need help 1987 GMC pickup

[shaperx]

I have a 1987 GMC pickup 350 auto with TBI
turns over will not start. Just did a tune up it ran fine drove 30 miles had the check engine light come on. Disconnected Negative side of the battery to reset the on board computer. Hooked battery backup truck turns over will not start.  Code is #12

[Captkaos]

12 means there is no codes, did it flash 12 6 times?

[1980c10]

check for spark and check for fuel getting to the cylinders (plugs should be wet). Then post your findings here.

[shaperx]

 I have dual tanks they are not switching from right to left. Replaced the switch on the dash still not switching. Can it be the FUEL TANK VALVE SELECTOR SWITCH? What is the best way to check this.

[Captkaos]

I am guessing you verified you are not getting gas to the TBI?

[shaperx]

No fuel to TBI. No electric fuel pump noise. Fuel gauge on dash not switching from right to left tank. If the fuel tank selectory valve is bad do the fuel pumps still work? here is a picture

[Captkaos]

not necessarily.

[VileZambonie]

Each pump is fed through the selector valve. They recieve current from the fuel pump relay which feeds the switch. The switch moves the current through the appropriate terminal on the selector valve which then goes to the selected pump.

[1980c10]

Although I don't believe there is a fuse for this may want to check anyway. Otherwise I do believe the circuit may run through a relay for your fuel pump and possibly an oil pressure switch; either of these could prevent your fuel pumps from turning on if defective. Maybe someone here can give you more model specific info.