Idle Control Wiring

Idle Control Wiring

This article explains the most common types of idle air control valves and how to wire them up using a Haltech Nexus or Elite ECU:

Bypass Air Control (BAC) Two Wire



This type of idle valve is the simplest to wire up, being a solenoid with just two pins. One pin connects to a switched +12V supply, and the other pin connects to an ECU output, typically a Digital Pulsed Output (DPO), but it could also be any ECU output capable of Pulse Width Modulated (PWM) control. Generally, the amount of idle air this valve bypasses depends on the duty cycle the ECU applies to the output. However, it is worth noting that some of these valves have a "wax pellet," which physically allows more air to flow into the manifold when the engine is cold and less air when the engine has warmed up. This means the same duty cycle command from the ECU will give you different idle RPM levels depending on the engine temperature, so you usually don't need to add a lot more duty cycle to the valve when the engine is cold compared to the average duty cycle percentages when the engine is at operating temperature. You can find this type of idle valve on many Ford, Nissan, Mazda, and Honda engines.


Note: Some solenoid-type BACs are not polarity-sensitive, but it is recommended to follow the factory wiring diagram for guidance on which pin should be supplied with power and which should be used for output control.

 

Bypass Air Control (BAC) Three Wire



The next type of idle valve is a three wire device which is a rotary motor actuator. On these valves, the middle pin connects to a switched +12V supply, and the two other (outer) pins connect to two ECU outputs capable of doing PWM. With no control applied to the valve, it flows about half of its maximum air capacity. When the ECU pulls one of the outer pins to ground, it closed the valve. When the ECU pulls the other pin to ground, it opens the valve. These are driven by alternately switching each side to ground at a specific frequency. With the valve wired using the correct polarity, engine should receive more air (higher RPM) with a higher duty cycle, and less air (lower RPM) with lesser duty cycle. If it responds inversely, simply re-assign the main and the secondary outputs in the NSP software the other way around. These valves are common on European engines (i.e. Bosch systems) 4-cylinder Toyota engines, and some Subaru EJ engines. 



On some engines with this type of idle valve, you might find that one of the outer pins is connected to ground instead of an ECU output. If the factory wiring is configured this way, you must use the same configuration, requiring only one output to be connected from the ECU. This type of three-wire BAC includes an internal driver circuit and can be set up similarly to a two-wire BAC.





6-Wire Stepper Motor


The next type of idle valve, which is also fairly common, is a 6-wire stepper motor. This idle valve has two coils internally, and each coil has a "center tap" to switched 12V, allowing the coils to be driven in either polarity with ECU outputs that pull to ground (e.g., DPOs). Four out of the six wires on this idle valve need to be connected to ECU outputs and must be carefully assigned and paired. This means the outputs assigned as Stepper Motor Pin A and Pin B must connect to either end of one coil, and the outputs assigned as Stepper Motor Pin C and Pin D must connect to either end of the other coil. You can test which coils are paired internally by performing a continuity test on the stepper motor connector using a multimeter. With the correct wiring, the stepper idle valve will rotate open or close in discrete "steps" as the coils are energized by the ECU in a specific sequence. If the stepper motor rotates in the opposite direction, you can toggle the "Stepper Invert" setting in the NSP software to correct the control. This type of idle valve is common on Mitsubishi, 6- and 8-cylinder Toyota, and some Subaru engines.


Common 6-wire stepper motor wiring configurations

Note: The software pin function (i.e., Stepper Motor Pin A-D) must be wired according to the pin locations as shown on the connector above. However, any spare ECU output (e.g., DPO, INJ, IGN) can be assigned in any order or combination to the 6-wire stepper motor pin functions in the NSP software. All of the Outputs' Active States must be set to "Low".


Note: The software pin function (i.e., Stepper Motor Pin A-D) must be wired according to the pin locations as shown on the connector above. However, any spare ECU output (e.g., DPO, INJ, IGN) can be assigned in any order or combination to the 6-wire stepper motor pin functions in the NSP software. All of the Outputs' Active States must be set to "Low".


4-Wire Stepper Motor



This type of idle valve operates the same way as a 6-wire stepper motor, except it doesn't have the center tap connection, resulting in just four pins. For this reason, this idle valve requires specific outputs from the ECU capable of driving both high (12V) and low (ground) for proper coil sequencing, enabling the motor to rotate in discrete steps. For Elite ECUs, these will be the stepper motor output pins. For Nexus ECUs, these will be the Half-Bridge Outputs (HBOs). Basic Elite ECUs, such as the 550, 750, and 950, do not have stepper motor outputs, so this type of idle valve is not supported with these ECUs on its own. 

The four outputs from the ECU connected to this idle valve must be carefully assigned and paired. This means the outputs assigned as Stepper Motor Pin A and Pin B must connect to either end of one coil, and the outputs assigned as Stepper Motor Pin C and Pin D must connect to either end of the other coil. You can test which coils are paired internally by performing a continuity test on the stepper motor connector using a multimeter.




Common 4-wire stepper motor wiring configurations


Note: The software pin function (i.e., Stepper Motor Pin A-D) must be wired according to the pin locations as shown on the connector above. However, any Stepper Output (Elite E1000/E1500/E2000/E2500) or Half-Bridge Output (Nexus ECU) can be assigned in any order or combination to the 4-wire stepper motor pin functions in the NSP software. All of the Outputs' Drive Type must be set to "Both".

DBW (Drive-By-Wire)


Idle control using an electronic throttle, also known as DBW, is an option for ECUs that support this functionality. Examples of these ECUs include the Nexus R5, Nexus R3, Elite 2500, and Elite 1500. There is no additional wiring involved apart from wiring the DBW system components themselves (electronic throttle and pedal). In this idle control strategy, the ECU controls the throttle within the first 10% to 20% opening to modulate the incoming air when the electronic pedal is not pressed, i.e., no throttle demand.

Ignition Only

This idle control strategy doesn't involve wiring as it simply uses ignition timing or ignition advance to increase or decrease the idle RPM to a certain degree. Idle control using only ignition timing is generally not recommended if there is a physical idle control valve on the engine that can be used. However, it is typical for the ECU to use ignition timing correction to help stabilize the idle RPM in conjunction with the above idle control systems.

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