As in all automotive electrical diagnostic procedures, we always begin at the battery. NEVER assume, by thinking, or reasoning, that your battery (even though it may be brand new) does not need charged or tested. It only takes 30 seconds to throw a volt meter on the battery and note the reading. If you skip this step, you can spend hours, even days, running around in circles and never quite get anywhere.
1. Place a voltmeter on the battery terminals. After the battery has sat undisturbed for several hours, it should read no less than 12.6VDC. If the battery passes this test you can proceed to step 2. An even more thorough test is to take a volt reading after the battery has sat all night (or at least 8 hrs). This will test if the battery has the in-ability to hold a charge, which is particularly more common in cold climates, or if there is a parasite drain on the battery (sometimes a factor when the vehicle is equipped with after market stereos and burglar alarms).
If you suspect battery not holding a charge, or parasitic drain, it takes a little time, but you can use the process of elimination to determine what the problem is. Unhook the negative battery terminal. Take and record a volt reading. (this must be a charged battery 12.6VDC or over). Take a reading after 12 hrs of exposure to climate. 12.6VDC and higher the battery is good.
Perform the same test with the negative cable installed. If the battery is below 12.6VDC, then you know the vehicle is draining the battery.
2. After battery quality has been established we need to take and record a voltage reading at the battery.
Note: Before performing tests underneath the vehicle, you must securely block (chuck) the non drive wheels and safely jack and secure the drive wheels off the ground.
3. Remove the push on spade or ring terminal from the solenoid on the starter. This is the small post on the solenoid, usually a 10MM nut holds it on.
Some vehicles have a solenoid relay to feed the main post on the starter. Some are hot all the time. Take a voltage reading at the main post. This is the large copper stud in the solenoid. Usually there will be a 13-14MM nut on the back of it. No need to remove it at this time. Lightly sand or wire brush a spot so you can get a voltmeter probe on it.
Lightly sand or wire brush a spot on the back of the starter body so you can get a voltmeter probe on it. Take and record a voltage reading. Sometimes physical access to the hot post on the starter is difficult. You could use a long screwdriver to touch the hot post in the desired location, then put the probe for the voltmeter at the bottom shaft of the screwdriver.
You may need to turn the ignition switch to the “crank” position because some Fords have an extra relay solenoid on the positive cable. The voltage drop of the cables may not exceed .2VDC. If it does, you have a bad cable or connection. If the voltage drop test passes, you need to take the starter off the vehicle.
4. Do not hit the starter with a hammer. The magnets will break. You can, however, tap lightly with a ¾” wrench the back of the starter or the solenoid while some one holds the key on for you. If this kicks the starter in, you can assume you have a bad connection on the solenoid, or bad brushes in the starter.