Mk7 Charging Issues

[geckomike]

Hi All,

I am currently stuck with trying to figure out electrical gremlins and why it isn't charging. Tried a different battery, replaced the alternator, all the fuses seem to be intact and I've moved relays around to see if anything changes (it doesn't, which to me suggests the relays aren't the issue).

Batteries hold charge at between 12.5 & 12.7 volts, when idling I get around 12.2 volts on the batteries, and I'm reading 0.5 volts directly off the back of the alternator. I've visually inspected the loom as best I can, and can't see anything wrong there (although I haven't taken the plastic cover off so there could be an issue inside, but can' think why there would be)

Can someone please tell me that I'm missing something easy and obvious!

[Altransit]

If it's a Euro 4, the battery should be charging at 14.4v pretty much at all times. If it's not, the fault could be with the new alternator (not unknown for them to be faulty) or a broken or misplaced wire in the charging circuit, maybe at the alternator end?
Does the charging light on the dash come on with ignition, and go out when the engine starts

[geckomike]

It is a euro 4 (2.4tdci) rwd.

I feel like it has to be a wiring issue as the old alternator and the new alternator put out basically the same volts at idle (12.2 on the batts and 0.5 on the back on the alternator).

I read on here somewhere that if you pull the smart charge plug off the back of the alternator it should kick out as if not a smart charge system (i.e. 13.8-14.4 volts). Only, when I do that, the output drops from 0.5 to less than 0.1 volts. The posts that I read refer to the 3 pin smart charge system, would you (or anyone!) know if that's the case for the 2 pin type (which mine is)?

Charging light comes on ignition, then goes out for a few seconds, then comes back on again.

[marcrbarker]

Good advice from Alltransit.

If it's a Euro 4, the battery should be charging at 14.4v pretty much at all times. If it's not, the fault could be with the new alternator (not unknown for them to be faulty) or a broken or misplaced wire in the charging circuit, maybe at the alternator end?
Does the charging light on the dash come on with ignition, and go out when the engine starts

That warning light coming back on after starting is a good clue.

Only thing I can add to above is 12.2 V at idle is way wrong.
And 0.5 V ? That's a peculiar voltage. Where are you finding that?
There's supposed to be full battery voltage round the back of the alternator the whole time. by a thick wire from there straight to battery plus.

[geckomike]

Hi marcrbarker,

I'm measuring 0.5 off the main output stud at the back of the alt while idling while the smart charger wires are plugged in, and 0.1 off the same stud when the smart charger wires are unplugged.

If I'm supposed to always be getting ~14v off the back of the alternator all the time, looks like evidence that both new and old alternators are faulty, as Altransit said?

Appreciate the advice - this van has me banging my head against the wall at the moment!

[Altransit]

It's a loooong time since I messed about with alternators, but bizarre as it may sound, an alternator won't produce any power unless it can see a +ve voltage from the battery on one of the small pins, nor will it charge if not connected to the charge light on the dash, so unplugging it whilst running, will stop it producing voltage, and may possibly damage the diodes in the rectifier pack.
Maybe there's a break in that wire going into the plug, or the pin itself is damaged or corroded

I don't believe that the Euro 4 has a "smart charge" system, because it normally produces a pretty flat 14.4v regardless of revs or electrical load, smart charge should produce a varying voltage depending on battery charge and load.

[marcrbarker]

Probably a blown fuse hidden in a missed place? Maybe when unbolting the alternator a tool shorted. With older vehicles there's a spark the tool gets hot you automatically drop it out hand, but modern vehicles there's a silent fuse blow and no spark and no burned hand to tell you something went wrong.

bizarre as it may sound, an alternator won't produce any power unless it can see a +ve voltage from the battery on one of the small pins, nor will it charge if not connected to the charge light on the dash

To add a bit, it's because the alternator inside needs current to magnetise its spinning magnet rotor, which it generates internally with a separate auxiliary generator. To kickstart this auxilary generator there's a gentle current coming in from the 'charge warning light', it's this tiny current feed via the warning light that kickstarts the full alternator into operation. This kickstart current can't come directly from battery + or it'll run the battery flat. Why it's via the key.

I'm posting here the charging system schematics for MK8, presuming MK7 is similar.
You can see (page 64) the red wire from the generator goes straight to the starter motor, then to battery junction box, battery juunction box and to 'A hot at all times'

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Can't be F1 blown because engine starter motor works.
I don't know 100% what the blue wire does that goes to GENMON on the PCM. Maybe it's the standard 'warning light' pin of a traditional alternator, if it is that then I expect it should be around a volt with key on no start and then go up to 12 V engine running.
The other big terminal (with red wire MK8) I expect always 12-14 V all times.

[marcrbarker]

Amusing 100% true story firsthand about charging problem. Friend owns a car repair business. For two weeks same BMW saloon kept coming back over and over again. Repaired numerous times. THREE new batteries installed, (each time tested the voltage was ‘wrong’, duh!), TWO Alternators (1st one considered defective goods), all cable connections and bolts cleaned several times. A special fuse was ordered in as a replacement to stall for time. Other things too irrelavent to the problem got replaced, i.e. complete set of fuses and probably were other things I don’t know about --- or want to know. Alternator cable replaced, twice. They were even thinking to replace the instruments panel which contains the charge warning light. So what was it in the end? The CRANKSHAFT PULLEY that drives the alternator's belt! The crankshaft was spinnning inside the drive pulley, but the pulley itself wasn’t turning at full speed. Completely true, I couldn’t make this up if I tried.

But getting back to the OP,

Charging light comes on ignition, then goes out for a few seconds, then comes back on again.

and I'm reading 0.5 volts directly off the back of the alternator.

That's not right. Should be 12 V-ish round the back whole time.

[andz327]

Have you by chance replaced starter and not put earth wire back on one of the bolts .....have you checked the engine earth cable?

Sent from my SM-G975F using Tapatalk

[marcrbarker]

Reminds me another alternator problem true story, replacement alternator's negative earth wire ring tag instead got attached to the alternator's +12V output post. No sparks or blown fuses, just didn't charge. The +12V tag that should had been connected was found floating not connected to anything. The alternator survived the abuse.

[geckomike]

You heroes! I hadn't been considering the starter motor at all (as I've not touched it). But having a look around there, one of the thick main red cables is completely severed. It's amazing it didn't fry something. Looks really corroded and I have just replaced the egr valve and must have caught it while moving around there (it's rwd so egr was a pig).

Now for a new connector hopefully that is the end of that!

[marcrbarker]

You're welcome. Was easy to work out with the good info you provided.

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Altransit was bang on from the beginning!

a broken or misplaced wire in the charging circuit, maybe at the alternator end?
:

[Altransit]

Altransit was bang on from the beginning!

a broken or misplaced wire in the charging circuit, maybe at the alternator end?
:

merely a lucky guess on my part, Andz was your man, spot-on