thanks for your responses Jerry. I tried what you told me, but no change. The ground wire from the battery is properly connected, I made several tests to other parts of the engine block and fenders,the connection is ok.
But I made some observations that I did not know before.
1. when engine is running, check lights on, and I turn the fan-motor on higher speeds, check lights go off(almost everytime, 4 out of 5 times).
2. with check lights off(above condition) and hit the gas, check lights come on again and headlights
and interior-lights go brighter for a moment. My foot off the gas and everything is perfect again, no check lights. I checked the voltage during all of this, all normal values except with higher RPMs charging voltage goes up to 16.2 or 3.
I can't make any tests tonight anymore. Since the negative cable connection is not the problem, is there anything else you can recommend, Jerry? Thanks for all the help so far. Can I work with you tomorrow again, how does this work?