Back to the Basics: Resistance or Peak Voltage
Resistance and peak voltage tests are both valuable for testing electrical components, but each has its strength and weakness. Resistance testing, measured in ohms is a static test that measures a component's ability to resist current flow. Peak voltage is a dynamic test used to measure a component's ability to function at an acceptable level while operating in its normal environment. A peak voltage test is often used with AC voltages or DC voltages that happen quickly. Both tests can provide useful information about a circuit or components when applied properly.
To perform a resistance or peak voltage test accurately, the correct meter must be used. Resistance testing is performed using an analog or digital multi-meter. Performing a peak voltage test required a special DMM adaptor or a meter capable of reading peak voltages.
Resistance is simply the opposition to the passage of electrical current. The higher an object's resistance, the less current it will allow to flow for a given voltage. If more current flow is required, a higher voltage is needed or the resistance of the component needs to be lowered.
When an ohmmeter is connected to a component, it creates a circuit. The meter becomes the power source and it then measures the amount of current flow through the component being tested. The measurement is used to calculate the resistance Ohm's law and the result is displayed numerically. This is why a component that is being tested needs to be isolated from its normal circuit; otherwise, the meter will calculate the current flow through everything that is connected, distorting the results. A resistance test is popular because it can be quick and easy, but a single resistance test may not always determine if a component is good or bad.
A crankshaft position sensor, for example, can pass a resistance test but may fail while in use. That's because increased temperatures during normal operation increase resistance and reduce the peak voltage produced. Resistance tests should be used to test components that do not produce voltage, such as switched, relay contacts, and components with resistances that vary based on movement, such as fuel senders.
Peak voltage is the maximum voltage that a component can produce dynamically. Since some components produce voltages that occur quickly or at an irregular frequency, a meter capable of reading peak voltage is required for testing. A standard voltmeter averages the voltages over a period of time; this typically produces an erroneously low voltage result.
A peak voltage meter will display the highest voltage reading of a component, unrelated to time or frequency. Peak voltage testing can be done either loaded or unloaded. Unloaded testing will produce higher results. An unloaded test should always be followed by a resistance-to-ground test. A compont that's shorted to ground can give acceptable peak readings unloaded, but it will fail a loaded test.
Peak voltage testing an be done with either AC or DC voltage sources. As a general rule, components that use winding and a magnet to produce an electrical signal, such as a crankshaft position sensor, will produce an AC voltage. The output of CDI units and some sensors, such as a camshaft position sensor, will produce a DC voltage.
One of the biggest advantages of doing a peak voltage test is that the component in question can be tested under its operating conditions.
To diagnose a failed unit properly, it is important to choose the correct test to perform. Also, always compare the test results to the specifications in the unit's service manual or, if the specification is not available, to those of a known good unit. Incorrect testing or not performing all the required testing can lead to unnecessary parts replacement and wasted diagnostic time.