Where it fits
You will only see this node after turning on Shift to First Enable in Transmission Control. It exists for two common situations:
- Upshift-only sequential cars, usually drag cars. The ECU fires the upshifts, but the driver pulls it back to first by hand. The ECU has to be told that it is in first again, otherwise its idea of the current gear drifts away from reality. It does not matter which happens first, the driver's shift or the ECU's condition, as long as both line up.
- Automatic setups configured to only shift up on their own. Something has to bring the car back to first, and the obvious moment is when it comes to a stop.
The priority order
When more than one thing is asking for a gear change, the Nexus decides in this order:
- Spool Up Gear, if enabled
- Shift to First
- Paddle shifts
- Automatic shifting from the auto mode shift points
That means a valid Shift to First request beats the paddles. Keep your conditions tight so it never fires while you are driving.
Setting the conditions
Every car is different, so rather than a fixed rule, you build up to three conditions from any ECU channel:
| Field | What it does | Example |
|---|---|---|
| Number of Conditions | How many checks to use, from 1 to 3 | 2 |
| Select | The channel to test | Vehicle Speed |
| Maths | Equal, not equal, greater than, less than, or the or-equal versions | Less Than |
| Value | What the channel is compared against | A walking pace or slower |
| Combiner | Whether all conditions must be true together | AND with a second condition, such as throttle closed |
A typical auto setup is "vehicle speed below a crawl AND throttle closed". A drag car might combine a switch input the driver uses when selecting first with low speed.
Shift to First usually happens with the car stopped. On a manual or sequential with a clutch, make sure the driver has the clutch in before the shift is made, or the engine will stall or the car will lurch.
Troubleshooting
| Symptom | Usually |
|---|---|
| Car shifts to first while rolling | Speed condition too loose, or combiner set so only one condition is needed |
| ECU still thinks it is in 3rd after a pass | Conditions never all true at once. Check each channel in a log |
| Paddles seem ignored | Shift to First conditions are being met and taking priority |
Related: Shift Inhibit and RPM Shift Point.
