Hi All,

I'm hoping for some help with an annoying issue on my MK2 Superb, whereby the battery keeps going flat! Most recently to the point where the car would not unlock (totally dead) after 5 days away. The details are as follows:

New battery, new alternator, both fitted by me.

Quiescent current when the car is locked in "deep sleep" is around 350mA (hence the battery going flat so quickly!) I would expect to see <50mA in deep sleep. Interestingly, If I disconnect the battery completely, then reconnect, unlock the car and lock it again, the quiescent current will drop to around 30mA which is about what I would expect. It's only after I start the car that the issue resumes. It's as if the act of starting the engine engages something or activates a module that never goes to sleep again ?

As far as trying to diagnose the issue, I've measured the voltage across every fuse in the engine bay and cabin compartment. In both cases, tricking the car into thinking the bonnet/ doors are shut. I've taken this even further and removed each fuse one by one, and the none of them make any difference to the 350mA quiescent current.

The only red flag is a cheap Chinese radio (A-SURE) that was in the car when I purchased it. However, I've both removed the fuse and removed the radio completely, the issue still persists.

At this point I'm totally out of ideas, I'm loathe to think it's a chafed wire since if I disconnect the battery completely, reconnect and do not start the engine, the issue goes away.

For what it's worth, I am an electronic engineer by trade, so use multi meters/ oscilloscopes etc on a daily basis and fortunately have access to good quality kit. So i'm pretty certain my measurements/ method are sound.

Are there any common issues regarding parasitic current on these cars ? And where could the current be going that not a fused circuit ?

Thanks very much in advance for your help, 🙂

"I've measured the voltage across every fuse in the engine bay"

Does that include the strip fuses on front side?

Only thing that's truly unfused, in my experience, is the starter motor.

May not get many mV across any of these, being fair-sized chunks of metal, but it sounds like you have the kit needed.

Designations (generic)

Hi Pete,

I retract what I said! I have not measured across the strip fuses. In fact, I didn't realise they were strip fuses. I will do that fist thing tomorrow and see what I get. I assume SA5 is the feed to the internal fuse box ?

I also read on another thread that someone had an issue with an electric seat, but this was a circuit breaker type setup as opposed to a fuse, not something I've heard of before but wondering if there is any mileage in that.

Yes, that's correct about SA5.

Not sure about heated seat fusing, might use thermal (self-resetting, bi-metallic strip) fuses.

Can check tomorrow.

Following please keep the thread updated seems some MK2 suffer this eventually

Morning Pete,

I've measured across the strip fuses this morning, all 0mV apart from SA1 (Alternator). Sure enough, I then removed the alternator wire and the current dropped to ~30mA. I then re connected the wire and it remained at ~30mA. Some more info on the alternator as follows (as mentioned, I changed this recently).

Winter 2023 the alternator completely died and I just about managed to limp the car home. On the run up to this point (for a few months), the battery light remained on after starting the engine. The light would turn off again after I started driving.

I swapped the alternator for a new NAPA part. However, this was never right. The alternator worked, but only started charging when the car was revved over ~1500RPM, at which point the battery light would turn off and when measuring across the battery terminals, the voltage would jump to a healthy 14.3V ish.

I ran the car like this for a year, giving the car a small rev on start up with no problems and in the meantime tried to find the issue. In the end, winter 2024 I changed the alternator AGAIN for an OEM refurbished part and the alternator now behaves as it should. i.e. I start the car and the battery light turns off straight away. I also managed to get a refund on the NAPA part which was nice. At this time I also put a new battery on the car just in case this was causing any issues.

Hopefully this gives some clues as to what might be going on,

I wonder if the NAPA part was never actually faulty? Despite the OEM refurb working as it should.

Thanks again for your help,

How odd.

Was the OEM replacement alt an identical part number (including suffix letter) to the original factory one? What is that part number? Refurbed by?

Can't see how it can 'alternate' (sorry!) between having a current drain and not, depending on disconnection of battery etc.

I guess it's a fairly high mileage car, for the original alt to have failed?

Just a thought on this, but when the battery and alternator was first replaced, was the system re-coded to reflect that both had been swapped, and also could all of this problem been caused by either jumping, or charging the battery incorrectly, by directly connecting to the battery terminals, when the -ve lead should really be connected to the earth point adjacent to the battery or indeed anywhere on the engine/body so the battery monitoring module is aware of the charging?

Pre-stop/start and fancy batts and alts Graham, I think.

@Breezy_Pete - That was the original alternator and failed at around 120K (car is now on 138K). The current unit is from Wigan Alternators, I can't remember the brand now but the part number was sourced from the VIN so I assume is correct.

@Graham Butcher No, the system was not re coded. I was fully aware of this but decided the worse case is the battery is being over charged and therefore has a shorter life. I didn't think it would solve the issue I'm seeing. However, I have always charged/ jumped the car directly from the battery terminals, I was not aware that this is bad practice. For what it's worth, I don't think I've ever charged/ jumped the car prior to this issue, but it's been jumped plenty of times since :)

As Pete has just posted (as i'm typing), I don't think mine has a BMM, since it's pre stop start. I'm not certain on that though.

3 minutes ago, trickyskoda said:

I have always charged/ jumped the car directly from the battery terminals

Doesn't matter at all without a BMM, although a slight safety gain by making last connection of jumpleads away from battery and any hydrogen it may have emitted.

1 minute ago, Breezy_Pete said:

Doesn't matter at all without a BMM, although a slight safety gain by making last connection of jumpleads away from battery and any hydrogen it may have emitted.

Good point about the hydrogen, I have seen a few batteries explode due to the spark igniting the hydrogen.

I've taken a video to show exactly what's going on:

Am I right in thinking that wire is straight to the back of the alternator ? Or is it spliced anywhere else ? This alternator has a two pin plug, not sure what they are doing either.

Thanks again,

The wire going to the strip fuse goes only to the alternator output stud, I would expect.

Let me know reg or VIN and I can look up the circuit diagram if you wish.

The two other wires are excitation (blue usually) and field monitor. First helps the alternator 'boot up' when first rotated by supplying a small current to the brushes. Second is a signal telling the engine ECU how much of its capability the alt is supplying, moment by moment.

It is not unknown for a diode to go rogue, its rare but is possible. This link will explain just how the alternator operates and it contains 6 large power diodes and judging from what appears to be happening and the wire that you are disconnecting is the main heavy wire from the alternator to the battery. If any of those 6 diodes has a slight leak, it could allow a small current to flow the reverse way through the alternator to ground.

How long ago was that alternator supplied and fitted? I'd be inclined to try and source another one just to swap it out on a temporary basis and see if it cures the problem, if it does, and I suspect it will, then complain to the supplier about your current one.

Some notes about three-phase rectification

Edited 31 May, 20251 yr by Graham Butcher

@Breezy_Pete that would be great if you could, KN60JVD. If you need the VIN let me know.

@Graham Butcher as mentioned this one is new (fitted a few months back). Is the diode pack/ regulator all part of the alternator unit ? If one of the diodes was leaking, why would the issue clear if the wire is disconnected then reconnected ?

Thanks again for you help

Edited 31 May, 20251 yr by trickyskoda

Quite simply because it could well be a heat issue, they will get warm when charging as they will be passing a fair amount of amps and that could be breaking down the internal junction. When you disconnect the power cable, you're breaking the connection just long enough for that slight leakage path to clear itself down and then reconnecting the power again, the leakage has gone due to the faulty connection inside the diode(s).

You could always test this out if at the end of the day after you have finished with the car, remove that cable from the strip fuse and leave it off until the morning or next time you want to use the car. See what the battery voltage is and see what the quiescent current is reading. It should be still around the 40mA as this will be current draw for the various ECU's the car has. If on the other hand the current has increased, then the alternator is not the issue. Don't forget to reconnect the alternator lead before using the car.

Thanks Graham,

I wonder if I could be so unlucky to have not one but two faulty replacement units, and it is the alternator after all ?

Further info:

This morning I have monitored the quiescent current whilst unplugging the two pin plug off the back of the alternator. This did not make any difference.

I'll discuss with Wigan alternators on Monday (where I sourced this part) but in the meantime, it would be good to know if that wire is spliced anywhere else as there could be something else at play if so,

Thanks again,

Private message sent. 🙂

5 minutes ago, trickyskoda said:

Thanks Graham,

I wonder if I could be so unlucky to have not one but two faulty replacement units, and it is the alternator after all ?

Further info:

This morning I have monitored the quiescent current whilst unplugging the two pin plug off the back of the alternator. This did not make any difference.

I'll discuss with Wigan alternators on Monday (where I sourced this part) but in the meantime, it would be good to know if that wire is spliced anywhere else as there could be something else at play if so,

Thanks again,

I really doubt that it is going to be spliced anywhere as it is on a 200A fuse and the next power load is the power steering at 80A, but I think @Breezy_Pete is going to confirm that for you later when he checks the wiring diagram.

Have you not experienced a diode failure to properly shut down and block the reverse back flow after the forward current ceases flowing? I must admit that the first time I came across it, I'd discounted it as a point of failure, but sure enough, after replacing the diodes the problem was cured.

Thanks very much @Breezy_Pete . It is a direct connection then. Looks like I’ll be putting a third!!! Alternator on it then, I’ll speak with wigan alternators tomorrow and get their take on it. Third time lucky perhaps? I wouldn’t mind as much but it’s a pig of a job on the superb to change.

Lots of stuff to move out of the way to create an exit/entry path, presumably?

Yes, it’s bumper off and front end pulled out an inch or so to make room for it. Not the end of the world but not a fun job!

Edited 1 June, 20251 yr by trickyskoda

4 hours ago, trickyskoda said:

Yes, it’s bumper off and front end pulled out an inch or so to make room for it. Not the end of the world but not a fun job!

I used to work at an auto electrical place and refurbished alternators, dynamos and starters of various sorts and sizes and have seen similar diode failures there. Every diode has a reverse voltage gradient, which is how diodes work. When the engine has been running, the battery voltage is higher than it is normally and the diode could, if it's going faulty, begin to conduct in the reverse direction, but it would have a far higher resistance than the forward resistance. When you break the battery power to the alternator, then the current cannot flow, and when you reconnect the power, the battery voltage is lower and so it cannot reach the level of breakdown point that had before. It is in effect almost like a zener diode, the magic word here is almost.

One thing is for sure, if left unchecked, then the diode is going to deteriorate faster and the battery will drain even faster.

Edited 1 June, 20251 yr by Graham Butcher