Care must be taken when measuring the resistance with, e.g., a multimeter. No, we don't mean that the circuit must be free of tension. That goes, so to speak, without saying. In this case, we'll be dealing with values indicated after an apparently successfull measurement.

Just one point, before we go on: The closer that the resistance to be measured gets to the internal resistance of the measuring instrument, the less accurate the measuring is. Indeed, there are also other reasons where in the measuring of resistance in a motor vehicle, problems can arise. The more one is aware of this possibility, the more likely it is that errors can be avoided.

On a printed circuit or in a cable-duct, even if the documentation is first class, unwanted connection between the two poles of the component to be measured can still occur. Therefore, the connection should be broken, at least on one side, either that or soldered out to avoid a faulty result. Under certain circumstances, the battery of the measuring instrument can cause the circuit do be damaged.

Here, this type of measuring is misused as a continuity test. So far so good, indeed, try this on the main cable from the battery to the starter, if instead of having at least 16 mm² it is frayed down to only a few strands of wire which keep up the connection. The multimeter shows a very low resistance. The question is however, how long will the cable still be able to do it's job properly?

Then there is also the infamous fault in the ignition coil, which is half burst open so that windings can almost be seen. The spark can strike without restraint, somewhere outside. Now, what does the multimeter have to say about the resistance value? As long as there is no actual short circuit in the windings, it shows perfectly normal values for both the primary- and the secondary windings.

Would you like another spectacular example? Hold the two measuring points of the cable in a damp cloth. Make a note of the measured resistance. Now plug the cables into each others sockets. Do you still get the same reading? No, of course not. Give some thought to the matter, a damp cloth basically represents a voltage source, which when correctly poled to the battery in the multimeter, is switched serially. Here's a small tip: Should this phenomenon in the resistance measuring not be comprehensible, then try it with a voltage measurement.

Finally there is the potentiometer or the path of resistance. Formerly, it was favoured for the air-volume meter. To get straight to the point, you should use an additional oscilloscope, because the path of resistance could have small interruptions, which you can hardly notice in the multimeter. With this type of so-called 'noise testing', interruptions or contact problems are shown in small peaks. These are in fact, what disturbs the smooth driving operation. 02/13