Ford Taurus Manual page 1038

UNDERSTANDING AND TROUBLESHOOTING ELECTRICAL SYSTEMS
Do NOT use a multimeter to probe electronic ignition spark plug or
coilwires. Small pin holes in secondary ignition wires will allow high
voltage toarc from the wire to a metal part, external to the secondary
ignition circuit.This arcing may cause misfiring, leading to a
driveability complaint.
Available Voltage Measurement
Set the voltmeter selector switch to the 20V position and connect the meter
negative lead to the negative post of the battery. Connect the positive meter lead
to the positive post of the battery and turn the ignition switch ON to provide a
load. Read the voltage on the meter or digital display. A well charged battery
should register over 12 volts. If the meter reads below 11.5 volts, the battery
power may be insufficient to operate the electrical system properly. This test
determines voltage available from the battery and should be the first step in any
electrical trouble diagnosis procedure. Many electrical problems, especially on
computer controlled systems, can be caused by a low state of charge in the
battery. Excessive corrosion at the battery cable terminals can cause a poor
contact that will prevent proper charging and full battery current flow.
Normal battery voltage is 12 volts when fully charged. When the battery is
supplying current to one or more circuits it is said to be "under load''. When
everything is off the electrical system is under a "no-load'' condition. A fully
charged battery may show about 12.5 volts at no load, will drop to 12 volts under
medium load, and will drop even lower under heavy load. If the battery is
partially discharged, the voltage decrease under heavy load may be excessive,
even though the battery shows 12 volts or more at no load. When allowed to
discharge further, the battery's available voltage under load will decrease more
severely. For this reason, it is important that the battery be fully charged during
all testing procedures to avoid errors in diagnosis and incorrect test results.
Voltage Drop
When current flows through a resistance, the voltage beyond the resistance is
reduced (the larger the current, the greater the reduction in voltage). When no
current is flowing, there is no voltage drop because there is no current flow. All
points in the circuit which are connected to the power source are at the same
voltage as the power source. The total voltage drop always equals the total
source voltage. In a long circuit with many connectors, a series of small,
unwanted voltage drops due to corrosion at the connectors can add up to a total
loss of voltage which impairs the operation of the normal loads in the circuit.
INDIRECT COMPUTATION OF VOLTAGE DROPS
1. Set the voltmeter selector switch to the 20 volt position.
2. Connect the meter negative lead to a good ground.
3. Probe all resistances in the circuit with the positive meter lead.
4. Operate the circuit in all modes and observe the voltage readings.
DIRECT MEASUREMENT OF VOLTAGE DROPS
1. Set the voltmeter switch to the 20 volt position.
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WARNING
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18.11.2005