The only function of the kickdown relay is to ensure that the transmission shifts instantly and reliably at redline. The relay receives an RPM signal from the ECU at redline. The relay activates, opening the normally-closed contacts. This breaks the signal from the kickdown switch under the throttle pedal, causing the transmission to upshift instantly. As soon as the transmission shifts, the RPM signal disappears, the relay deactivates, the N-C contacts close, and you are back in kickdown mode (eager to downshift, reluctant to upshift).

Wally Plumley
928 Specialists