How to test and replace the solenoid on the starter on a 5.4L Ford F150

    The starter on my 2000 Ford F150 5.4L failed several times to start the engine. Each time I was able to tap the solenoid with a hammer to get the engine started. A starter for this truck costs from $125 to $175. The starter mounted solenoid is available for a mere $27. The solenoid is often the failure point on a starter, due to either a stuck plunger, bad switch or open coil.  In this post I will test the solenoid and starter out of the vehicle and replace the faulty solenoid.


Remove the starter. I have a separate post for that:  How to remove the starter from a 5.4L Ford F150

Testing out of the truck
    If desired test the starter and solenoid out of the vehicle. With the battery disconnected from the truck,  I used a universal battery terminal and attached a 12 gauge wire to it. The other end went through a 10A toggle switch and from there to the small (10mm) terminal on the solenoid. Using a set of jumper cables, I attached one end to the ground terminal on the battery and the other to the casing on the starter. Flip the switch and the solenoid should energize and the pinion gear move out. Mine was intermittent. Bad solenoid.

With power applied (11.85V) the solenoid failed to operate. Bad solenoid. 

    To test the entire starter, connect the positive cable on the jumper cables from the plus terminal on the battery to the larger (13mm) lug on the solenoid.

Using jumper cables and  a 12 gauge wire, hook up like this to test solenoid and starter. Toggle switch not shown. 

  
Remove the old solenoid.
1) Unwire the switch lug from the last (13mm) terminal on the solenoid.
2) Using an impact screwdriver or other impact tool break the two Phillips screws loose.  Turn them out till they stop. They are retentive screws.
3) Slip the solenoid plunger over the forked end of the pinion actuator and remove.

4) Now the switch in the solenoid can be tested with an ohm meter. Put the meter leads (in resistance mode) on the two large terminals on the solenoid. An open circuit will be measured now. Now manually push the plunger all the way in. At the extreme end of travel the meter will read less than an ohm. This is where the 12V (B+) from the battery will energize the starter windings.

The low ohm reading at end of travel tells us the internal switch is working

Procure a replacement solenoid. I used Napa part #ECH ST421  $27

Install the new solenoid.
1) Pull the pinion gear outward by hand and engage the disk end of the plunger with the fork of the pinion actuator.
2) Rotate the solenoid until the correct hole lines up with a screw. Make sure it is oriented so that the switch wiring fitting will fit over the 13mm switch lug on the solenoid.
3) Tighten the 13mm nut back on the switch terminal.
3) Turn in the solenoid mounting Phillips screws by hand and then give each a couple of good taps with the impact driver.

Give the impact screwdriver a couple of taps with a hammer

Remount the starter on the truck and test.

                                      Cheaper than                      Handy to have

                                         a starter!