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Alternator testing
Shamrock GS - 19/4/08 at 02:59 PM

Is there any way to test if an alternataor is working?

Mine was working fine until last week and suddenly nothing. I have checked the wiring and it seems ok but dont want to replace the unit and find its a problem somewhere else in the circuit.

Thanks

Gary

big_wasa - 19/4/08 at 03:11 PM

what alternator is it ?

britishtrident - 19/4/08 at 03:43 PM

If it has the standard Lucas/Bosch 3 bladed connector block first stop is always the connector block -- the large spade terminals tend to arc (they carry a big current) and fritz the alternator reculator.
Sometimes just cleaning them up with nail board and giving the connector spades a slight squeeze is enough.
After that do a voltage check --- if you don't get more than 13.2 volts across the battery with headlights on and engine at 2000 - 2500 rpm the alternator is on the fritz.

You can check for blown diodes by checking for reverse current through the alternator when the ignition is off -- diodes usually aren't easy to fix.

However 9 times out of 10 it is just the brushes or the regulator pack -- if so for spares try.
http://www.alternatormart.co.uk

daviep - 19/4/08 at 05:46 PM

Very simple to test alternator independently of the vehichle wiring, you need a volt meter, a 12v test lamp and a piece of wire(and some crimps) .

1: Remove all wiring from alternator
2: Check alternator is correctly earthed
3: Connect 12v test lamp between battery positive and the warning light terminal on the alternator

At this stage the test lamp should light if it does proceed to step 4. If it doesn't connect the test lamp between battery +ve and the alternator casing (it should light) to verify the alternator is correctly earthed. If it is correctly earthed then there is a problem with the alternator.

4: Connect a wire (heavyish if possible) between battery +ve and the output (large) terminal of the alternator, nothing should happen

5: Start the engine, the test lamp should go out now to indicate the alternator is charging, if it stays lit again problem with alternator. If it goes out the alternator is working correctly which can be verified by checking battery voltage. In this case there is a problem with the vehicle wiring.

Sorry this is so long winded.

Davie

Shamrock GS - 19/4/08 at 07:09 PM

Fantastic - very clear instructions

Thanks

G

britishtrident - 19/4/08 at 07:42 PM

quote:
Originally posted by daviep
Very simple to test alternator independently of the vehichle wiring, you need a volt meter, a 12v test lamp and a piece of wire(and some crimps) .

1: Remove all wiring from alternator
2: Check alternator is correctly earthed
3: Connect 12v test lamp between battery positive and the warning light terminal on the alternator

At this stage the test lamp should light if it does proceed to step 4. If it doesn't connect the test lamp between battery +ve and the alternator casing (it should light) to verify the alternator is correctly earthed. If it is correctly earthed then there is a problem with the alternator.

4: Connect a wire (heavyish if possible) between battery +ve and the output (large) terminal of the alternator, nothing should happen

5: Start the engine, the test lamp should go out now to indicate the alternator is charging, if it stays lit again problem with alternator. If it goes out the alternator is working correctly which can be verified by checking battery voltage. In this case there is a problem with the vehicle wiring.

Sorry this is so long winded.

Davie


That procedure won't test the alternator --- what it will do is identify a wiring fault -- testing the wiring by substitution.

Also if the warning light goes it out --- it dosen't follow the alternator is charging many alternator faults won't show up this way. Even if the warning light goes out and a reasonable charging voltage is shown across the battery the alternator can still be U/S.

02GF74 - 19/4/08 at 07:57 PM

^^^^ well although it may not guarantee the alt is working, if it faislthe above simple test, then it will show up the alternator as duff.

daviep - 19/4/08 at 10:04 PM

Yes sorry as pointed the correct way to test an altenator is to remove it from the vehicle and strap it to your test bench. Everybody has one of these in their garage don't they???????


quote:

That procedure won't test the alternator --- what it will do is identify a wiring fault -- testing the wiring by substitution


That's true how ever if you know the wiring is good and the battery still isn't charging then you can be reasonably sure the fault lies with alternator.

It's a far moe comprehensive check than sticking a voltmeter on to the battery and then talking crap about diodes which is of no use to anyone.

Davie

[Edited on 20/4/08 by daviep]

:{THC}:YosamiteSam - 21/4/08 at 07:05 AM

quote:
Originally posted by daviep
Yes sorry as pointed the correct way to test an altenator is to remove it from the vehicle and strap it to your test bench. Everybody has one of these in their garage don't they???????


quote:

That procedure won't test the alternator --- what it will do is identify a wiring fault -- testing the wiring by substitution


That's true how ever if you know the wiring is good and the battery still isn't charging then you can be reasonably sure the fault lies with alternator.

It's a far moe comprehensive check than sticking a voltmeter on to the battery and then talking crap about diodes which is of no use to anyone.

Davie

[Edited on 20/4/08 by daviep]


i got my brother to replace the diodes once in my pinto'd dax - they are a common cause of alternator failure - first sign is light stays on

to be honest if you know its the alt that is duff - an exchange one is only around £35 new (well recon) not worth repairing