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PCM in 1990 just ran engine systems and had some transmission control(if automatic)
Lights and fan have no connection to PCM
My guess would be you some how disconnected the ground for the fan motor, and motor is using running light ground so those bulbs glow when fan is on
Check fuel pump fuse/fusible link in engine bay, also check fuel pump relay
If your PCM was in the passenger side kick panel, then that could explain a few things, fan and fuel pump grounds are often in that kick panel, so you may have left them disconnected or pulled out a wire.
PCM also uses Ground to turn on Fuel Pump Relay
So common relationship in all this is....................................Grounds
And just another heads up, there should be a Ground strap from engine head to the firewall, this is the Main Ground for all cab electrics, it is often left off and forgotten, until you start having odd electrical issues
All body parts are painted first and then assembled, so bolts to other body parts or the painted frame are NOT good grounds, bare metal, clean connectors make for good grounds.
And if in doubt..........add a good ground, no such thing as too many grounds
Battery Ground to Engine, biggest cable, for starter motor and alternator
Battery ground to inner fender, smaller wire, for relays and starter solenoid, Jumper ground wire from this to Rad Support, for head lights
Ground strap from engine to Frame, for tail lights/trailer lights and some times fuel pump motor and fuel gauge sender grounds
Ground strap from engine to Firewall, for all cab electrics
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