This article was first published in 2009.
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.
Car Measurement: Measuring the Airflow Meter Output Signal
the multimeter to measure Volts DC.
sure that the leads are positioned to measure Volts. This is normally the
remove the boot covering the back of the airflow meter plug.
the negative lead of the multimeter to the chassis (ie ground).
the airflow meter plug terminals, one after the other in sequence.
5.1 volts can be found on one plug terminal. That’s the regulated power supply
to the meter (it’s meant to be 5.0 volts!).
on this terminal can be found a voltage that changes with engine load. It’s 1.3V
here but when the throttle is opened, it immediately rises. This is the airflow
meter output signal.
why would you want to find the airflow meter output signal wire? You could be
wiring in a fuel interceptor, installing a voltage switch that operates on
engine load, or tuning the action of the airflow meter.
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!)
Car Measurement: Measuring Current Draw
the multimeter to measure Amps DC.
the leads to the Amps measurement position. Normally, the negative lead
(circled) stays in the same socket but the positive lead is moved.
Fluke multimeter clearly indicates on the display that the leads need to be
altered if Amps measurement is selected but the leads aren’t shifted.
fuse for the circuit being measured is pulled.
multimeter probes are applied to either side of the fuse.
current flow through the meter is then shown – here it’s 3.2 amps.
why would you want to measure current flow? If you’re installing a fan or pump
you generally want to know how much current it draws so you can size the wiring
and fuse appropriately. For example, a pump drawing 10 amps shouldn’t use 5 amp
wiring or have a 5-amp fuse. Another common use is to see if a battery charger
is working properly.
how do you use a multimeter to measure watts? In short, you can’t!
we described last week, watts are calculated by multiplying amps by volts. So
what you do is measure the volts, then measure the amps, and then multiply the
in the above section, we measured a current flow of 3.2 amps. If the car’s motor
was off, the battery voltage would have been about 12.5V.
x 3.2 = 40 watts.
accuracy you’d not just estimate the battery voltage like I just did, but
actually measure it under load, ie with the current flowing.)
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.
Example Car Measurement: Measuring Sensor Resistance
the multimeter to measure resistance.
sure that the leads are positioned to measure Resistance. This is normally the
the sensor, or as has been done here, remove it completely. Connect the probes
one to each terminal and measure the resistance.
sensor resistance is the most common way of fault-finding throttle position
sensors and temperature sensors, including coolant, intake air and cylinder head
temp sensors. The resistance values are compared with the data in the workshop manual. Resistance values are also measured when selecting certain
electronic components, like resistors and potentiometers.
So the key points to remember are:
can measure other factors that are often very important in car modification.
These includes duty cycle, frequency and temperature. However, if you can
confidently measure volts, ohms and amps, the rest easily follows.
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
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
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.
The parts in this series:
Part 1 - background and tools
Part 2 - understanding electrical circuits.
Part 3 - volts, amps and ohms
Part 4 - using a multimeter
Part 5 - modifying car systems with resistors and pots
Part 6 - shifting input signals using pots
Part 7 - using relays
Part 8 - using pre-built electronic modules
Part 9 - building electronic kits
Part 10 - understanding analog and digital signals
Part 11 - measuring analog and digital signals
Part 12 - intercepting analog and digital signals
Part 13 - the best approaches to modifying car electronics and the series conclusion