ECU Detects [P]/[N] or Other

The ECU controls idle speed based on whether the neutral signal is input (grounded). To account for torque converter drag, it increases the amount of bypass air and raises the idle speed. In the AT circuit, the [P] and [N] signal wires from the shift position console switch are combined inside the combination meter before being input to the ECU. This means the control changes when the position is [P] or [N].
After an AT-to-MT conversion, simply removing the AT selector lever leaves the ECU neutral signal ungrounded, which is equivalent to having the shift position in [1], [2], [3], or [D].
In other words, for engine control, the ECU distinguishes between [P]/[N] and all other shift positions.

Fixed Position Causes Problems

If the neutral signal is not grounded and the ECU determines that the shift position is something other than [P] or [N], normal driving is still possible, but the idle may run high and fluctuate, and engine speed may be slow to drop when the clutch is disengaged.
On the other hand, grounding the neutral signal so the ECU detects [P] or [N] resolves the idle and normal-driving issues, but apparently because the ECU interprets this as free-revving with no load, the rev limit is restricted to 7,000 rpm and VTEC will not engage (※).
Therefore, to operate the car normally with the ECU still configured for the AT, it must detect [P] or [N] while stopped and switch to another position ([1], [2], [3], or [D]) once the car starts moving.
If retaining the AT wiring, the ground connection at connector [[430]] can be switched between [N] and [D], but manually operating a switch while driving is inconvenient.
It should also be possible to switch the connection automatically once a certain vehicle speed is reached, but if going that far, rewriting the ROM data for the MT may be quicker.

※ If the shift position is anything other than [P], the key-lock solenoid operates and prevents the key from being removed, so [N] is the best position to use after the MT conversion (there is no particular difference if unnecessary parts such as the key-lock solenoid and interlock control unit have been removed).
Also, regardless of AT or MT, vehicle speed must be at least 5 km/h and coolant temperature at least 60°C for VTEC to engage.

1. Shorting the [N] signal and [アース] on the 12-pin connector ([6] and [8] of [[430]]) grounds the ECU neutral signal, causing the ECU to detect neutral.
   To remove the driving restriction, disconnect [6] and [8] according to vehicle speed so that the neutral signal is no longer grounded.
   If retaining the AT wiring, one possible method is to switch the connection for [8] from [6] (shift position [N]) to [1] (shift position [D]).
2. The inhibitor switch conducts electricity when the shift position is [P] or [N], preventing the engine from starting in other shift positions where it does not conduct.
   Shorting the vehicle-harness side ([1] and [2] of [[431]]) allows the engine to start.
   

The backup-light wiring is completely different between the AT and MT, so the lights will not illuminate when reverse gear is selected without modification.
On the AT, the lights illuminate when the shift lever is placed in [R]. On the MT, the backup light switch inside the transmission detects when reverse gear is engaged and turns on the lights.
It may seem that new wiring must be run for the lights, but the issue can actually be solved with surprisingly simple wiring.

1. After the AT-to-MT conversion, four connectors near the MT are left unused. Using the 2-pin connector for the "Lock-Up Control SOL.V" makes it easy to route the newly added backup light switch wiring into the cabin.
2. Modify the wiring near connector [[33]]. Short the Lock-Up Control SOL.V [2] wire (red/white) to the [3] wire (green/black) leading to the backup lights.
3. Connect the Lock-Up Control SOL.V [7] wire (white/black) to IG power. IG power uses a yellow wire, and several can be found nearby. I connected the power wire for the TCS, which was no longer needed.

On the AT, a signal is sent from the shift console switch to the ECU when the shift position is [P] or [N], while on the MT the signal is sent to the ECU from the neutral switch on the transmission itself.
If the ECU remains configured for the AT, the absence of a neutral signal can cause problems such as unstable idle, so it would be undesirable for the signal to be determined simply by whether the transmission itself is in neutral.
Therefore, the transmission neutral switch is connected to the ECU when the ECU has been replaced with an MT unit.

The AT also has no wiring for the neutral switch, but as with the backup light switch, one of the unused connectors can be used.

1. Use the 3-pin connector for the "Linear Solenoid" (a 3-pin connector with only two pins used) to route the wiring into the cabin.
2. Connect the [9] wire (gray) of connector [[33]] to the wire (light green) leading to the ECU through connectors [[4]] and [[154]], and connect the [8] wire (black/white) of connector [[33]] to body ground.

The clutch switch signal is used by the ECU to determine whether the clutch is engaged or disengaged.
In the factory ECU's MT control, this signal changes the deceleration fuel-cut control. When the clutch is disengaged, fuel is controlled to match the condition in which the engine is disconnected from the drive wheels.
If the clutch switch is not wired after an AT-to-MT conversion, the ECU cannot recognize that the clutch has been depressed, so deceleration fuel-cut control may continue under conditions that assume the clutch is still engaged. As a result, when the clutch is disengaged while transitioning from throttle-off to idle, the timing for recovery from fuel cut differs from the intended MT control. This can cause engine speed to drop unnaturally or temporarily dip and become unstable as idle resumes.

Normal driving may still be possible without wiring the clutch switch, but the factory ECU's MT control distinguishes between clutch ON and OFF. To reproduce the intended MT control, the switch should therefore be wired.

There are two types of clutch switch: a 3-pin type for the Coupe and a 2-pin type for the NSX-R. I wanted to retain cruise control, so I used the 3-pin Coupe type, which provides two signal outputs.
One of the two signal wires connects to the ECU, and the other connects to the cruise control unit.

1. On an AT vehicle, the wiring to the cruise control unit runs from connector [[430]] of the shift position console switch to the unit.
   Connect the clutch switch NC contact ([2] of [[416]] if following the wiring diagram) to [5] of connector [[430]].
2. On an AT vehicle, the wiring to the ECU ends at connector [[4]] immediately before it, so run a new wire from the clutch switch NC contact ([3] of [[416]] if following the wiring diagram) to terminal [12] of connector [[4]].
3. Connect [1] of clutch switch [[416]]) to ground (the wiring diagram specifies ground point G401, but any suitable ground is fine).

1. The interlock control unit is mounted to the left of the AT control unit behind the seats when viewed from the front.
   This unit is fitted only to AT models. It only imposes shift-position-related restrictions, such as preventing the engine from starting unless the shift position is [P] or [N], and preventing the key from being removed unless the shift position is [P]. It is no longer needed after the MT conversion, so it is removed.
2. The key-lock solenoid physically locks the key to prevent it from being removed and is mounted below the driver's key cylinder. It may be retained if you have another use for it, but because it is fitted only to AT models, I removed it.
3. Removed unnecessary parts. In addition to the interlock control unit and key-lock solenoid, I also removed the backup chime mounted to the back of the cover beneath the steering column (the chime that sounds when the shift position is placed in reverse).