multi-meter

[Alpine]

I've just bought a muti-meter so i can try to trace why my battery keeps going flat.

could someone just cofirm how i use it? Do i disconnect the negative ternimal and link the meter from the lead i have taken off to the battery?

Is there any way of doing it without having to disconnect the battery? (can i not just touch it to the positive terminal and earth it through the body of the car)

sorry to ask this - i'm not very technically minded !

thanks

 

[berg944]

Take one lead off the battery (usually earth) then connect the meter between the bare battery pole and the lead you have just removed. ALL current then goes through the meter (set to 10 amp.)

DO NOT start the car or connect positive and negative as you will damage the meter (or blow the fuse) unless it is the very expensive type designed to do this.

Mike

 

[Guest]

Well, you could also get a shunt and ampmeter gauge and make a more robust measuring device for pretty cheap. But it would have to be pretty darn big to take the starter draw. Like several hundred amps as the starter can suck alot of juice.

If his multimeter is a cheaper type and doesn't have a fuse provision built in I would rig up an inline one as if he makes a mistake and turns on the ignition key or the radiator fans kick on that meter could be a gonner.

 

[nfearn]

An alternative to measuing the current drain from the battery is to remove each fuse in turn (with the ignition off) and test for current across the fuse holder (use the spiked points of the probes). The meter should be set to the 5 to 10A range, which should give you enough sensitivity to read current draws as low as 0.01A.

Go through the whole fusebox, noting the number of any fuse positions which show any current draw, no matter how low.

Most 944 batteries have a capavity of around 70Amp hours ("Ah" for short, and it will be written on the battery in this form). So, the battery in this case (70Ah) can in theory supply 70A for one hour, or 1A for 70 hours. In practice, the battery will be flat for the purposes of starting the car before these time limits are up.

So, if you find you have a fuse position with a 0.2A current across it - again, I emphasise with the igition off - you have a component that will flatten the battery in around 70 / 0.2 = 350 hours. This is about 15 days or so, although the battery won't have enough power left to start the battery before that limit is reached.

Once you've identified the circuit, check to see what component on that circuit is taking the power by disconnecting each component on the circuit in turn. Watch out for accessories that might have been powered from that circuit as well.

Hope that helps

 

[berg944]

Your meter should be fine, if used on a cold car your fan will not come on.

I left the meter connected and removed each fuse one at a time (ignition off), just look for a figure drop then write down how much it dropped and the fuse number. When you have done all the fuses you can work out how much is being used by which circuit.

Mike

 

[Alpine]

thanks very much for all your replies - I'll let you know how i get on !

 

[Alpine]

I've just linked the multi meter up to my battery to try to find out why my car wont start after a week of sitting in the garage.

The results are:

- with everything off (alarm off) it draws 0.03A
- with the alarm switched on it reads 0.08A

My battery is a 66Ah.

I disconnected the fuse that runs the clock, and then took out each fuse in the hope that it would drop to zero at some point. However, it didn't !!

I think that this might be to do with the 2 flashing lights by the gearleaver, one for the alarm and one for the immobiliser. They still flash even with the alarm off. None of the fuses affected these lights, but they did go off when I disconnected the battery.

Do they run off a seperate fuse, not under the bonnet.

Does 0.08A with the alarm on sound ok?

 

[Guest]

Those amp draws are just fine and normal. Acutally, pretty good at 0.03 with the alarm off. I don't know how your alarm is wired but I'm sure there is a fuse somewhere in the power supply.

You will not be able to drop draw to Zero as there are some minor draws that will always be there like the radio memory, clock memory and I think the ECU even has a minor draw.

So, either your battery is approaching the end of its life or you have some mysterious intermittent draw that pops up. As far as an intermittent draw, if you have a dirty battery top or connectors along the main positive battery cable that can cause a soft short when humidity is high or they get wet. The current will flow through what is basically now an electrolyte. I once fell off a boat and into the sea with an electric drill in my hands ...... man did I get a shock. Good I was young and had a strong heart or that may have killed me. Anyway, the same type of thing can happen with a car's electrics.