A bad starter relay on your Ford vehicle will prevent the necessary electrical power from reaching the starter motor; fail to cut off power; or keep the motor from starting at all. Fortunately, all Ford models have a remote relay near the battery, which makes it easy and convenient for troubleshooting. This guide will help you determine whether the starter relay on your Ford has gone bad, using a jumper wire and a multimeter.
Disable the ignition system. If your Ford comes equipped with a distributor, unplug the ignition coil from the distributor, and ground it to the engine with a jumper wire. If your vehicle is not equipped with a distributor, you can remove the fuel pump relay or fuse for these tests.
Have a helper turn the ignition key to "Start." You should hear a clear click coming from the relay. If not, make sure there is a good connection at the control circuit wire on the relay; this is the small wire on the terminal marked with an "S." If you hear a weak or chattering click, continue to Step 3. If you hear a good click, skip to Step 4.
Connect a jumper wire from the positive battery terminal to the "S" terminal on the relay. Ask your assistant to turn the ignition key to "Start." If you can now hear a good click, the relay is working properly. If there is a weak or chattering click, or no sound at all, make sure all the connections on the relay are clean and tight. The relay mounting bracket should make good contact with the vehicle body. If all the connections are clean and tight, yet the starter relay still does not work, then replace the relay.
Check the voltage drop on each cable connection at the starter relay. Touch the probes of a multimeter on both sides of the same connection at the relay. The positive probe on your meter should always be on the side of the connection leading to the positive side of the electrical current. Ask your helper to again turn the ignition key to "Start." The multimeter should register no more than 0.2 units of voltage drop on each connection, according to Ken Freund in "The Haynes Automotive Electrical Manual." Otherwise, make sure the connections are clean and tight. If you still read a high voltage drop, replace the relay.
Unplug the wire at the "S" terminal on the relay. Check the resistance between the terminal and the relay mounting bracket using the multimeter. If there is more than 5 ohms between these two points, clean the mounting bracket. If you still read a high resistance, replace the relay.
Check for incoming voltage at the relay "S" terminal. Connect the red probe of your multimeter to the relay terminal and the black probe to a good ground such as a metal bracket or bolt attached to the engine block or cylinder head. Ask your helper to turn the ignition key to "Start." Your meter should register voltage during the test; otherwise, make sure the "S" connection and wire are clean and well-connected. If your voltmeter still registers no voltage, replace the relay.