Parasitic drain - multimeter choice and use

[vanman1936]

Hi Guys

Have a parasitic drain on my Maser Gran Turismo.

Total amateur in car electronics, any recommendations for a good value multimeter and how do you use them on a car battery to test.

Cheers

P

[Disco_Biscuit]

Any multi meter with amp meter function built it will do the job, doesn’t have to be expensive.

[cuprabob]

Any multimeter will measure DC Current.

Pull fuse for each circuit in turn and connect probes to either side of the fuse socket and measure the current in each circuit to track down which one is causing the issue.

If you want to measure the total current being drawn from the battery you will need to disconnect one of the battery terminals and put the meter in series.

[PositronicRay]

Don't forget some circuits will take ages to go to sleep, and may not if you've left a bonnet/boot open for fuse box access.

ETA

Crocodile clips on leads and insulation piercing probes are useful to have.

Edited by PositronicRay on Tuesday 2nd January 08:30

[Stenasev]

PositronicRay said:

Don't forget some circuits will take ages to go to sleep, and may not if you've left a bonnet/boot open for fuse box access.

As per the above if your car has a simple plunger switch for the bonnet then tape it down while testing or unplug it as many are open circuit with the bonnet closed. You can buy multimeter leads with fuse adapters on the end which makes things a bit easier especially if waiting for circuits to go to sleep.

[DuraAce]

Voltage drop test over each fuse can also be a useful way, saves you having the battery disconnected (or if you blow the low amps fuse in your meter... Ask me how I know!)

https://us.autologic.com/news/testing-parasitic-dr...

http://info.powerprobe.com/fusechartsdownload

[GreenV8S]

Rather than connecting the ammeter directly into the circuit, I recommend that you start with the two leads of the meter shorted together (so the meter is bypassed) while you reconnect the battery and power up the vehicle. This avoids the meter seeing the inrush current, which can be enough to damage sensitive meters. Once the battery is reconnected you then move the leads apart so that the steady state current goes through the meter. Then you can go ahead and pull fuses etc.

[vanman1936]

GreenV8S said:

Rather than connecting the ammeter directly into the circuit, I recommend that you start with the two leads of the meter shorted together (so the meter is bypassed) while you reconnect the battery and power up the vehicle. This avoids the meter seeing the inrush current, which can be enough to damage sensitive meters. Once the battery is reconnected you then move the leads apart so that the steady state current goes through the meter. Then you can go ahead and pull fuses etc.

Sorry V8, my car electrical knowledge is very light....could you clarify the above, if you can be arsed a step by step description much appreciated.

[DuraAce]

It'd be quicker and easier just to voltage drop test across each fuse IMHO
Nothing to disconnect or disturb so you aren't upsetting anything, you only need a millivolt capable multi meter.

[Classy6]

Get a reasonably priced meter with amp function and some croc clips. To cheap you might be chasing your tale with incorrect readings. I'll also forewarn, drains are nightmare jobs. They are extremely labour intensive, you will need full access to fusebox designation and if you're really unlucky, wiring diagrams.

If access to doors/under bonnet & boot are required to reach fuse boxes make sure lock latches are tripped over / door switches pressed and held in to simulate locked position. Get rid of keys if keyless entry etc.

Lock car. Leave for at least 45-60 minutes to allow everything to shut down. Setup meter to read amps and connect cables/clips to meter. Next bits fiddly, but short of making up your own set of cables with isolator switch maybe practice a few times first because if you get it wrong you restart the 45-60 minute wait.

Lift up the car '-ve' cable off the battery terminal slightly maintaining contact at all times. Connect negative croc clip to the base of the battery terminal then clip your positive croc clip to the -ve car cable. Only when both croc clips are connected with good contact can you fully release the car -ve cable from the battery terminal. If you manage to knock it etc, you'll know as car alarm will go off due the battery being disconnected, even momentarily.

Once meter is connected in series, you will now have a live reading of whatever your drain is in amps. Take note of this, check it against spec of what the drain is supposed to be. Stand and watch it for 5-10 minutes, see if it's fluctuating or decreasing/increasing. If it's over spec, meaning you do have a drain - work out how much by. This can sometimes give you a good idea of what it might be which can enable some time saving. Before starting have a walk round just make sure nothing obvious is on visually, or audibly. If nothing, then you can commence the fun.

Methodically start pulling each fuse out from the fusebox, see if drain drops. Note which fuse your pulling, as some components will still be active with slight draw - alarm etc. If/when you find the fuse that's causing the drain you then go into a component level. Find what components are fed from that fuse and begin disconnecting one by one, until you find the drain disappear (obviously make sure you put the fuse back in!).

Sounds simple, is an absolute ballache. Consider a battery maintainer if available/feasible if car drains over weeks rather than days.