I see a couple of posts where someone is trying to install a CNG unit. I have just the opposite problem. I have a 1994 GMC, 5.7L throttle body, with a dual fuel CNG unit attached. I am removing the unit, because there is no CNG available in my area. Everything comes off fine until I get to the four wire plug that goes from the wiring harness to the pressure gauge on the CNG unit. When I unplug this, it shuts the fuel off to the vehicle and a few seconds later it dies. Plug it back in and its fine. Any ideas on what the problem is? Thanks, Larry

 

Sure. there's bound to be an interlock there that interfaces to the fuel pump.
Using an ohmmeter, probe each pin on the gauge side for connection to gauge body brass fitting, mark any that have zero resistance as "gnd" on the corresponding plug wire

then probe each pin on the gauge side for connection to each other pin. There will probably be two that show zero ohms. Mark the corresponding wires on the harness side as A and B

Now jumper A and B and see if the truck runs. If so, then you're done. If it still doesnt, connect the wire marked Gnd to the chassis. It shoudl run then.

 

I did just what you said and the vehicle started up. Thanks for taking the time to answer my question.
Larry