Need help on 66 engine harness wiring

[ScottH]

I'm in the process of replacing the engine harness that runs from the firewall plug to the starter, temp gauge, coil. PO had bypassed the purple wire to the starter so I've testing the wiring inside the cab.

I've noticed that 1 of the wires on the cab side, the black 1, has constant power going to it. That wire goes to the starter terminal that the positive battery cable connects to. The constant power is present when battery is connected, ignition off, and it is not connected to the engine harness being replaced.

If I'm reading the wiring diagram correctly it appears to originate from the gauge cluster. The truck does have factory gauges, thinking may have something to do with the amp gauge.

So my question - should the black wire have constant power, especially considering it is eventually connected directly to the positive battery cable?

Any help, suggestions, or info would be most appreciated.
TIA

[ray_mcavoy]

Yes, that is correct. That black wire that's connected to the starter solenoid (same terminal as the positive battery cable) is for the amp gauge in the factory gauge cluster.

And yes, it normally has power all the time.

This circuit was not protected by any fuses from the factory, which isn't exactly the safest thing. So I usually recommend adding a 4 amp inline fuse just like GM did with the 67-72 trucks. You'd get the most protection of the wiring by locating the fuse down near the starter but that isn't always practical so the next best thing is placing it up near the firewall plug.

The amp gauge circuit also connects to the bus bar on the horn relay (via a black wire with a white stripe). That side of the circuit is live all the time and not fused either. So again, adding another 4 amp inline fuse (near where that wire connects to the horn relay) will make the circuit much safer.

[ScottH]

Ray, thank u so much for that info, it was exactly what I was hoping for.
Really appreciate it, happy & healthy holidays to u & urs.

Scott