Last week we covered making the normal measurements with a multimeter –
volts, ohms, amps, duty cycle and frequency. This week – in the last of our
three-part series – we take a look at some more automotive uses you can make of
Measuring Voltage Drops
Voltage drops occur in all wiring but are much more likely when high currents
are flowing. A voltage drop means that the voltage available at the battery is
not reaching the load – there is resistance along the way.
Voltage drops can be easily measured with a multimeter, especially one that
has a Max/Min hold function. Simply set the meter to read Volts DC and then
connect one probe of the meter to the positive battery terminal and the other to
the positive supply at the load. Any voltage that shows on the multimeter scale
indicates that there is a difference in the voltage between the two points. No
voltage drop equals no difference – IOTW, no voltage is being lost between the
battery and the load.
This can be a good check to perform on headlights which don’t seem to be as
bright as they should be. Here the voltage drop to the high beam of a
mid-Eighties BMW 735i is being measured with the engine running. As can be seen
on the meter, the voltage drop is 0.434V. Obviously zero would be better but
anything less than about 0.5V is usually deemed to be satisfactory.
Another voltage drop that can be measured is during engine cranking. This is
an especially good test of the earthing system of the battery and engine and
again, it can be easily tested.
Place one probe of the meter on the negative battery post and the other on
the engine earth strap. Prevent the car from starting (eg by pulling the main
EFI relay) and then crank the engine with the starter motor. In this BMW the
voltage drop between the neg terminal of the battery and the engine earth strap
was 0.333V during cranking. Again, this is below the 0.5V rule of thumb. If it
had been a high reading, you could move the multimeter probe around (the one
that was connected to the earth strap) and try it next on the negative battery
clamp, for example. If there was a
major voltage drop across the clamp/battery post connection, this would indicate
that cleaning of this junction was required. By making the measurement at
different points it’s easy to isolate the area in which the major voltage drop
Measuring Other Stuff
Once you get used to using a good multimeter in car modification you’ll start
hankering after a meter than can measure more than just electrical values. Like,
wouldn’t it be great if you could measure peak boost, or other physical
parameters? Well, you can. We already briefly touched on temperature (many
multimeters will take the direct input of a thermocouple) but there are also
adaptors available that will measure nearly anything that’s measurable and then
turn it into units you can read on your multimeter.
Unfortunately many of these adaptors are quite expensive so you’re unlikely
to have them all in your toolkit – but having one or two becomes viable if you
do a lot of development.
DC current clamps are devices that have a moveable jaw that clamps around the
wire carrying the current. The clamp – which has its own internal battery – can
sense the amount of current flowing through the wire (yes, without even touching
it, let alone requiring that it is cut!) and outputs a precise voltage per amp
that it measures.
For example, a clamp might have an output of 1 millivolt per amp. This makes
measuring the clamp's output easy - if the multimeter shows a measurement of 5
millivolts on its voltage scale when connected to the operating clamp, the
current flowing in the wire is 5 amps. If the voltage displayed on the
multimeter is 100 millivolts, the current flowing in the wire is 100 amps.
Current clamps should be used primarily for measuring currents too great for
a conventional multimeter to handle – eg over 10 or 20 amps. This is because
their accuracy at small currents isn’t a strong point. But one of the beauties
of using them is that you don’t have to worry about blowing the fuse in your
multimeter if you’re trying to measure an unknown current.
For more on these designs, see Current Clamps.
Adaptors are available that output a precise voltage per unit of pressure –
for example, 10 millivolts per psi, or 1 millivolt per kilopascal. In the same
way as with a current clamp, you plug the device into your multimeter and then
read off the pressure. The beauty of the approach is that (if your meter has the
functions, of course) you can very easily measure Maximum, Minimum and Average
pressure levels. Turbo boost, fuel pressure, brake fluid pressure, oil pressure
– all can be measured in this way. If you have a data-logging multimeter (either
internal or through a PC interface) these measurements can also be logged.
As with the current clamp, is pretty well impossible to damage the multimeter
by too high an input (although of course you can always blow up the pressure
adaptor if you exceed its limits!) and the system is portable and safe. WRT the
latter, when measuring fuel pressure for example, the adaptor can be inside the
engine bay with long leads connecting its output to the multimeter in the cabin.
This means that dangerous fluids aren’t brought into the cabin.
The usefulness of pressure measuring is very high, primarily because you can
see details that would otherwise be impossible to find. For example, shown here
is the pressure measured between the throttle and the turbo after the throttle
has been abruptly closed. Without a blow-off valve, pressure waves bounce back
and forth as the pressure gradually dies away. This pattern of pressure change
is nothing like the one most people imagine happening in turbo cars without a
It’s not going to be top of your list, but a carbon monoxide probe is ideal
if you need to keep a watch on engine exhaust levels inside a workshop. Like the
other probes, it simply plugs into the multimeter, with carbon monoxide levels
read off as 1 millivolt equalling 1 part per million CO levels. Again, max, min
average and trends can be logged with the right multimeter and/or PC
Over this series we’ve shown you how to select and use a multimeter – from
the basics of on-car measuring of volts, ohms and amps to the more sophisticated
uses including measuring physical parameters like boost and temperature.
Summary? A good multimeter is one of the most effective tools in your