Testing Continuity with a Test Lamp
Always refer to the diagnostic procedure when testing low reference circuits. Low reference circuits are often capable of carrying only a minimal amount of current. Using a test lamp on certain low reference circuits may be inappropriate. Diagnostic procedures will identify the correct tool, DMM or test lamp, to use when testing low reference circuits.
When using a test lamp, it is important to understand the impedance of the test lamp. A test lamp with high impedance, such as an LED, will illuminate with very little current flow. A test lamp with low impedance, such as an filament bulb, will require more current flow to illuminate.
When testing any ground or low reference circuit, you must understand the purpose and current carrying capacity of the ground or low reference circuit being tested when choosing an appropriate test lamp. If the ground or low reference circuit is for a high current device, such as a blower motor, using a high impedance test lamp may indicate a good ground or low reference circuit because the current required to illuminate a high impedance test lamp is much less than the current required to operate the component. You can determine the current carrying capacity of a ground or low reference circuit by referencing the circuit gauge in the schematic or connector end view. Larger gauge wires have a high current carrying capacity.
The recommended test lamp and the test lamp referenced in all diagnostic procedures is EL-35616-200 . This is considered a high impedance test lamp that utilizes a filament bulb, but does provide more load than an LED.
Test lamps do have limitations when measuring a high resistance condition:
- High impedance test lamps may illuminate with up to 200 Ω present on the ground or low reference circuit
- Low impedance test lamps may illuminate with 20 - 30 Ω present on the ground or low reference circuit
It is important to remember that if a test lamp illuminates, there still may be a high resistance condition present. This is especially important when using a high impedance test lamp to test any ground or low reference circuit or when using any test lamp to test a ground circuit with a high current carrying capability.