- 1). Park the Ford truck and turn the engine off. Allow the engine to cool for an hour. The engine must be completely cooled to provide a proper warm-up time to test the glow plug relay and receive an accurate reading.
- 2). Locate the glow plug relay under the hood of the truck. Look behind the fuel filter, on the passenger topside of the engine; this is where the relay is installed. You can identify the relay by two large wires, one yellow and one brown, attached to a large post.
- 3). Turn on the multimeter and switch the dial to the appropriate settings. The unit should be set to "DC Volts" and "15 Volt range." Connect the positive lead to the wire relay post. The positive lead is colored red. Connect the negative lead to a ground point, for instance a battery terminal. The negative lead is colored black.
- 4). Turn the ignition to the "On" position. Do not crank the key to start the engine, as you want the engine off. This will allow the computer to transmit a signal to the relay and allow the relay to start warming up.
- 5). Monitor the multimeter. You should hear a click and see some activity. Read the multimeter gauge or display; it should read between 10 and 15 volts. If you notice no activity or the volts are below 10, the relay should be replaced.