Alternator or battery?

MiskyBoyy

Lads, battery light came on in my VW Passat B6 1.9 TDI

I tested the battery with multimeter and it's just about on 12v whereas it should be 12.6 according to Google.

I then tested it with engine running and it was lower again about 11.8v or so.

Would this definitely be the alternator or is it a possibility a new battery is all that's needed?

Sam Quentin

I'd guess the battery not taking a charge, have you a car battery charger? I would connect it up just to see if it's charging the battery anyway, and if it is then it's your alternator!? it would be unusual(I stand to be corrected)for the alternator to die/stop working just like that, unless you were already having intermittent issues...

JohnBoy26

It a charging problem, most likely the alternator and they can fail suddenly.

antodeco

When did this happen? Very cold weather could have tipped the battery over the edge

JohnBoy26

It could but a bad battery won't cause the battery light to illuminate.

MiskyBoyy

I must add, the battery icon only comes on when I rev it to 2k revs and even then it tends to flash more than stay illuminated.

Also, would the car not die if I left it on long enough if it was the alternator?

I had it running earlier for 15mins and it was fine. The trouble is getting it started, it took a bit more effort than usual.

Before this, it was always 1 tip start. No problems leading up to this.

Mech1

Its the alternator, but a bad alternator can murder a good battery. you may end up needing both.

toyotafan

Sounds alternator to me. 11.8v is too low. If alternator, I would suggest you to purchase a new one rather than rebuilt, as you could get the same problem with rebuit one after months. I've recently purchased a Denso one with a great price from a supplier in the UK, as I rebuit one @ €100 and it stopped working after 11 months. Got new one around €150.

antodeco

Have the engine running and disconnect the negative terminal. If it does, dead alternator. If it doesn't, dead battery.

toyotafan

antodeco wrote: »

Have the engine running and disconnect the negative terminal. If it does, dead alternator. If it doesn't, dead battery.

Well... if you do like this, your ECU might be gone.

Mech1

antodeco wrote: »

Have the engine running and disconnect the negative terminal. If it does, dead alternator. If it doesn't, dead battery.

Do not try this, terrible advice.

dooroy

Would agree that it's the alternator that's failed/failing - and do not disconnect alternator with engine running or you can cause much bigger problems.
If the alternator was charging as it should you should see 13.5 - 14 volts with engine running.

jca

antodeco wrote: »

Have the engine running and disconnect the negative terminal. If it does, dead alternator. If it doesn't, dead battery.

Do not do this. Your alternator has failed.

antodeco

Sorry lads. I've been told that so many times from different people I just assumed it was he norm!

Steel rain

Sounds like the alternator to me too,and if possible I would go for a new off the shelf alternator over a reconditioned unit just for peace of mind.

MiskyBoyy

Any suggestions on where to get one at the best price? Any which amps per hour one do I go for?

biko

I suggest you call in to a local mechanic to get a second opinion and a quote from them before you go buying stuff.

toyotafan

MiskyBoyy wrote: »

Any suggestions on where to get one at the best price? Any which amps per hour one do I go for?

I've recently purchased one at the OpieOils in the UK with a Denso brandnew alternator after online shopping around. They offer a great discount with best price at the moment so you could look at this as an option.

From my pain experience, highly recommend to purchase a new one, and don't use a rebuilt.

martyc5674

Go into a motor factors who will test your battery for free.
The info you have to hand isn’t enough to conclusively say it’s one or the other.

MiskyBoyy

Cheers for the replies so far. I'm of course going to bring it to my local garage as soon as I have a chance. Very busy this week.

I had a quick look at it last night, had disconnected the battery the previous night.

Reconnected battery, jump started it. Drove for 15 mins. No sign of any warning or battery lights.

...but multimeter is still giving a low reading whilst engine is on or off.

Still assuming it's alternator?

jca

MiskyBoyy wrote: »

Cheers for the replies so far. I'm of course going to bring it to my local garage as soon as I have a chance. Very busy this week.

I had a quick look at it last night, had disconnected the battery the previous night.

Reconnected battery, jump started it. Drove for 15 mins. No sign of any warning or battery lights.

...but multimeter is still giving a low reading whilst engine is on or off.

Still assuming it's alternator?

If the voltage isn’t rising with the engine running it’s a charging issue. Get the car checked over first as it could be a broken or burned connection either. Never jump to conclusions with electrical issues.

martyc5674

jca wrote: »

If the voltage isn’t rising with the engine running it’s a charging issue. Get the car checked over first as it could be a broken or burned connection either. Never jump to conclusions with electrical issues.

Your more than likely correct. However a bad battery can draw so much current from the alternator it can pull the voltage down. That’s why it’s not clear cut in this situation.

JohnBoy26

martyc5674 wrote: »

Your more than likely correct. However a bad battery can draw so much current from the alternator it can pull the voltage down. That’s why it’s not clear cut in this situation.

If the voltage isn't rising at all with the car running it's a charging issue. It sould increase even with a bad battery.

martyc5674

JohnBoy26 wrote: »

If the voltage isn't rising at all with the car running it's a charging issue. It sould increase even with a bad battery.

No not always, most likely but definitely not always.

jca

martyc5674 wrote: »

Your more than likely correct. However a bad battery can draw so much current from the alternator it can pull the voltage down. That’s why it’s not clear cut in this situation.

Maybe, but if the battery was that bad it would never start a diesel engine, I agree though that logic is the only route to follow with what might look to be a straightforward problem.

MiskyBoyy

jca wrote:

If the voltage isn’t rising with the engine running it’s a charging issue. Get the car checked over first as it could be a broken or burned connection either. Never jump to conclusions with electrical issues.

Alright, problem solved.

Turned out to be neither the alternator OR battery but a broken connection alright, on the starter motor!

The negative/black had broken but was still tipping off it, so that might explain why I found it harder to start and why the battery symbol light was flashing on and off instead of staying on.