Last week in How to Electronically Modify Your Car, Part 3 we looked at volts, ohms, amps and watts. Knowing what these terms mean – and how they inter-relate – is simply vital for any electrical or electronic work. And just as important is this week’s content – how to use a multimeter.
So how do you go about using a multimeter to measure volts, amps and ohms?
When measuring volts the meter should be connected in parallel with the voltage source. In all car systems the voltage potential is with respect to ground – the car’s chassis. This means that normally you will ground the black lead (eg by using a crocodile clip to connect it to a clean bolt) and then use the positive probe to connect to the car’s wiring. That applies when measuring any voltage on a car – battery, engine management sensors, stability control sensors, and so on.
For example, say you're trying to find a 12 volt supply for an accessory, or measuring the voltage output of a sensor. In either of these cases, the meter would be set measure DC voltage. The negative probe is then grounded and the positive meter probe inserted into the connected wiring.
If the polarity is wrong (you've used the negative probe to the positive supply line) then no damage will be done - the meter will simply show negative volts instead of positive volts. In fact, it is pretty well impossible to damage any systems by just measuring their voltages with a multimeter – the only possible problem is if you short together multiple pins with the meter probe.
When measuring voltage, remember the circuit does not need to be broken - the meter is inserted in parallel.
Measuring current (amps) requires that the circuit be broken and the meter placed into it (ie in series), so that all the current flows through the meter.
If you're measuring currents above milliamps, the meter will often require that the positive probe plug be inserted into another socket on the multimeter. Failure to do this will result at best in the blowing of an internal multimeter fuse, and at worst in damaging the meter. This means you always need to be very careful when measuring amps!
Breaking the circuit to measure current flow can sometimes most easily be done by removing a car fuse, and inserting the meter at that point. Obviously, though, if that circuit has a short circuit then the original fuse protection will be lost and the meter's fuse could be blown instead. (Note: some multimeter fuses are very expensive!)
Resistance measurements require that the device is isolated from its normal circuit, otherwise the measurement could be false. In the case of an engine management sensor (like throttle position) this means that it needs to be unplugged. If you try to measure resistance with the sensor still plugged into its circuit, the reading will usually be wrong!
Always first check that the multimeter indicates zero resistance when its leads are touched together - if the meter doesn't show zero resistance, here should be a meter adjustment available that resets the display to zero.
So the key points to remember are:
It’s simply no use looking at the above pics and text and then thinking that you know how to use a multimeter. What you must do is actually put the theory into practice.
Get used to using a multimeter in the same way as you might use a socket set – whenever you need the tool.
So, when you wonder if the battery in your torch or child’s toy is getting flat – measure its voltage with a multimeter. If you’re looking at a car for which you don’t have a workshop manual wiring diagram, and you wonder what each pin on the airflow meter is for, ground the negative lead of the multimeter and back-probe each pin in turn, measuring its voltage. If you’re using a wiring diagram and you figure that on the car a particular wire must have battery voltage on it (ie 12V), always measure it to be certain.
An industriously used multimeter can tell you an incredible amount of information, especially if the car is being driven. I once modified the four wheel torque split control system on a Nissan Skyline GTR – without having a wiring diagram. The first step was to drive the car hard while measuring the voltage outputs of the ‘g’ sensors; that let me see what signals the ECU was using. (When doing this, use an assistant to either drive or read the meter.)
More recently, I came up with a way of monitoring air/fuel ratios by using the output of a factory-fitted wide-band exhaust gas oxygen sensor – most of the research involved just the careful use of a multimeter.
Use, use and use a multimeter until you’re confident and quick with the tool: it’s extremely important to success in electronically modifying cars.
Next week we’ll look at one of the most useful components you can use when electronically modifying a car.