I get asked quite frequently about issues that people are having with their fuel gauge in 97 trucks that are LS swapped.
I'm going to try and compile all of the info that I have into this new thread to make it easier for everyone and so that I won't forget steps.
Ok I know this info to be fact for 97 trucks and blazers, I don't know about 96 models and I know that 95 and older and 98 and newer are NOT the same.
-- THE BASC ISSUE
97 trucks have a fuel level buffer module that controls the gauge as the gauge is NOT controlled by the PCM.
-- HOW TO FIND THE MODULE
The fuel level buffer module is located under the dash on the passenger side of the truck. You can access it by removing the lower cover under the passenger side of the dash. The buffer module is a circuit board inside of a black plastic housing, wrapped in some foam crap, and stuck somewhere to the back side of the glove box.
First off, if your gauge worked with the stock engine, but not with the LS engine, then there is a 97% chance that the issue is in the wiring.
The pin letters are marked on the plastic casing if I remember correctly. These are the wires that you need to check at the board.
(Pink - Pin A) - Ignition switched 12v - Check this for 12 volts with the key on.
(Gray - Pin B) - 5v reference from PCM - Check this for 5 volts with the key on.
(PPL/WHT - Pin D) - Level sensor wire from tank. - Check the resistance on this wire. This one comes directly off of the level sensor in the tank. It has a resistance that varies from 20ohm to 250ohm (roughly). More fuel in the tank = a lower resistance and less fuel in the tank = a higher resistance. You can also shake the truck and watch the resistance move because it's unbuffered at this point so the fuel will slosh around and move the float.
(BLK/WHT - Pin M) - Ground this to the chassis. - Check and make sure that this is grounded. This wire goes out to a 2 pin connector (C121) right beside connector 120 under the hood behind the pcm, near the blower motor. There are two wires in this plug but you only need to hook the ground up.
Here is a quick diagram that I thew together a while back for someone to help them with the wiring. Notice the pin letters beside the wires, very helpful in finding the correct wires.
Pin B on the buffer module is the 5v reference signal from the PCM. It goes through the dash and into Connector 120, in Pin R (GRY), located behind the PCM near the blower motor under the hood. From there it ties into the MAP sensor 5v reference on the PCM. Which pin this is on the PCM I guess depends on the PCM you are running.
So in short you need:
1. 12 volts going to the module to power it.
2. Ground on the module via Pin M.
3. 5v input from the MAP 5v reference on the PCM.
4. Level sensor input from the level sensor in the tank.
5. Output to the gauge.
-- ADDITIONAL INFO
The computer doesn't need to know how much fuel is in the tank for a 97 model truck unless you have something like an overhead console with a mode button that shows fuel left in the tank and what not.
If you want the computer to be aware of the amount of fuel for some reason, then just hook up Pin D from the buffer module, into your fuel level sensor input pin on your PCM. Pin D on the buffer module runs through the dash and into Connector 121. It's the purple wire that shares the 2 pin plug with the ground wire for the buffer module.
I know this is poorly written and very long but hopefully it will help clarify somethings and help everyone out.