vehicle tagged above. This should be incredibly simple, and it could be. But the GM wiring diagrams don't match the truck, and the flowchart for the DTC P1637 use circuit "names" that aren't on the wiring diagram.

Flowchart refers to "generator turn on signal circuit". Wiring diagram lists two wires "generator field duty cycle signal" (GRY) and "charge indicator control" (BRN).

There is also a THIRD wire at the connector not on the diagram. It is blue.

DTC P1637 says "Generator L circuit"

hmmmm. Pretty sure it just a bad alt, but, anyone know how the names are supposed to match the "other" names ?

I THINK "charge indicator control" = "Gen L" = "Gen turn on" after about an hour of research.

Geoff from Hawaii