12-18-2012, 12:49 PM
Join Date: Oct 2009
Mein Auto: car
Originally Posted by BMWFatherFigure
7...8...9...OUT! Check current drain from the battery terminal to battery lead. Trust me on this RB, no point in telling fibs here.
Well I have done my checks, and here are the results.
1. Yes you can indeed use the ammeter's leads on BOTH ends of each fuse to determine if there is a current flow, with a low current draw setting which means high internal resistance selected on your multimeter ("mm"). If current registers on the display, you can then check if it makes sense eg clock circuit, or it doesn't eg luggage compartment light when the boot is closed, and take actions accordingly.
2. Why this method works perfectly is because you are placing your mm in parallel with each circuit. 3 aspects of electrical theory make this safe :
(a) The total current going into a junction of parallel circuits is the same as the total current coming out of it. I.e. the total current entering the junction splits up according to the various resistances in the parallel circuits and rejoins after passing through each line in the parallel circuit.
(b) If you place a high resistance device in parallel with a live circuit, though it offers an alternated pathway for current, it will draw very little current due to its high resistance, so you needn't be concerned
(c) The mm will not fry as well because the voltage in the car's circuit can never be higher than 12.6volts due to the battery's design and this is obviously safe for the mm.
Let me use a worked example that we can all relate to. The sparkplug. The actual spark itself is the current in the circuit. The potential difference between the plug's leads is the voltage. The spark will not kill us, its a ridiculously small current. However, the voltage of 20k-40k volts (I forget) will. So that's why its dangerous to humans. [ Always be mindful when around active ignition coils. As a rule, only work on and around it with the engine off and the ignition at key0. ]
On the car, the only device that can step up voltage like a transformer would be the ignition coils. And the voltage is stepped up solely within the coil itself and not in the external circuit that supplies it with the base current/standard voltage. So there is no risk that you will fry your mm if you test all of the car's circuits via the fuse box.
Thus, if you put your mm in series with the battery after disconnecting a terminal, you will destroy the mm's internal fuse (usually 10A) if the current draw somewhere else in the circuit happens to be higher than that. Put it in parallel.
Thus, the Bentley manual's method of searching for current draws is impractical to the point of being dumb when compared to the alternative. It involves waay too much work and time and risks your mm's fuse, as compared to simply connecting your mm's leads to both ends of each fuse in your fuse box, one after another. It just shows how powerful general superstitions can be that stuff like this makes it to the manual.
Remember, we are not trying to measure the current draw accurately. We are just trying to see if there is any current draw to begin with...when the engine is off with key0 on the ignition. So an accurate reading is not important.
FF, I have snatched victory from the jaws of defeat.
Last edited by robertobaggio20; 12-18-2012 at 01:56 PM.