What Shift Inhibit is for
Automatic shift logic reacts to what it sees. On a drag car, a burst of wheel spin looks like a big jump in speed, and the controller may try to grab the next gear far too early. On a road car you might never want a particular down shift to happen above a certain speed. Shift Inhibit lets you name specific shifts and set a condition each must meet, and the ECU blocks any listed shift that doesn't meet it.
It only acts when it's enabled and when the shift in question is on the table's left axis. Every other shift is left alone. It can veto any shift request, with one exception: spool-up gear shifts aren't subject to it.
How the table works
This isn't a normal lookup table. The left axis is locked to the Shift Inhibit Axis, and each row on it is a specific shift, such as 1 → 2 or 4 → 3. The value in each row is compared with your chosen Shift Point Variable:
- Up shifts are allowed only when the variable is at or above the value in the table.
- Down shifts are allowed only when the variable is at or below the value in the table.
You can mix up shifts and down shifts in the same table. Any shift that isn't listed skips the check entirely.
A drag race example
Suppose you use Race Time as the variable and list only 1 → 2, 2 → 3 and 3 → 4. If the 1 → 2 row holds 1.2 seconds, the car can't take second until 1.2 seconds into the run, no matter what wheel speed does. Set the other rows in the same way to match where each shift normally happens on a good pass. Because 4 → 5 and every down shift aren't in the table, they behave exactly as they would without Shift Inhibit.
Skipping gears
When the controller wants to jump more than one gear, the ECU checks every step along the way. A request from 2nd to 4th is tested as 2 → 3 and then 3 → 4:
- If 2 → 3 is blocked, nothing happens and the car stays in 2nd.
- If 2 → 3 passes but 3 → 4 is blocked, the car goes to 3rd and stops there.
Reading the Shift Inhibit Status channel
When a shift doesn't happen the way you expected, log the Transmission Control Shift Inhibit Status channel. It reports the result of the most recent check:
| Log value | Shown as | What happened |
|---|---|---|
| 0 | No Request | Nothing has been checked since power up |
| 1 | Allowed | The last shift went through as requested |
| 11 | Not All: skip disabled | A multi-gear jump was asked for, Allow Gear Skip is off, so it moved one gear only |
| 12 | Not All: Down-shift Over-Rev | Fewer down shifts than requested, limited by over-rev protection |
| 13 | Not All: Shift Inhibit | Fewer gears than requested, limited by this table |
| 22 | Blocked All: Down-shift Over-Rev | No shift at all, stopped by over-rev protection |
| 23 | Blocked All: Shift Inhibit | No shift at all, stopped by this table |
| 24 | Blocked: Gear Doesn't Exist | The requested gear isn't there, such as Reverse on a box without one |
Our advice
- Don't use Shift Inhibit to stop over-revving on down shifts. The Block Down-shift Over-Rev feature is built for that and is simpler to set up.
- For drag cars, Race Time is the natural variable. It stops wheel spin from triggering early shifts, which is the most common reason drag racers use this table.
- List only the shifts you need. Every row is a shift that can be refused, so an overfilled table is an easy way to end up stuck in a gear.
- Log the status channel on the first runs so you can see which feature stopped a shift, rather than guessing.
Troubleshooting
| Symptom | Usually |
|---|---|
| Table seems to do nothing | Function not enabled, or the shift isn't listed on the axis |
| Car stuck in one gear | An up shift value that's never reached, or a down shift value set the wrong way round |
| Status reads 11 on a skip | Allow Gear Skip is disabled, not Shift Inhibit |
| Spool-up shift ignores the table | Expected. Spool-up gear shifts can't be inhibited. |
Related transmission pages
Shift Inhibit sits alongside transmission control, auto mode shift points, rpm shift and shift to first. The race timer page covers the Race Time channel.
FAQ
Can Shift Inhibit block down shifts as well as up shifts?
Yes. List the down shift on the axis and it will only be allowed while the variable is at or below the table value.
Why did my car take 3rd when I asked for 4th?
The 2 → 3 step passed but 3 → 4 was blocked. The status channel will show 13 in that case.
