89 kenworth starting problems

Make sure all your cables are tight ,if one is loose and not clean they wont get a good contact to the starter.

 

I assume the starter is a 12volt Delco 40MT, 42MT, or 50MT.
Battery positive on large stud on top of the solenoid.
Battery negative on the large stud on the rear of the starter. Also earth wire from here to a small stud on the solenoid.
Starter energising wire on another small stud on the solenoid. This wire goes to a starter slave relay, usually nearby on the chassis or firewall etc. This relay has a positive cable on its other stud. The start button operates this relay which closes its contacts and powers up the energising wire to the main starter solenoid. The starter will then crank the engine.

To test the system connect a voltmeter to the main positive and negative studs on the starter. Voltmeter should read at least 12volts.
Press the start button and see if the voltage drops. If the starter doesn't work, but the voltage drops substantially you have problems in the main battery cables, isolation switch, or batteries.
If the voltage doesn't drop by much, move the positive voltmeter lead to the solenoid energising wire.
Press the start button again.
If the voltmeter reads about 12volts with the start button pressed, the starter is faulty.
If the voltmeter doesn't show any voltage it means that the relay is not working. This could be a faulty relay or wiring to it from the start button.

Check for dirty connections on the copper link from the solenoid output stud to the starter fields. I have also seen this link break on Cummins engines.

 

You can remove the stater solenoid, inside there is a contact disk which makes contact at 9:00 & 3:00. You can rotate this disk one quarter turn to engage new contact surfaces to get you going.

Good Luck