Trying to chase battery discharging issues on the Passat. It was around 0.5A but somehow, with me just wiggling things and clearing codes, it’s gone down to 90mA which isn’t bad. But according to a TSB it’s meant to be 40mA. So I tried the suggested method of tracing it which is to connect a multimeter set to mV range across the back of each fuse. However, to find 50mA of current drain I need to detect about 0.1-0.3 mV. I didn’t detect anything, so I’m wondering if my cheapo multimeter is the problem?

Is it likely to be expensive to find a multimeter capable of detecting voltage drops this low? Can you get microvolt ones? Or should I just give up? 🙂

Biggest problem will be getting a reliable connection to be able to take valid readings.

should I just give up?

…and deny the world so much Passat-tinkering entertainment?

I think not. Charge on.

I think this is what you need.

I can’t see that measuring the voltage across a fuse will tell you anything (unless it has blown), the voltage drop will be tiny. You need to put your meter onto the current setting (might need to move the where the leads plug in) then take the fuse out and put one probe on each terminal of the fuse holder. That should give you a true measure of any current flowing. You can get meters specifically for this which plug straight in (eg https://www.ebay.co.uk/itm/Car-Fuse-Current-Battery-Amp-Meter-Leakage-Detector-Mini-Blade-Fuse-Type/261107871862 ) but easiy bodged with a normal multimeter

Ooh, that fuse replacement one is darn clever 🙂

I can’t see that measuring the voltage across a fuse will tell you anything (unless it has blown), the voltage drop will be tiny.

It’s the standard method, lots of stuff online about it. There’s enough resistance to identify significant current draw with a tiny voltage drop. The problem I have is that the current draw is now so small that the voltage drop is indeed tiny. And yes, getting a good connection is the key. If I touch one metal bit and then the plastic of the fuse the meter reads more voltage than the values I am looking for!

The problem with removing fuses is that it powers things down and causes the CAN gateway to go a bit nuts and all sorts of things to wake up and start consuming power. So you might save 0.5A by taking out the right fuse but cause another 0.5A consumption by waking something else up.

I have the fuse replacement ammeter, but as said you’d have to replace each one then wait for ten minutes each time for everything to settle down – maybe more. The Bentley TSB says you have to leave the car untouched for 2 hours before you can be sure everything’s gone to sleep.

However, reading up, seems like measuring microvolts is pretty difficult in general, never mind rummaging through a car fusebox with a multimeter.

Have you tried one of:

with one of these:

Just checked my multimeter and although it will measure down to .1mV I’m not sure I would trust it much there. Do you have the option of setting the voltage range or is it automatic? Sometimes that gives better results, especially if the current is in spikes rather than DC

Measuring volts across the fuse is just fine to get a rough idea

I’ve got two – one has a 200mV range, and the other one is auto but seems to be the same 200mV. I think it’s just going to be too difficult.

Car started this morning and didn’t drain despite not being fully charged, so I think the 0.5A drain has gone away.

why not just measure your total draw and start pulling fuses until it drops to 40mA

there’s your culprit!

As I said – when you pull fuses, it wakes up the CAN gateway and then other stuff wakes up. I tried it. If I waited for it to quiesce again I’d be there for hours.

There’s one fuse I pulled out that started a relay clicking on and off behind the dash for ages!

It’s bad enough trying to measure mV in our power lab at work, let alone 0.1mV!

If there isn’t another way, eg. pulling fuses etc, then try and get hold of a clamp type current meter. I’ve just had a quick look and Maplins do one for about 30 quid which claims to measure down to 10mA DC.

Ammeter won’t help, because like I said disturbing the fuses causes activity. I already measured the current.

I’ll just leave it I think.

Mol… it’s a clamp type, you open the jaws and put it round the cable, you don’t need to disturb any fuses.

https://www.maplin.co.uk/p/uni-trend-ut203-dcac-current-clamp-meter-n41nc

sounds like the perfect excuse to buy a proper meter:

34470A

😆

Mol… it’s a clamp type, you open the jaws and put it round the cable, you don’t need to disturb any fuses.

So that tells me the current drain – so how do I then figure out what’s draining the current?

If I do that then pull a fuse, the CAN gateway wakes up which draws more current than I would have saved by deactivating that circuit. I already measured that with a regular ammeter.

@maxtorque.. there’s no price on that, it just says ‘get a quote’ 😆

Meter is about #2600 – you can rent one if you want to.

mol… how were you hoping to figure out what was draining the current from poking around in the fusebox?

Surely you’d just use the current clamp on the wires to/from the fuses, instead of the fuses themselves…

I am presuming you have a wiring schedule for the car at this point.

Ah, see what you mean about using the clamp-on to check the incoming wires. Tricky, but not a bad idea.

The technique with the voltmeter: a car fuse usually has the ends of the blade contacts exposed on the back of the fuse. So you can touch these with the multimeter probes and detect the voltage drop across the fuse and use a table to determine the current flowing through it. Described in this TSB from Bentley (that’s the US vehicle manual publishers, not the car manufactuers!)

As you can see, at the lower end of the current range it’s a very low voltage drop that needs to be measured. I think it’s too low for my cheap kit.