Testing Output Devices

A few rules apply to testing output circuits:

NEVER use a test light with a hot resistance of lower than 20 ohms when testing output signals to computers.
Back-probing terminals is done when live circuit measurements are required. It is always better to back-probe a terminal than it is to pierce a wire's insulation to obtain a measurement. Insert a long straight pin, T-Pin, or meter probe onto the back to the connector to contact the desired terminal. Be careful not to damage any weather seals on the connector or terminal.

A live current reading taken over an extended period of time can check for an output device that malfunctions when warm or hot. Typically the output device's resistance will increase (current will decrease) as it gets warmer; however, if its resistance decreases (current increases), it indicates a problem with the device, usually an internal short. Use ohm's law to calculate the hot resistance of the device to determine wether or not the device resistance is out of specification.