Technical Bulletin about parasitic current draw measurement

[Breezy_Pete]

Came across this interesting VW document about a 'modern' method of measuring parasitic currents. Original link no longer available, so go here to download: https://www.briskoda.net/forums/topic/377587-technical-bulletin-about-parasitic-current-draw-measurement/?do=findComment&comment=5408205

 

 

Here's the rationale:

Due to the state of the CAN Bus communications in the vehicle, it is no longer acceptable to pull each individual

fuse one at a time to try and identify which circuit is consuming current. Removal! reinsertion of a fuse while

vehicle is in a sleep state may wake the bus of the vehicle. and invalidate the test. Identifying "consuming" circuits

must be done by measuring a voltage drop across fuse and aligning with the value in matrix located at end of this

document.

 

An initial overall measurement at the battery is done to establish whether the 'sleep current' is more or less than a criterion of 40mA (after 2 hours of not touching the vehicle).

If >40mA, the individual circuits are checked not by pulling the fuses and watching the overall consumption, but by (voltage) probing across each fuse, in-circuit, to see how many mV are being dropped across the fuse itself. Then there's a big old look-up table for both mini and standard blade fuses, showing what current flow through the fuse corresponds to what mV reading seen. 

 

Cunning. 

Edited 26 February, 2022 by Pete_Ex-Wino
Fixing dead link

[rum4mo]

Thanks for that, document now safely saved for future use, this is still a subject causing me issues with daughter's late 2009 Ibiza SC, though now that I've found "mouse attack" under the bonnet (chewing foam blanking pieces, no doubt for nest building), I've been diverted from my "is it or is it not going to discharge its battery" - so far, while being instrumented, it has not suffered from higher than normal parasitic current draw, or discharged its battery!! Though CTEK battery charger/tester now says "battery not good" when cranking with a fully charged battery!

Edited 6 November, 2015 by rum4mo

[Breezy_Pete]

You're welcome.

[Breezy_Pete]

I tried the first bit of this test (overall current draw from battery) on my Polo earlier, and initially made a bit of a schoolboy error. 

 

Before locking the car, it was taking about 100mA, and I thought "that's only gonna go down as things go to sleep", so put the meter probes and dial in the 200mA-range position  to try to get a nice accurate reading. Used some long leads so I could still see the meter with everything including bonnet shut.

 

Guess what happened to the meter fuse when I flicked the remote button to lock up and set the alarm? 

 

Yep, the doorlocks all take a little surge as they do their thing, and the indicators all flash simultaneously. I can confirm that the current in that moment is ample to blow a 315mA quick-blow fuse in the multimeter.   

 

Switching to the 20A range I found that the current settled down to somewhere between 10 and 20 mA sometime during the next couple of hours, good news. 

[cheshire_cat]

that's really useful  cheers :sun:    I'v taken to using a cheep (thirty bob) and voltmeter from fleabay  (china)  which plugs into  the lighter socket(always live)  and can be easily read from outside the car, they seem to be surprisingly accurate too    +_  .2volt

[erindad]

Those flea bay cheeps are quite expensive at thirty bob , is that for one or a bag of   ?  

[mrgf]

On 06/04/2018 at 08:52, erindad said:

Those flea bay cheeps are quite expensive at thirty bob , is that for one or a bag of   ?  

Thirty bob (£1.50, to those who can't remember real money), IS cheap. Mind, coming from a Bonny Scot! Ill let you draw your own conclusions. 

 

I would expect the battery to be quite weak by now, though! That'll be a lot more then thirty bob!

Edited 9 July, 2018 by mrgf

[J.R.]

I have saved that usefull technical info guide and noticed while reading it that the photo of the fusebox shows the meter probes being connected to 2 different fuses.

 

Or it could be that I need to go to Specsavers!

[Breezy_Pete]

Sorry, Specsavers, I think. You're only looking up to the lighter coloured insulated portion of the left-hand probe, the tips are both going to the right hand 10A fuse.

[VWD]

Looking at the print of the statement, it means that we'll need even more exotic test gear, such as a DVM with a recording facility to record voltage over a time. Or a DVM with a separate output to drive a portable recorder. That and plenty of time. But I'm a bit sceptic as VAG love any excuse to get the labour bill up  to pelican level. ( Ed China comment).

[EMF]

Hi

The link to Bently publihers is no longer working, has anyone saved the document mentioned (voltage drop accross fuses due to current draw) if so could you send me a coppy?

many thanks in anticiption Michael

[Breezy_Pete]

Try this:

vw.current.draw.tb.pdf

[Camlobe]

(Really, this isn't in the right place, there is a more up to date thread where it should have gone: https://www.briskoda.net/forums/topic/489055-how-deactivate-door-open-sensor-octavia-2011/)

 

I doubt it will help anyone, but just in case,here are a couple of things to consider:

 

If you've got a clamp-on current meter (for this kind of thing, it is helpful, I keep meaning to look in to one, but the need hasn't yet arisen) you can monitor more than one line at once.

 

Anywhere you can find a cable/big bundle of cables, you can put your clamp around. If you find a bundle that is taking current, you can start a 'binary division' approach with that bunch - divide it in to two, and keep dividing 'till you get to a single wire.

 

Err, while not what VW recommend, I'd even start on the cables, say, after five minutes, to get a 'short list' of wires that are most likely to need further attention.

 

One further point, at one stage (late 90's/early 2000s?) VW did have a problem with something slightly weird like Marmoset attacks in Austria taking out ignition cables, and causing cars not to start/misfire (I don't remember the exact details).

 

Hence, or otherwise (as they used to say in exam papers), I would have a look around the area of the mouse attack, just in case the mouse fancied something more substantial to go with the foam. 

Edited 2 February, 2021 by Camlobe
post in to old thread; see link

[grahambulmer]

I know this topic is over a year old but I thought I'd post what I'd found. Not having used the car much lately I had the problem of the battery going flat. The first time was after about five weeks so fair enough, but has also gone flat within two weeks. The battery is only a couple of years old so hopefully OK. I did some measurements of the battery current and found that after about five minutes it would drop to either 400mA or 170mA - on subsequent tests with the same conditions This was with the car unlocked. If I locked the car it would drop to about 40mA with a slight pulsing, which I think is correct. I do have a habit of forgetting to lock the car as it is kept behind locked gates. I know I should wait longer  during the tests but I wondered if anyone else has noticed this?

[PetrolDave]

When the car is unlocked many of the electronic modules stay awake only going to sleep when the car is locked, so to minimise current drain the car should always be locked when left unused for a long(ish) time.

[grahambulmer]

Thanks, that makes sense. The other thing during these investigations I found the alarm had failed, presumably due to the battery. I wonder if periodically the system tries to charge this battery, but of course it never achieves it, thereby draining more current from the main battery. I did my tests both with the alarm fuse in and out, with no difference, but I probably didn't do the test for long enough to observe it trying to charge the alarm battery. I'll leave the fuse out until I've fixed the alarm.