My 02 2.5ts has been having an intermintent problem starting the last couple of moths. It would start fine first thing, but after it was warm and taking short trips it would not start back up. Syptoms when it wont start were it would crank very slow for about half a second and then nothing. No clicks or cranking. Battery is 1 year old and tested perfect and alternator is not new, but did test out good. So I'm thinking the problem is the starter and/or starter solenoid? How can I know which one it is or where the problem is otherwise? all connections from what I can see to the battery/alternator/starter seem tight.

So when you have the problem the starter won't crank at all?  Does it occur only when the engine is "hot"?  If you leave it sit for a little bit have you noticed the problem?

 

The windings in the starter or the wiring going to the starter may be starting to degrade and once it heats up may give you intermittant issues.  I'd also suggest checking all the grounds coming from the battery to the chassis and starter as well as the power lead.

If you haven't already thoroughly cleaned the battery connections that needs to be done first. If that doesn't help then you need to check the main battery cables for bad connections on the other end and check for internal wire corrosion at the battery end. Voltage at the starter solenoid end should be around 10 volts while the starter is cranking the engine. It is also possible that the battery may have an internal connection problem when it is heated up. Checking the battery voltage while the problem is occurring will show where the trouble is at.

Ok, so it was only when it was hot at first and after I let it sit for a while it would be fine again, but now it just doesn't work. When cold it will turn over for maybe half a second and then nothing. All the lights and fan are on but no crank and no clicks. I will look into those suggestions though. Thanks

Since there is signs of things getting power the problem could be with the starter solenoid contacts. They can be replaced for around 12 dollars if you do the work yourself. If more than 10 volts is getting to the main battery lead tied to the solenoid and the solenoid is working then the contacts are suspect. If the solenoid isn't working in the START position then the inhibit or safety switch circuit needs to be checked along with the ignition switch connections to that circuit area.

does it start normally if jumped fom another car connected to the battery?

 

if not, def investigate the cables and solenoid/starter. If it does, more likely to be the battery or charging system.