Checking Wiring and Components
RESISTANCE OR CONTINUITY
Checking a portion of the wiring harness or a component as shown indicates whether or not there is a continuous conductive path-whether current can flow between the two test points. The example illustrates troubleshooting the idle and full-throttle switches in the fuel injection system.
CAUTION: Resistance measurements and continuity checks must always be made with all power to the circuit or component switched Off. When testing continuity in a circuit that is always powered (fuse/relay panel "30" circuit for example) disconnect the battery before testing.
Always use a digital (low current) meter. An ohmmeter, or the ohms scale of a multimeter, measures resistance bypassing a small amount of current through the circuit or component being checked.
Improper testing may damage sensitive electronic components.
Continuity (approximately 0 Ohms): Little or no resistance indicates that there is a continuous conductive path between the two test points. As shown, this is correct for the normally-closed idle switch in the idle position (switch not actuated). Also check that the switch opens (no continuity) when actuated by the throttle.
NO Continuity: There is resistance to current flow through the switch. In the example, this indicates that the normally-closed idle switch or the wires between it and the connector (T3) are faulty.
CAUTION: Direct contact with meter probes at the connector terminals can easily damage the small contacts, causing poor connections and risking future intermittent faults.
Audi Special Tools include adapters for making test connections safely and preventing connector damage. Tools and Equipment
Continuity (approximately 0 Ohms): Little or no resistance indicates that there is a continuous conductive path between the two test points. In the example, this indicates that the normally-open full-throttle switch is faulty.
NO Continuity: There is no connection-an open circuit. As shown, this is correct for the normally-open full-throttle switch. Also check that the switch closes, completing the circuit, when actuated by the fully-open throttle.
Voltage Drop
Checking voltage drop across connections or components as shown will indicate whether there is abnormal resistance creating an additional load in the circuit-consuming power and dropping the voltage available to other parts of the circuit.
NOTE: Voltage drop measurements can only be made when the circuit is powered and there is normal current flow.
The example illustrates troubleshooting the back-up light switch. The switch is in the circuit to switch power to the back-up lights On and Off. When Reverse gear is selected and the switch is closed, it should have very little resistance and not be a consumer.
If dirt or corrosion on the switch contacts creates resistance, some of the available battery voltage goes into overcoming that resistance. Less voltage is available to light the back-up lights, and they will not be as bright.
NOTE: An ohmmeter can measure resistance (or check continuity) only when the circuit is not powered, i.e. when there is almost no load.
CAUTION: Direct contact with meter probes at the connector terminals can easily damage the small contacts, causing poor connections and risking future intermittent faults.
Audi Special Tools include adapters for making test connections safely and preventing connector damage. Tools and Equipment
Low voltage: A low voltage reading across the two switch contacts indicates almost no difference in potential. Resistance across the switch is low-most of the battery voltage is passing through the switch and is still available to power the lights.
High voltage: Any significant voltage reading indicates a difference in potential across the switch contacts. Excessive resistance is loading the circuit, causing a voltage drop-voltage is consumed overcoming the resistance of the switch, and less is left to power the lights.
Maximum allowable voltage drops recommended by the Society of Automotive Engineers are:
- 0.0 volt for small wire connections
- 0.1 volt for high current connections (example: fuel pump, headlights)
- 0.1 volt for ground connections
- 0.2 volt for high-current cables (Example: battery/starter cable)
- 0.3 volt for switch or relay contacts