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How to Electronically Modify Your Car, Part 4

Using a multimeter

by Julian Edgar

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At a glance...

  • Using a multimeter to measure...
  • ...volts...
  • ...amps...
  • ...ohms
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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?

Volts

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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.

Example Car Measurement: Measuring the Airflow Meter Output Signal

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Set the multimeter to measure Volts DC.

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Make sure that the leads are positioned to measure Volts. This is normally the default position.

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Carefully remove the boot covering the back of the airflow meter plug.

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Connect the negative lead of the multimeter to the chassis (ie ground).

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Back-probe the airflow meter plug terminals, one after the other in sequence.

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Here 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!).

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And 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.

And 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.

Amps

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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!)

Example Car Measurement: Measuring Current Draw

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Set the multimeter to measure Amps DC.

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Move the leads to the Amps measurement position. Normally, the negative lead (circled) stays in the same socket but the positive lead is moved.

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This 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.

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The fuse for the circuit being measured is pulled.

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The multimeter probes are applied to either side of the fuse.

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The current flow through the meter is then shown – here it’s 3.2 amps.

And 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.


Watts?

So how do you use a multimeter to measure watts? In short, you can’t!

As 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 two together.

So 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.

12.5 x 3.2 = 40 watts.

(For 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

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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

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Set the multimeter to measure resistance.

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Make sure that the leads are positioned to measure Resistance. This is normally the default position.

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Unplug the sensor, or as has been done here, remove it completely. Connect the probes one to each terminal and measure the resistance.

Measuring 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.

Rules

So the key points to remember are:

  • To measure voltages, the circuit does not need to be unplugged or broken – the multimeter probes go in parallel with the circuit. Measuring voltages is the measurement done most often.

  • Measuring current (amps) requires that the circuit be broken to allow the meter to be inserted – that is, the meter is used in series. One multimeter plug normally needs to be moved on the meter and it’s easy to blow up the meter’s internal fuse if you make a mistake.

  • When measuring resistance, the component being measured needs to be isolated (eg unplugged) from its circuit.

Other Parameters?

Multimeters 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.

Doing It

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.

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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.

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

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