1996 chevy 3500 truck fuel gauge will read 1/2 when first started then in 30 seconds with go to empty and stay

Could be the gauge in the cluster, or a wiring problem, or the sending unit in the tank. The sending unit grounds the gauge.

It could be a bad fuel gauge in the dash unit, but....
Usually it's "The bad".
The fuel level sending wipe arm and windings on the fuel sender wear and send erratic readings.The only cure is a replacement fuel gauge sender.
The check engine light should not be related to the erratic fuel readings.
"The good".
If you have over 200,000kms or 120,000 miles on your van you need a new fuel pump anyway,so have a complete assembly put in which will have a new fuel sender on it.

If this was not a problem before the pump replacement then the sender in the tank may of faied or is contaminated with sulfer which is corrosive to the silver electrical contacts.

Do a search on sulfur corrosion and fuel sender units. Gm has big problem with this. Its possable the stress from removing the old and installing the new pump created an issue with contacts. If this is the case you will now need a sender unit.

chevy and gmc trucks have an on going problem with this,turn the key on,and get a rubber mallot or a hammer and hit the bottom of the fuel tank a couple of times,this will unstick something in the fuel pump/sending unit.

COULD BE THE FUEL SENDER OR YOU HAVE A POOR GROUND POINT IN THE CABIN.
WEIRD AND MULIPLE MALFUNCTIONS ARE USALLY A POOR GROUND.

Fuel levels are equaled in the tanks via a frame mounted transfer pump that transfers fuel from the rear tank to the front tank where the main pump is. Your transfer pump needs replaced, and should run about 60 bucks. Should find the pump mounted to the frame between tanks.

Could be a
1. Fuse - not likely, I'd expect other instrument panel problems.
2. Broken wire from fuel gauge
3. Bad fuel sender in fuel tank. Normally requires dropping the tank.

Vehicle is not that old, would think it is still under warranty?

sounds like the float isnt sitting proplery in the tank

Is......the problem--"the gas gage reads empty when there is fuel in the tank. OR, The gage moves when the key to turned off ?

If it's the first,
You will need to test the gage and through process of elimination, determin if the Sending unit inside the fuel tank has gone Bad.

If its the second ? - Electrical gauges will move when powered down. Normal

Testing the gauge may not be easy - just depends on your ability.
It is a simple 2-wire gauge circuit, when disconnect (usually need the fuel tank) the gage reads in one direction (either full or empty)
then when shorted-the gage reads in oppisite direction.

Of course you must select the correct wires or you may short out the wrong circuits!! Most GM vehicles use a purple wire & black wire.

A quick method I have used to check the in-take part of the gauge is this.
1st. know there is gas - 1/2 tank works best.
2nd driver's door open, key in run position, engine not running.
3rd gently lift up on the door then let if come back down, gently.
just enough to cause the gas to slosh around.
4th - keep an eye on the gas gage, a bad sending unit in the take will show up as a "twitchy" needle.

Of course, if the gage never leaves empty - sloshing won't work.
I have a known good sending unit I can plug into the vehicle and simulate different levels of fuel - if the gas gage works, I know the problem is in the tank.

Let us know.

Dave

This is a normal concern.
The instrument cluster uses 4 different operating modes to calculate the fuel level:
Anti-slosh (default mode)
Key OFF fueling
Key ON fueling
Recovery
After a fuel fill up, the time for the fuel gauge to move from empty (E) to full (F) ranges from 2 seconds to 55 minutes depending on which operating mode the fuel gauge is in.
The default fuel gauge mode is called the anti-slosh mode. To prevent fuel gauge changes from fuel slosh (gauge instability due to changes in fuel sender readings caused by fuel moving around in the tank), the fuel gauge takes approximately 55 minutes to go from empty (E) to full (F).
The key OFF fueling mode (2 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The key must be in the OFF position throughout the entire refueling of the vehicle.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
The instrument cluster must receive a valid key ON fuel sender reading within 1 second of the key being put into the RUN position. The key ON sample readings are considered valid if the fuel sender reading is between 15 ohms ± 2 ohms and 160 ohms ± 4 ohms.
If these conditions are not met, the fuel gauge stays in the anti-slosh mode, which results in a slow to read full (F) event.
The key ON fueling mode (approximately 90 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The transmission is in PARK (P).
The key is in the RUN position.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
In key ON fueling mode, a 30-second timer activates after the transmission is put into the PARK (P) position (automatic transmissions). When the 30-second time has elapsed and at least 15% of the vehicle's fuel capacity has been added, the fuel gauge response time is 90 seconds to read from empty (E) to full (F). When the transmission is shifted out of PARK (P), the fuel gauge strategy reverts to the anti-slosh mode. The key on refueling mode prevents slow to read full events from happening if the customer refuels the vehicle with the key in the RUN position.
Recovery mode is incorporated into the instrument cluster strategy to recover from a missing fuel level input during a refueling event. Missing fuel level inputs result from intermittent opens in the fuel sender or its circuits. Recovery mode (empty [E] to full [F] approximately 20 minutes) is initiated when the following 2 conditions are met:
The instrument cluster is in the anti-slosh (default) mode.
The actual fuel level in the tank is greater than what is being displayed by the fuel gauge