4th wire on 94 Blazer Fuel Sending unit?

[usar17]

Ok, Some may remember im currently swapping out the entire driveline and wiring from a 94 5.7L blazer into my 85 K20. This maybe the wrong website to ask this but...

The wiring that goes to the fuel sending unit has a 3 prong plug. So I google'd 1994 fuel pumps to see if the 3rd prong was just ground. Well a 1994 sending unit has a 3 prongs connecter AND a stand a lone ground wire, making 4 wires from the top of the sending unit. Any idea what this 4th wire is for? The connector goes Purple for tank level, Grey id assume is positive still, then there is another black. My assumption is 2 grounds...

 

[RetroC10Sport]

My diagrams are not showing the standalone wire so no idea what that is for but the black/white wire is ground and gray is 12V+.

 

[usar17]

The rain here finally stopped and I was able to trace it. So there are 3 wires that go from the engine compartment back. Purple signal wire from the instrument cluster, Grey wire from the Fuel Pump Relay, and Black wire from a gang ground. Apparently the Fuel sending units on the GMT400s had 2 grounds, 1 from the frame near the tank and one from the engine block.....hell if I know why.

 

[RetroC10Sport]

That is kind of weird, I don't remember if my 1989 is like that.