TRUCK LOSSED ELECTRIC POWER , WAS DRIVING AND THE VOLTAGE WAS GOING DOWN

(1)EVAP Purge Solenoid Ignition Positive Voltage
(2)To Intake Manifold Port
(3)Fuel Tank Pressure Sensor
(4)Fuel Tank Pressure Sensor Ground Circuit
(5)Fuel Tank Pressure Sensor Signal Circuit
(6)Fuel Tank Pressure Sensor 5V Reference Circuit
(7)Fuel Filler Neck
(8)Rollover Valve
(9)Fuel Tank
(10)EVAP Vent Valve
(11)EVAP Vent Valve Control Circuit
(12)EVAP Vent Valve Ignition Positive Voltage
(13)EVAP Canister
(14)EVAP Purge Solenoid
(15)EVAP Purge Solenoid Control Circuit

check this out on your vehicle. also make sure there is no air lines leaking on the engine.

Your serpentine belt tensioner may be bad,not holding enough tensioner and not letting your alternator keep a charge on your battery,get some one to sit in your vehicle,open hood,have them start it while you watch the belts turn and have them rev the engine up to see if the belt turns loosely.another thing to look at is the A.C.compressor locking up .Power steering pump could be locking up,just check it out while it's running and see.

Are you getting a spark from the coil wire ?
Do you have voltage going to the coil and a ground signal from the computer ?

When you turn the ignition to run, the fuel pump only runs for three seconds, or until the starter is engaged or the vehicle starts. If you are turning the key on and crawling under the truck to check power, you won't have any. Have a buddy crank the truck while you are checking voltage to see what you really have going on. You are correct, the purple wire is for the fuel gauge. The grey wire is the fuel pump power. Also check to make sure the wire harness going to the fuel pump did not get pinched between the fuel tank and the frame. Have seen this happen after a fuel pump replacement.

No. It is a bad alternator. When they test the alternator they turn it at faster rpm. When it turns at faster rpm it is charging fine. What the problem is the voltage regulator in the alternator is going bad. When you turn your blinker on I bet the voltage shows on the gauge. Moves up and down with the blinker.

You could have one of several problems. Could be an electric fuel pump going bad or it could be your distributor module. More than likely it is the distributor module.

I don't see any problem unless you are having a starting problem or dim headlights..
What you are describing is pretty normal behavior.  The voltage regulator senses the battery condition and the electrical load on the battery.
If you are having problem anew battery will solve it.

sounds like your relay under the bonnet has gone im fraid, not too hard to change. if this is not the cause then the problem might be a little more difficult to sort out as this will prob be the power steering motor and is a bit of a swine to change.

i would suggest checking your fuel pressure first

It sounds like either alternator or computer, I f you can get it to run on its own pull the positive battery cable off the battery and if it dies its your alternator. And this is no real way to check the computer except going up to a AutoZone and they can check codes for free