B737-300
Flight Crew Air Transport Pilot (ATP)
Confusion; Distraction; Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Critical
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Returned To Departure Airport; General Maintenance Action
Aircraft
Took with flaps 15 no bleed takeoff with 900 foot stopping margin.  At 3;000 FT; the First Officer reconfigured the bleeds and packs.  Climbing through 4;000 FT got a Main Pressurization Controller failure.  Noticed we were not pressurizing; so I slowed my assent and ask to level off at 8;000 FT.  First Officer ran checklist for Main Controller failure and went to standby and got a green light.  I saw there was no bleed pressure from either engine.  Went back to APU bleed and had bleed pressure.  Still had no pressurization; so I checked the outflow valve was closed and cycled the pack and noticed no APU bleed pressure drop to the left pack. Tried the right pack and no bleed drop.  At 8;000 FT and the cabin was at 5;000 FT climbing.  Checked engine bleed circuit breakers and they were in.  Thought it was a bleed trip problem so ran the checklist and could not get the engine bleeds back with trip reset or engine anti-ice on. Still had APU bleed but I could not get the packs on.  Asked the First Officer if he had any other ideas and he said no.  The First Officer ran the landing numbers for our weight and we returned to the departure airport.  Notified Operations; Dispatch; and the Flight Attendants.  Made a 500 foot per minute descent and landed.  Maintenance found a computer in the EE compartment had come un-racked.  They re-racked the computer and the bleeds and packs worked again.
After departure a B737 bleeds and packs would not operate to pressurize the aircraft. After troubleshooting at 8;000 FT they returned to the departure airport where Pressurization Control Computer in the E&E was found unracked.