Tools needed
Get your self a test light and a jumper wire. Next time the starter won't turn over, unplug the yellow wire going to the starter solenoid (small round thing attached to starter). Have someone turn the key to start and hold it there. Use the test light to make sure the wire is giving a signal to the starter when the key is turned. If so, the starter may be bad or need more amperage than the ignition switch is supplying. This is real common with rebuilt starters. If the yellow wireis not getting power when the key is turned, trace that back to the switch, On a 900, there are three troublesome connectors on the left kickpanel that bring the wiring in from the fuse box. Unplug them from under the dash and make sure there are no pushed out or burnt pins. The white connector there has bigger terminals off to one side that carry the starter signal and if they loosen up, they can burn enough to cut the amperage to the starter. If all that looks good, you may need to replace the electrical portion of the ignition switch in the shifter housing in the center console. That is a pretty big job for the first timer. On a 9000, usually on 90 or earlier, the electrical part of the ignition switch can also wear out. Symptoms it gives are stiffness when the interior of the car is hot or intermittant no starts and funny electrical problems. it is easily replaced by removing the four screws in the bottom of the steering column covers and then removing the two allen set screws holding it to the column. It lives directly behind the ignition cylinder (the part the key goes in). The electrical connector to it can be tight to remove. Note that you have to have the notches in the new switch aligned with the ignition cylinder to reinstall. A drop of LocTite on the set screws will keep them from backing out.
Automatic transmission cars have a neutral safety switch that can come out of adjustment. If the signal to the starter is missing when the key is turned on an automatic, move the shifter around Neutral or Park and see if the signal changes. If so, you need to remove the covers on the center console and adjust the switch so the signal is there when the car is in neutral or Park. Usually, if it is missing in Park, pushing forward on the shifter makes the switch make contact and sends the signal on to the starter.
If the yellow wire is lighting the test light every time the key is turned and the starter is not responding, take the test light and touch it to the big red wire on the starter. If it lights there, use the jumper wire from the big red wire on the starter to the terminal where the yellow wire attaches. Make sure the car is in neutral (or park if automatic) If the starter now works, it is simply requiring more amperage to pull down its solenoid than the ignition switch is supplying. You should first make sure that the battery cables (both positive and negative) are tight and clean. Trace them to their ends and clean and tighten any connections. If you have a voltmeter, a voltage drop test will pin point weak cables. Place one end of the voltmeter to each end of the cable and if the voltmeter shows more than 0.3 V DC, you should replace the cable as it has developed excessive resistance.If this doesn't eliminate the problem, and the starter does hit every time you touch the red battery wire to the terminal for the yellow wire at the starter, you can add an inexpensive generic Ford solenoid to the circuit and solve the problem for about $8 and 10 minutes time. Pep Boys or AutoZone or similar chain parts stores will have a Ford solenoid for less than $10. You will need some eyelet terminals to fit the terminals on the solenoid, ample wire to run from the battery to the solenoid and then to the starter and a female terminal end to fit the starter terminal that the yellow wire goes on. An inline fuse for the wire coming directly off the battery positive is not a bad idea either.
To wire in the solenoid, first find a suitable mounting point for the solenoid. The chosen place should allow room for the wiring so the wiring will be in no danger of grounding out and the solenoid should be able to be firmly and securely mounted so it won't move and so the mounting points provide the solenoid with a ground.
After the solenoid is securely mounted, run the yellow wire that went to the starter to one of the smaller terminals on the solenoid. Run a larger gauge wire directly from the positive battery terminal to one of the larger terminals on the solenoid. Finally run a wire from the remaining large terminal on the solenoid to the terminal onto the starter where the yellow wire was originally positioned.
We have done this to a number of cars and it works well and saves a lot of $$ and worry. Some rebuilders, notably Bosch are now including a label on their rebuilt units noting the amperage requirements to energize the solenoid. The trend we are noting is that the starters work ok when first installed, but after a few months, require more amperage and start the intermittant no start, especially when the car is or outside temperature is hot.
Modified Starter Wiring With Ford Solenoid