Chapter 42 - Circuit Testers and Digital Meters
Electrical Power and Grounds
A 12 volt test light can be used to check the following:
Logic Probe
A ____________________ must first be connected to a power and ground source such as the vehicle battery. This connection powers the probe and gives it a reference low (ground).
High-Impedance Test Light
A __________________________________ has a high internal resistance and therefore draws very low current in order to light. High-impedance test lights are safe to use on computer circuits because they will not affect the circuit current in the same way as conventional 12 volt test lights when connected to a circuit.
Supply Power or Ground.
A fused jumper wire can be used to help diagnose a component or circuit by performing the following procedures. _______________________________________If a component, such as a horn, does not work, a fused jumper wire can be used to supply a temporary power and/or ground. Start by unplugging the electrical connector from the device and connect a fused jumper lead to the power terminal. Another fused jumper wire may be needed to provide the ground. If the unit works, the problem is in the power side or ground side circuit.
Fused. Alligator Clip Ends. Good-Quality Insulated Wire.
A technician-made fused jumper lead, which is equipped with a red 10 ampere fuse. This fused jumper wire uses terminals for testing circuits at a connector instead of alligator clips. It should include the following features.
Grounds.
A test light can be used to check for grounds by attaching the clip of the test light to the positive terminal of the battery or any 12 volt electrical terminal. The tip of the test light can then be used to touch the ground wire. If there is a ground connection, the test light will light.
Fused.
A typical fused jumper wire has a blade-type fuse that can be easily replaced. A 10 ampere fuse (red color) is often the value used.
DC Volts (DCV). AC Volts (ACV). and Range.
A voltmeter measures the pressure or potential of electricity in units of volts. A voltmeter is connected to a circuit in parallel. Voltage can be measured by selecting either AC or DC volts.
Unacceptable = 1.00% Okay = 0.50% (1/2%) Good = 0.25% (1/4%) Excellent = 0.10% (1/10%)
Accuracy for a DMM is usually expressed as a percent of reading. An accuracy of ±1% of reading means that for a displayed reading of 100.0 V, the actual value of the voltage could be anywhere between 99.0 V and 101.0 V. Thus, the lower the percent of accuracy is, the better.
Frequency (hertz, abbreviated Hz) Temperature probe (°F and/or °C) Pulse width (millisecond, abbreviated ms) Duty cycle (%)
Additional features for advanced automotive diagnosis include:
Alligator Clip Ends.
Alligator clips on the ends allow the fused jumper wire to be clipped to a ground or power source while the other end is attached to the power side or ground side of the unit being tested.
Root-Mean-Square (RMS)
Alternating current voltage waveforms can be true sinusoidal or nonsinusoidal. A true sine wave pattern measurement will be the same for both _______________________________ and average reading meters.
LED Test Light
An ________________________ uses a light-emitting diode (LED) instead of a standard automotive bulb for a visual indication of voltage. An LED test light requires only about 25 milliamperes (0.025 ampere) to light; therefore, it can be used on electronic circuits as well as on standard circuits.
OL, meaning over limit or overload. Flashing or solid number 1. Flashing or solid number 3 on the left side of the display. Flashing or solid number 4 on the display.
Different meters have different ways of indicating infinity resistance, or a reading higher than the scale allows. Examples of an over limit display include:
cycles per second
Frequency is a measure of how many times per second a signal changes. Frequency is measured in a unit called hertz, formerly termed ___________________________
Mass Air Flow (MAF) Sensors for Proper Operation. Ignition Primary Pulse Signals when Diagnosing a No-Start Condition. Checking a Wheel Speed Sensor
Frequency measurements are used when checking the following:
Electrical Power.
If the test light lights, then there is power available. It will not, however, indicate the voltage level or if there is enough current available to operate an electrical load. This indicates only that there is enough voltage and current to light the test light (about 0.25 A).
Mega (M)
If the value is over 1 million (1,000,000), then the prefix ________________ is often used.
Pickup Coils, Half-effect sensors, and Magnetic sensors
Most logic probes also make a distinctive sound for each high- and low-voltage level. This makes troubleshooting easier when probing connectors or component terminals. A sound (usually a beep) is heard when the probe tip is touched to a changing voltage source. The changing voltage also usually lights the pulse light on the logic probe. Therefore, the probe can be used to check components such as:
Good-Quality Insulated Wire.
Most purchased jumper wire is about 14 gauge stranded copper wire with a flexible rubberized insulation to allow it to move easily even in cold weather.
100% pulse width indicates that a device is being commanded on all of the time. 50% pulse width indicates that a device is being commanded on half of the time. 25% pulse width indicates that a device is being commanded on just 25% of the time.
Pulse width is the amount of time in a percentage that a signal is on compared to being off.
Milli (M)
Sometimes a circuit conducts so little current that a smaller unit of measure is required. Small units of measure expressed in 1/1,000 are prefixed by ______________.
The meter applies roughly a 3 volt DC signal to the test leads. The voltage is high enough to cause a diode to work and the meter will display: 1. 0.4 to 0.7 volt when testing silicon diodes such as found in alternators 2. 1.5 to 2.3 volts when testing LEDs such as found in some lighting applications
The meter is able to test diodes by way of the following:
Range.
The range is automatically set for most meters but can be manually ranged if needed.
A 3 1/2-digit meter can display three full digits ranging from 0 to 9, and one "half" digit that displays only a 1 or is left blank. A 3 1/2-digit meter will display up to 1,999 counts of resolution. A 4 1/2-digit meter can display up to 19,000 counts of resolution. It is more precise to describe a meter by counts of resolution than by 3 1/2 or 4 1/2 digits. Some 3 1/2-digit meters have enhanced resolution of up to 3,200 or 4,000 counts.
The terms digits and counts are used to describe a meter's resolution. DMMs are grouped by the number of counts or digits they display.
DC Volts (DCV).
This setting is the most common for automotive use. Use this setting to measure battery voltage and voltage to all lighting and accessory circuits.
AC Volts (ACV).
This setting is used to check for unwanted AC voltage from alternators and some sensors.
DC volts AC volts DC amperes (up to 10 A or more is helpful) Ohms (Ω) up to 40 MΩ (40 million ohms) Diode check
Try to purchase a digital meter that is capable of reading the following:
OL
_____ meaning over limit or overload
Meter Resolution
___________________________ refers to how small or fine a measurement the meter can make.
Electrical Units
_______________________measured in numbers such as 12 volts, 150 amperes, and 470 ohms.
Diode Check
a meter function that can be used to check diodes including light-emitting diodes (LEDs).
AC/DC Clamp-On Digital Multimeter (DMM)
a useful meter for automotive diagnostic work.
Logic Probe
an electronic device that lights up a red (usually) LED if the probe is touched to battery voltage. If the probe is touched to ground, a green (usually) LED lights.
Inductive Ammeters
do not make physical contact with the circuit. They measure the strength of the magnetic field surrounding the wire carrying the current, and use a Hall-effect sensor to measure current.
Kilo (K)
means 1,000
Ammeter
measures the flow of current through a complete circuit in units of amperes.
2 Volt Test Light
one of the simplest testers that can be used to detect electricity.
Continuity Light
similar to a test light but includes a battery for self-power that illuminates whenever it is connected to both ends of a wire that has continuity or is not broken.
Test Light
simply a light bulb with a probe and a ground wire attached.
Digital Multimeter (DMM) & Digital Volt-Ohm-Meter (DVOM)
terms commonly used for electronic high-impedance test meters.
Meter Accuracy
the largest allowable error that will occur under specific operating conditions.
Fused Jumper Wire
used to check a circuit by bypassing the switch or to provide a power or ground to a component. A fused jumper wire, also called a test lead, can be purchased or made by the service technician.
