away on consequently the low cost gear disappears and fancy all singing all dancing stuff like battery monitors costing several £hundred proliferates. The cheapskates who prefer to keep the shed doors firmly locked are but a small minority.

Still available as far as I know is the Axon LED battery monitor though SEE HERE but at a price of £40 that will cause serious medical problems for the cheapskates.

 

[Refueler]

Re: Alternator Testing - Idiots\' Guide Please

Draper guy I know reckoned not so long ago it was still available. But also why I mentioned that cheaper are available for less than a fiver.

It's quick, easy, effective enough for most people who just want to know if alternator is working or not. Mine showed me just by dabbing on battery while engine running that first regulator was on it's way out and charge was dimly lighting 3rd LED, little later when diodes went in it - it showed it immediately. After auto-lecky and £20 repair to regulator - it then showed excellent charging.

I totally agree that even a cheapo M-meter in good hands can be one of the most versatile tools out ... I wouldn't be without one - but wouldn't also be without my LED tester !

For less than the £40 odd quid quoted item - you can get a battery load tester !

Me ? Well known cheapskate who hunts out bargains and alternative ways to do it ...

 

[lw395]

Re: Alternator Testing - Idiots\' Guide Please

I don't think it's the diodes you have to worry most about when testing off load, I would be more concerned about the regulator, although these things are pretty robust.
Be aware that the charge warning light needs to be present and working for most alternators to start. If it is dim or not working, check the brushes for wear or sticking in their housings.
Check the output at medium revs stays at 14 to 14.4 volts with a reasonable load on, e.g. the headlights if its a car, I have a little 10A/12V fan heater which is a useful test load!
Those led testers sound like a good idea, I like the idea of no switches to corrode like on a multimeter. The Maplins 'yellometer' is worth having though. I also find a moving coil meter that comes on when the engine is running can be useful.
Good luck.

 

[DinghyMan]

Re: Alternator Testing - Idiots\' Guide Please

These are cheap and cheerful and easy to use. Similar idea with the leds for level and charging