Which wires should I keep?

[Steve Cole]

Which engine bay wires need to stay and which can be deleted when doing a LS swap? I'm thinking about wiring for the factory tach, HEI ignition, starter, temp sensor, alternator, etc. Mostly concerned about wires coming through the firewall. I will be using a wire harness from BP-Automotive but I'm not to that point yet.

Thanks

[mongocanfly]

your tach wire will feed from ecm...I used the wire going to hei for my "switch on" wire [needed by ecm]...same wires for starter on sb will work on LS...temp sensor can still be used on passenger head, the ecm will use temp sensor on driver side head....the ecm wire harness will have its own alternator plug..but the "big" wire still needs hooked up,i connected that to the junction box on the firewall.......

[Steve Cole]

Thanks for the reply Mongo! Can you provide a little more info on the "switch on" wire? What does it connect to? Also, on my tach, does the "coil" wire connect to the ECM and then I just provide the 12V and Ground wires?

Thanks,

[mongocanfly]

when you get your harness youll have 4 wires that have to be hooked to the truck...youll have a constant 12v, a switch on 12v, and a couple grounds...in my swap harness there was a tach wire but I don't have a tach so im not sure exactly how it hooks up
for my switch on wire I just used the wire that went to the hei...I pulled it back thru the firewall...

[davepl]

The ohly wires I can remember keeping are the purple crank (solenoid). Everything else is supplied from or to the ECM harness.

Tach will go to expansion header on the harness.

You might have a wire or two like the coolant sender, but very, very little.