1990 Ford Aerostar check engine light won't always start

I have 1990 aerostar 3.0 v-6 battery was hooked up backwards replaced starter solenoid and fusible link ran fine for 2 months fuel pump went bad and replaced at same time check engine light came on when I turn key on check engine light doesn't always come on then van wont start have to keep turning ignition switch on and off till light comes on then it will start! Also replaced fuel pump relay and pcm relay tried reading codes from computer but reader says cannot read codes in either mode tried both! Also when its running check engine light flickers fast. Can power control module be bad and how can I test to be sure?