oil light

Low oil pressure at low engine rpm. Could be main bearings, oil pump, plugged pickup screen, or just bad sender (snap switch).

To remove any doubt put a mechanical gauge in and check the pressure.

Coasty

here is another question: i have a 89 dodge dynasty 3.0l with more than 208kmile (it stop working at that time)

Just after i change oil, i notice my oil pressure sensor work for a couple seconds and dies. I check the oil and i confirm that my sensor says truth. I donto get oil on the top of the engine after a couple of seconds. did anybody see anything like this.

How can we detect a worn oil pump?

Don't know what you mean by "says truth". Use a mechanical gauge at the oil pressure sender port to check pressure. If it shows say 80 psi cold, replace the sender.

what i did is , i loosen the oil filter as the car runs? :) initially, first time i start the car, i get oil pour down, laterm as it runs lets say 1 minutes after, it did not pour as i loosen oil filter. I check the oi on the top of the engine by loking through the oil cap.

PS: I already change the pressure sender twice(from donors out of junk yards)

if you don't get oil pouring out of a loosened filter with the engine running, either the oil pump is gone, or the pump screen is totally plugged. Either way, the engine's probably now ruined. A diagnostic paper:

A good link

Another good link