Electrical Test Equipment
VOLTMETERA voltmeter is used to measure electromotive force (EMF) or voltage (pressure) in an electrical circuit. Voltmeters are used to measure such things as battery voltage and alternator Output voltage. Voltmeters are used to check for voltage in a circuit, locate an open, or test for a voltage drop.
Voltmeters must be connected to a circuit in parallel. The voltmeter is connected in the circuit so that it parallels the existing circuit. The voltmeter senses and displays the voltage drop across the meter's resistance unit. A voltmeter has high internal resistance so that little current will flow through it.
Voltmeters can be used to measure the voltage drop across a load device or conductor. Voltage drop is the loss of voltage caused by the flow of current through a resistance. Increases in either resistance or current flow will increase the voltage drop. The total of all the voltage drops in a circuit should add up to the voltage of the battery.
Voltage drop is the most important electrical troubleshooting test
To check the actual voltage drop, use a voltmeter connected between the points where the voltage drop is to be measured. The positive lead (red) of the voltmeter is connected to the more positive (+) side of the circuit to be measured (the part of the circuit closest to the positive (+) side of the battery). The negative lead (black) is connected to the more negative (-) part of the circuit (the section closest to ground).
Another method of measuring voltage drop is to measure the voltage at two different points in the circuit and subtract one from the other. Whenever checking voltage drop, current must be flowing in the circuit.
Excessive voltage drop indicates high resistance or excessive current flow in the circuit. Voltage drops are commonly made to check wires and connectors for excessive resistance. A voltage drop of less than 0.1 volts is an acceptable value for a length of wire.
AMMETER
Ammeters measure the amount of current flowing in a circuit. An ammeter can be used to determine if current is flowing in the circuit or if excessive current is flowing due to a short circuit.
Ammeters measure current flow in amperes or amps. Current flow of less than an ampere is measured in milliamperes (1/1000 of an ampere).
The ammeter is connected in series in the circuit. In order to measure current flow, current must be flowing in the circuit.
Unlike the voltmeter, the ammeter has very low internal resistance. Therefore, connecting an ammeter in parallel with a circuit may cause too much current flow through it, damaging the meter.
AMPERAGE
Amperage can be a very useful way to determine what the total resistance of an electrical circuit is. To do this you must determine the current flow (Amperage) of the circuit. If an amperage value is not available, an approximate amperage value may be obtained by using the fuse rating. In a properly designed circuit the current flow (Amperage) should be approximately 75% - 80% of the fuse rating. A circuit with a fuse rated at 10 Amps should have approximately 7.5 - 8.0 Amps of current flow. This approximation will change if there is an electric motor in the circuit. Most electric motors will draw about twice the current when they start as they do when they are running, therefore an adjustment must be made for this. A circuit containing an electric motor that has a 10 Amp fuse would initially draw about 8 Amps when the motor first starts and then would draw about 4 Amps after the motor is running.
Once the approximate Amperage of the circuit has been determined, the resistance of the circuit can be found by dividing the measured battery voltage by this Amperage. Although this is an approximate value it can be a useful tool to determine if the resistance of the circuit is correct. This approximate resistance value may now be checked against the actual tested resistance of the circuit to determine if there is a problem.
Amperage tests will indicate a problem but will not pinpoint the exact location of the high or low resistance within the circuit. Additional testing (i.e., voltage drop) will be necessary to locate the actual resistance.
OHMMETER
An ohmmeter measures resistance in ohms. The ohmmeter is self-powered and must never be connected to a circuit that has current flowing or the meter will be damaged.
To measure the resistance, connect the meter leads to the terminals or leads of the conductor or load device. An infinite reading indicates an open circuit; a reading of zero indicates no measurable resistance which is often referred to as continuity. Continuity is little to no resistance and is measured on a higher scale of an ohmmeter. A reading between zero and infinity is the component's actual resistance to current flow. To obtain the actual resistance, multiply the meter reading by the ohmmeter scale. Example: a meter reading of 10 R x 1000 scale is 10,000 ohms of resistance.
NOTE: It is important to ensure that the ohmmeter is set to the correct scale for the device or item that you are measuring.
CAUTION: DO NOT CHECK CIRCUITS OR COMPONENTS THAT OPERATE ON MILLIAMPERES (COMPUTER CIRCUITS)WITH 12 VOLT TEST LIGHT OR SELF POWERED TEST LIGHTS. THE LOW RESISTANCE OF THE 12V TEST LIGHT BULB OR THE CURRENT OF THE SELF POWERED TEST LIGHT WILL DAMAGE THE CIRCUIT OR COMPONENTS. ONLY LED TEST LIGHTS OR A HIGH IMPEDANCE MULTI-METER SHOULD BE USED ON THESE CIRCUITS.
LED TEST LIGHT
An LED test light is used for checking voltage signals on components where an ordinary test light cannot be used, as it will not damage computer circuits. This is the only type of test light that should be used for testing any electrical circuit.
ECU CIRCUIT TESTER
TEST LIGHT
J-34730-2
This is another type of LED test light which is a bulb type tester that will plug into most two-pin electrical connectors to test the signal of an ECU circuit. This type of tester is ideal for fuel injection diagnoses and will not damage ECU's. The number listed is Kent- Moore's part number for this tester; however, they should be readily available through other tool and equipment sources.
JUMPER WIRE
A jumper wire is the simplest type of electrical troubleshooting tool, and is used to bypass a portion of a circuit. The jumper wire can be used to bypass switches and apply power directly to a load device. They can also be used to provide a good ground for the circuit or load device.
NOTE: A fused jumper wire would ensure that tested circuits arc not damaged.
CAUTION: NEVER USE A JUMPER WIRE TO BYPASS THE LOAD DEVICE. THE LOW RESISTANCE OF THE JUMPER WIRE IN PARALLEL WITH THE LOAD DEVICE WILL CAUSE EXCESSIVE CURRENT TO FLOW THROUGH THE CIRCUIT, WHICH WILL OPEN THE FUSE OR BURN THE WIRE.
THE INDUCTIVE AMMETER MULTIPLIER (x10)
An inductive ammeter multiplier (x10) can be used to improve the measurement of small current levels. For example, most ammeter scales will not visibly register the movement between 2 and 2.5 amps. Using the inductive ammeter multiplier (x10), this movement would read the change as 20 to 25 amps. The multiplier makes one amp look like ten amps to the meter. Therefore, any change in amperage appears ten times any actual change.
TO MAKE THE MULTIPLIER, use ten feet of #16 gauge wire, two alligator clips (25 amp type), and electrical tape. Proceed as follows:
1. Wrap the wire around a round tube (like an old ignition coil) exactly ten times, leaving approximately a foot of wire on each end for the alligator clips.
2. Remove the wire from the form and wrap it with electrical tape (the final product should look like a doughnut).
3. Attach the alligator clips to the two ends of the wire.
Figure 1 - Finished Inductive Ammeter Multiplier (x10)
USES OF THE MULTIPLIER
A. The inductive ammeter multiplier (x10) is installed in series with the item whose current flow is to be measured. For example, if a blower motor is to be measured, follow these steps:
1. Disconnect power lead of blower motor and connect one end of the multiplier to the motor.
2. Connect the other lead of the multiplier to the disconnected power lead.
3. Clamp amps probe around multiplier wire.
4. Ammeter will show the amps draw times ten (x10). If the meter registers to the negative side, reverse the amps probe. Readings will be ten times the actual amperes draw.
B. Check the draw of any accessory to compare it to specifications. For example, to test a rear window defogger, follow these steps:
1. Disconnect defogger's power lead.
2. Connect one end of multiplier to the power lead and the other end to the defogger.
3. Clamp amps probe around multiplier.
4. Turn on defogger.
5. Draw will register on meter ten times actual current draw.
6. Compare reading to specifications and take corrective action, if necessary.
C. To find a small current leakage, follow these steps:
1. Turn off all accessories.
2. Disconnect battery negative (-) cable.
3. Connect one end of the multiplier to the negative (-) battery terminal and the other end to the negative (-) battery cable.
4. Clamp amps probe around multiplier.
5. If any current is detected, disconnect individual fuses to find which circuit is causing the draw.
6. Once the circuit is found, go to Procedure D to find the source of the current leakage.
D. To use the multiplier in place of a fuse to check total current draw of each circuit, follow these steps:
1. Remove fuse from circuit and connect multiplier to fuse holder.
2. Clamp amps probe around multiplier.
3. Disconnect each accessory in circuit until amps draw drops to zero.
4. If reading does not drop, the leak will have to be isolated in the wiring harness.
5. If reading drops to zero, the last accessory disconnected is probably causing the leak.
CAUTION: DO NOT INSTALL MULTIPLIER IN A FUSE CONNECTION THAT HAS A BLOWN FUSE IN IT.