How to check Alternator for function

[MudRunnerTD]

Hi Gents,

After our recent snow trip and winch recovery where i fried my cranking battery (End of life i think) i recharged the battery and got a week out of it before it left the wife stranded. RACV sorted her out. I swapped out the battery for an old new battery (bought new for the GQ but sat in the shed a couple of years and recharged for maintenance a couple of times) and she got 3 days before having a flat battery again.

She called me yesterday as she was nearly home to tell me a number of dash lights and come on (Sub tank, Glow, and a couple of others) while driving. She got home and parked it. When i got home i checked the battery and it was reading 12.1v. I have had it on charge all night and it shovel be good to go.

I think the lights coming on suggest the Alternator is gone? can anyone suggest otherwise? Note the car has 275k on it and this is the first alternator so I've had a good run.

Can someone explain how i go about checking the Alternator for function? Do i just rev the car and see the voltage output on the battery terminals or do i need the check the back of the Alternator? If i need to check the Alternator what and where am i checking and what settings on the multimeter and i looking for please.

Anyone got a good price on an Alternator for a 2005 TD42Ti

Thanks in advance.

[4bye4]

Side of the road quick check; with the engine stopped put a volt meter across the cranker battery terminals and check the reading. If the battery is good you should see 12.4 or so volts. Start the car and at idle read again, the voltage should be 12.4 or better. Rev the engine and watch the meter, you may need help or have very long leads or legs:wink:, the voltage should increase with revs to a max of about 13.5 to 14 volts, then stabilise. If the volts go above 14 volts and keep increasing, stop the car. The regulator is stuffed and it will cook your brand new battery. If the alternator is faulty the battery voltage will just decrease from the static voltage you read first. Hope that makes sense. Incidently, odd combinations of dash lights or all dash warning lights is nearly always an indication of a stuffed alternator.

[PeeBee]

I would say that a good battery condition is 12.68 - 12.72 with a charge rate at idle of circa 13.8 - 14.0, and fast idle of 14.2 - 14.3, with a boost voltage to top up straight after crank of say 14.5- 14.6. Above this, as 4bye4 has stated its getting into damage territory for the battery and the electronics I understand. Further than that I head to the auto elec as he is skilled better than I am to trouble shoot. Hope this fills in a piece of the puzzle.

[MudRunnerTD]

Thanks gents, Just took it off charge and the battery held 13.7v. I started the car and it dropped to 12.4v. Revved the engine for no discernible change at all. It would seem there is Zero signal from the alternator.

[Softy]

On the alternator there will be the "thick" post on the rear of that, that wire is the one that runs to the battery to charge it.. Hook the multimeter up to that with the motor running.. Just be aware of the fan belt etc because they can quickly eat fingers or said multimeter if you not paying attention. lol

[jff45]

It's a good idea to check the output at the alternator post to be sure that the connection between alternator and battery is not broken.

[MudRunnerTD]

Helpful thanks guys, when i revved the engine there was not even a tickle of change, not a flicker on the Multimeter reading to 2 decimal points. I thought that a little odd even for a dead alternator. I will have a good look and see if i can find a broken wire thanks guys. Missus has the car now.

[GQtdauto]

If the alternator really shits itself it can and will drain power , peebee is spot on with his voltages , as for new alternators try either Patrolapart or auto8 .

[Hodge]

It's a good idea to check the output at the alternator post to be sure that the connection between alternator and battery is not broken.

This +1 .
Always check alternator volts at the alternator. I found out the hard way when I had similar issues with my Navara. What had happened, the alternator was charging fine, but the cable between the Alt and battery was so annealed, it was letting very few volts through, giving me a false reading at the battery end.

[MudRunnerTD]

This +1 .
Always check alternator volts at the alternator. I found out the hard way when I had similar issues with my Navara. What had happened, the alternator was charging fine, but the cable between the Alt and battery was so annealed, it was letting very few volts through, giving me a false reading at the battery end.

How did you find that Eric?

[MudRunnerTD]

Just did a check off the back of the Alternator and there is no change from idle damn it. New Alternator it is. Thanks gents.

[Hodge]

How did you find that Eric?
Sounds like you got it pin pointed Darren.
I meggared my lead and that's how I found high resistance . But when I physically touched the lead near the motor it was obvious. Anealed copper cable was greeny almost corroded Color and very brittle.

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[dom14]

Just did a check off the back of the Alternator and there is no change from idle damn it. New Alternator it is. Thanks gents.

Darren, comparing to aftermarket alternators, the original Nissan alternator may be still a better one once you have it professionally rebuilt. Usually the rebuild is about replacing the diodes or the regulator, but the auto sparkies usually replace the bearings & brushes as well, as it doesn't make sense to have an alternator with failing bearings or brushes few months down the track after the rebuild. Aftermarket ones are a hit and miss I think. You can end up with a good aftermarket one or a dud. You get warranty but PITA to deal with malfunctioning ones. Genuine ones are pretty pricey I reckon. I usually prefer rebuilding the old genuine one, either by myself or by an auto sparky.

If you left a new battery in the garage bench for two years and only charged it twice for those two years, I reckon the battery might already have some damage and on it's way. I maintenance charge batteries once a month or so when they are on a bench. I also maintenance charge the one on the vehicle once a quarter or so. My batteries give me many years of life 'cos my maintenance habit. Multi stage chargers or pulse chargers are the best.
My pulse charger & the multi stage charger each cost me less than fifty bucks. There are better brandname chargers out there as well. From my experience, maintenance charging can pretty much double the lifespan of a battery, be in the vehicle or on the bench.

[MudRunnerTD]

Darren, comparing to aftermarket alternators, the original Nissan alternator may be still a better one once you have it professionally rebuilt. Usually the rebuild is about replacing the diodes or the regulator, but the auto sparkies usually replace the bearings & brushes as well, as it doesn't make sense to have an alternator with failing bearings or brushes few months down the track after the rebuild. Aftermarket ones are a hit and miss I think. You can end up with a good aftermarket one or a dud. You get warranty but PITA to deal with malfunctioning ones. Genuine ones are pretty pricey I reckon. I usually prefer rebuilding the old genuine one, either by myself or by an auto sparky.

If you left a new battery in the garage bench for two years and only charged it twice for those two years, I reckon the battery might already have some damage and on it's way. I maintenance charge batteries once a month or so when they are on a bench. I also maintenance charge the one on the vehicle once a quarter or so. My batteries give me many years of life 'cos my maintenance habit. Multi stage chargers or pulse chargers are the best.
My pulse charger & the multi stage charger each cost me less than fifty bucks. There are better brandname chargers out there as well. From my experience, maintenance charging can pretty much double the lifespan of a battery, be in the vehicle or on the bench.


Thanks Dom,

I bought an up spec 125amp one of Patrolapart today for $400. the OEM was around $1000 which was crazy and cheap ebay for $140. I didn't really think about rebuilding given it has 280k on it, i figured it had done a good job.

I am using a Noco Genius G15000 smart charger which is a beast and I'm really happy with it. https://no.co/g15000

yeah i was a little slack with the maintenance of the replacement battery as it sat a while. I have been a bit more active now i have a decent smart charger and been rotating my huge collection of battery across the charger regularly.

The 2 Optima D31A batteries that i had under the bonnet of the GUIV lasted 7 years which is awesome and the accessories battery died 2 months ago. I should have replaced the drivers while i was there but thought it was ok, as soon as i put winch load on it up at the snow it crashed to zero so it was due. I wonder if that crash caused damage to the alternator too as it happened basically straight away?

I have the charger topping up the battery every other day until the alternator arrives (this week i hope) then we are good to go.

[dom14]

Thanks Dom,

I bought an up spec 125amp one of Patrolapart today for $400. the OEM was around $1000 which was crazy and cheap ebay for $140. I didn't really think about rebuilding given it has 280k on it, i figured it had done a good job.

I am using a Noco Genius G15000 smart charger which is a beast and I'm really happy with it. https://no.co/g15000

yeah i was a little slack with the maintenance of the replacement battery as it sat a while. I have been a bit more active now i have a decent smart charger and been rotating my huge collection of battery across the charger regularly.

The 2 Optima D31A batteries that i had under the bonnet of the GUIV lasted 7 years which is awesome and the accessories battery died 2 months ago. I should have replaced the drivers while i was there but thought it was ok, as soon as i put winch load on it up at the snow it crashed to zero so it was due. I wonder if that crash caused damage to the alternator too as it happened basically straight away?

I have the charger topping up the battery every other day until the alternator arrives (this week i hope) then we are good to go.


Yeah, for $400, you can get a decent one for sure. Ebay ones can be nothing but trouble unless you're lucky. Rebuild is usually about $200. I had mine rebuilt by local sparky for $160 many years ago. It's been really good ever since, except I recently upgraded the alternator to a 115amp one. Mine already had 300k on it when I rebuilt it, so I reckon your old one is still good for a rebuild if you want to.

It makes sense the alternator dying after battery dying from the winch draw. The winch would've tried to draw the lacking amps from the alternator and the old tired regulator might have cooked as a result. I've been told, for winches, more CCAs you have the better, so the optima spiral wound core batteries are the best I reckon for their high CCAs.

You can leave the smart charger connected to a bench battery indefinitely. The float amps it would draw in should be pretty low and I doubt it would affect the electricity bill to something you would notice(unless of course the battery is already stuffed :) ). Ideally, the smart charger should cut off the amps to nothing when the battery is 100%. I don't leave my batteries connected to the smart charger all the time 'cos I tend to do it manually.

7 years is a pretty good lifespan. No argument there.