Chrysler Repair: is it my starter or battery?, safety interlock switch, solenoid switch

Question
i have and '87 buick century and when i try and start it, it won't turn over or do anything.  There is power everywhere else, so i was thinking if it was my starter or battery or something else.  
thank you for your time and knowledge.
JB

Answer
Hi Jeff,
I will have to assume that G.M. uses the same wiring concepts as Chrysler. To begin with, many times when the starter won't operate it has to do with the safety interlock switch that is operated by the gear shift lever. If it is slightly out of adjustmet relative to the lever then the switch won't close so as to allow the starter relay to energize and send power to the solenoid switch on the starter motor. So first try slightly off-setting the gear shift lever from the park and the neutral positions while turning the key to the start position. If that causes the starter to now work then that switch needs to be moved slightly in its mounting. It is usually on the transmission where the mechanical connection from the lever in the cabin connects to the shift rod protruding from the transmission (have a helper mover the lever while you look at the trans to find out what moves and then see if you see a switch with wires that is associated with that motion.
Then electrically there are numerous possiblilities:
The battery cable clamps have to be clean and in good contact with the battery posts, so remove the - post first, clean the surfaces, then remove the + post and clean its surfaces, then replace the + post cable, finally replace the - post cable (in that order, for safety). One of the wires from the + post cable goes to the starter motor where it provides current, and it has to be attached firmly to the motor by a nut. So if you can follow that wire to the starter motor do so and make sure that it isn't loosely attached. But don't tighten the nut with a wrench without first removing the - post clamp from the battery, again for reasons of safety.
Then there is the ignition switch, the starter relay, and the starter solenoid. The relay is probably mounted on the inner fender wall inside the engine compartment on the driver side. It clicks softly when the ignition key is turned to "start" position. So open the hood and see if you can hear a soft click when the helper tried to start the car: if not, then the ignition swiitch or the relay may be bad. If so, then the wire from the relay to the solenoid switch mounted on the starter motor or the solenoid switch itself may be bad. If you hear a louder click from the area of the starter motor when the helper tries the key then everything up and too the motor is o.k and the problem will be either the starter motor, the switch contacts of the solenoid, or the battery is too weak to cause motor to crank. You can only determine which with a battery tester or a voltommeter to test the motor proper.
Here is how I summarized the similar problem on a Chrysler:
"In general, if you know the starter and the battery are good, then the other components are: ignition switch, starter relay, transmission neutral safety switch, and the wires that connect the whole system together. You might try verifying that the safety switch is not out of adjustment: try shifting the gear selector slightly off center from its resting position in Neutral (or Park ) while trying the 'start' position of the key switch. If that doesn't cause the starter to respond, then open the hood and look for a row of relays   
Have a helper try to start the car and see if you hear a soft click from any relay, if so it is the one for the starter motor. Then if so, then try the key again and listen for a louder click coming from behind the engine, which is the starter solenoid. If both are clicking then the main power wire from the battery to the starter is loose or has a poor connection or the starter motor is bad. If neither click then the starter relay or the ignition switch is bad. If only the relay clicks, then the solenoid is bad or the wire from the relay to the solenoid is "open". But write back and tell me about the 'click' noise test, for further instruction."
Those are the basic components, so open the hood and do some listening and some checking of the electrical connections, and of course check the shifter safety switch which may be out of adjustment.
Roland