Jump to content
nige8021

Testing your Alternator

Recommended Posts

nige8021

This is a guide to testing your alternator I found in Practical Classics.

1) Connect a multimeter across the battery terminals, it should read at least 12volts with the engine not running, Start the engine and it should then read 14~15volts, Switch on the headlights full beam, Heated rear window & heater fan on full speed, the voltage should still read 14~15volts

2) Test the Rectifier/Regulator, Put your multimeter to the 20 volt AC range, and with the engine running put the probes on the battery terminals, If the rectifier is faulty there will be an AC voltage shown on the meter, you shouldn't see more than 0.1~0.5 volts any more than this and the rectifier/regulator is faulty.

3) Test the diodes, Ignition OFF, disconnect the battery NEG terminal & output lead/plug from alternator, set the multimeter to DC amps and connect it between the output terminal of the alternator and the wire/plug removed before, reconnect the battery NEG terminal if the meter reads more than 3mA the rectifier diodes are "leaking" causing a drain on the battery and over time with no use the battery will go flat!

4) If the CHARGE light is ON and dims as the engine is revved the alternator is suspect, If the light brightens as its revved the battery is suspect

  • Like 2

Share this post


Link to post
Share on other sites
apertures

Not trying to put you down as this is a very useful guide.

But for anyone whom doesnt have a multimeter and would like their charging system fully checked, pop down to any local halfords store and ask them to test your battery, this is a FREE service and tests the Battery, Alternator and Cranking time on the battery, useful to anyone looking to just check their car, There is no obligation to buy any batteries from them and they will give you a print out of the data free of charge.

As i said, I'm not trying to put your thread down but not everyone has a multimeter or doesnt want to put things across the alternator wires.

(If you want me to shoo this post, just say.)

Share this post


Link to post
Share on other sites
kbs

A good guide. I'd be happy with a voltage lower than 14v though - 13.5 or above will do me.

Having a free check done locally can be a useful option but only if you get printed results that you can post back here. Many 'mechanics' are simply incapable of interperating the results correctly. The kind of reject burger flipper (that may be employed by some of the big names) can mean well but inadvertantly feed you a concentrated diet of horse manure.

I had to replace a battery about a year ago.

My local battery supplier wanted £40 with a 3 year warranty.

Halfords wanted £85 with a 3 year warranty

The RAC wanted £120 (but would deliver and fit it for 'free').

When looking at what the 3 year warranty meant (if my battery failed in the third year);

my local battery supplier would give me a new battery - no expense at all.

Halfords and the RAC would only SELL me a new battery at a small discount (still more expensive than binning their overpriced unit and buying a new one from an honest supplier).

This means Halfords and the RAC would profit if my battery failed in the third year (by selling me another at a 'discount')

Only my local (honest) battery supplier would actually benefit from selling a battery that would actually last three years!!

So Halfords and high street suppliers may be convienient but if you MUST use them, be prepared to bend over and grease up.

  • Like 1
  • Sad 1

Share this post


Link to post
Share on other sites
apertures

Who told you that if the battery failed in Halfords they wouldn't replace it within warranty?

This is not true.

As long as you have your receipt or battery warranty slip or even a bank statement we will exchange the battery if our equipment determines the battery is faulty.

If we fitted the battery at the original time (will be on the receipt) the store should be more than willing to exchange and refit the new battery.

Share this post


Link to post
Share on other sites
kwijibo_coupe

I bought a battery 6 months ago from Halfords, if you have access to a Trade Card they are just as cheap as any other supplier.

Anyway last week the battery gave up the ghost, wouldn't hold a charge at all.

Returned it to Halfords and within 5 minutes I walked out with a new replacement, so I don't see why this wouldn't happen anytime within the 3 years. Nothing on my receipt states otherwise anyway.

The only downside is the warranty is only ever valid from the date of purchase of the ORIGINAL battery. So after 2 years and 364days you could get a replacement battery but it would have no warranty at all.

  • Like 1

Share this post


Link to post
Share on other sites
kbs

Who told you that if the battery failed in Halfords they wouldn't replace it within warranty?

This is not true.

As long as you have your receipt or battery warranty slip or even a bank statement we will exchange the battery if our equipment determines the battery is faulty.

If we fitted the battery at the original time (will be on the receipt) the store should be more than willing to exchange and refit the new battery.

My appologies,

Last time I spoke to halfords, the deal was;

Failire within 12 months - free replacement

Failiure after 12 month but before 24 months (meduim) discount if you buy a new battery - still cheaper to buy from a battery supplier

Failiure after 24 month but before 36 months (small) discount if you buy a new battery - far cheaper to buy from a battery supplier

So are Halfords now offering a true 36 month warranty on new batteries? A step in the right direction - good news.

Last new one I bought was an 013 - halfords wanted around twice the price of a battery supplier - still better than the RAC though.

Share this post


Link to post
Share on other sites
apertures

I work in Halfords and in my store and across the company policy. If we offer a 3/4/5 year guarantee. If it fails within that time due to a manufacturer defect. We will exchange it. Even 1 day before the 3/4/5 year guarantee.

Share this post


Link to post
Share on other sites
funkster

This is a guide to testing your alternator I found in Practical Classics.

1) Connect a multimeter across the battery terminals, it should read at least 12volts with the engine not running, Start the engine and it should then read 14~15volts, Switch on the headlights full beam, Heated rear window & heater fan on full speed, the voltage should still read 14~15volts

2) Test the Rectifier/Regulator, Put your multimeter to the 20 volt AC range, and with the engine running put the probes on the battery terminals, If the rectifier is faulty there will be an AC voltage shown on the meter, you shouldn't see more than 0.1~0.5 volts any more than this and the rectifier/regulator is faulty.

3) Test the diodes, Ignition OFF, disconnect the battery NEG terminal & output lead/plug from alternator, set the multimeter to DC amps and connect it between the output terminal of the alternator and the wire/plug removed before, reconnect the battery NEG terminal if the meter reads more than 3mA the rectifier diodes are "leaking" causing a drain on the battery and over time with no use the battery will go flat!

4) If the CHARGE light is ON and dims as the engine is revved the alternator is suspect, If the light brightens as its revved the battery is suspect

Gave my belt a little squirt of wd40 the other day and now the charge light comes on at idle but dims when revs pick up.

Tried to tighten the belt but it was awkward to get enough leverage on the alt to get it tight enough (5mm according to haynes) Polished belt?

Thanks

Edited by funkster

Share this post


Link to post
Share on other sites
nige8021

Putting any oil on the belt is the last thing you would do !! it will just make the belt slip as the electrical load increases

  • Like 1

Share this post


Link to post
Share on other sites
funkster

indeed, don't I feel silly now... I know where I read about it now too, twas cam belts not drive belts........mup

Share this post


Link to post
Share on other sites
steveo3002

never spray wd40 on a cam belt either

Share this post


Link to post
Share on other sites
apertures

Same as putting WD-40 on tyres to blacken them, it is no good. It causes the rubber to dry out and crack prematurely.

Share this post


Link to post
Share on other sites
funkster

Wow, even I'm not dumb enough to put it on tyres.....lol

Thanks guys

Share this post


Link to post
Share on other sites
Rocky6n2

Haha I heard if you pour some coca cola on it that it helps a slipping belt by making it sticky! Obviously a temporary fix tho!

Share this post


Link to post
Share on other sites
MikeyMK

No disrespect to my local Halfords, but i doubt the average 16yo even knows how to use a multimeter.

Share this post


Link to post
Share on other sites
nige8021

No disrespect to my local Halfords, but i doubt the average 16yo even knows how to use a multimeter.

I watched one of their "technicians" trying to fit a battery to an original Mini, he had the bonnet open for 10 mins scratching his head !!

Share this post


Link to post
Share on other sites
kwijibo_coupe

No disrespect to my local Halfords, but i doubt the average 16yo even knows how to use a multimeter.

I watched one of their "technicians" trying to fit a battery to an original Mini, he had the bonnet open for 10 mins scratching his head !!

I've gone in asking for a price on brake discs and pads and was greeted with just a blank look. I had to explain what they were!

And to think I applied for a job there when I was 16....never even got an interview!

  • Like 1

Share this post


Link to post
Share on other sites
wizardmanjm

I was turned down for an interview to an apprenticeship in a motorbike repair shop...he asked me "why are you wearing a shirt, trousers and smart shoes?" I said "erm...it's an interview". Then he replied with "well you won't be wearing those types of clothes in a bloody garage"

Yeah because when I worked in an mot and service garage and when I was a tire fitter I done it wearing a full blown tux.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. Terms of Use Privacy Policy Guidelines