Low-impedance injectors, also referred to as peak-and-hold injectors, have a very low internal coil resistance, typically between 2 and 6 ohms (most commonly 2.5 to 3 ohms on vintage Honda setups).

Overview#

Unlike saturated circuits, peak-and-hold injectors are designed to open as fast as possible. Because of their low coil resistance, they allow a large amount of current to flow through the circuit when 12V is applied.

To control this current, true peak-and-hold ECU drivers use a two-stage current profile:

1. Peak Phase: The ECU hits the injector with a high current pulse (around 4 amps) to snap the heavy internal pintle open quickly.
2. Hold Phase: Once open, the ECU reduces the current to a lower holding level (around 1 amp) to keep the valve open for the remainder of the pulse width.

If you connect low-impedance injectors directly to a standard saturated driver (found in most OBD1/OBD2 Honda Civic and Integra ECUs) without current limitation, the excessive current flow will overheat and burn out the ECU's internal driver transistors within minutes.

The Resistor Box Solution#

To safely run low-impedance injectors on an ECU designed for high-impedance (saturated) injectors, you must install an inline resistor box (also called a resistor pack).

The resistor box adds approximately 6 to 10 ohms of resistance in series with each injector's power supply wire. This raises the total circuit resistance to around 10–13 ohms, mimicking a high-impedance injector. This limits the current to a safe level (~1.2 amps) and protects the ECU drivers, though it sacrifices a small amount of the opening speed advantage of a true peak-and-hold driver circuit.

Specifications#

Below are common Honda factory vehicles that came equipped with low-impedance injectors and their corresponding specifications:

Resistor Box Wiring Reference#

An OEM Honda resistor box (typically found mounted on the driver-side firewall of a CRX Si or Prelude) has 5 wires coming out of it:

• 1 Power Wire (Red): Receives +12V switched power from the main relay.
• 4 Output Wires (Black): Connect to the power side of each of the four fuel injectors.

                  +12V Switched Power (from Main Relay)
                            |
                     [ Resistor Box ]
                       (Red Wire)
                     /    |    \    \
                  [10Ω] [10Ω] [10Ω] [10Ω]  (Internal Resistors)
                   /      |      \      \
               (Black) (Black) (Black) (Black)  (Output Wires)
                 |        |        |        |
            [Inj 1]   [Inj 2]   [Inj 3]   [Inj 4]
                 |        |        |        |
             (To ECU Ground Trigger Pins A1, A3, A5, A2)

Installation Procedure#

Wiring a Resistor Box into a Saturated Harness (e.g., OBD1 Civic/Integra)#

1. Locate the Injector Power Joint: In a factory saturated harness, all four injectors share a common +12V power source wire (usually Yellow/Black) that splits near the intake manifold.
2. Cut the Power Wires: Cut the individual Yellow/Black power wires going to each injector plug. Leave the ground return wires (which go to the ECU) untouched.
3. Mount the Box: Mount the aluminum resistor box securely to the firewall.
4. Connect Switched Power: Connect the single Red wire of the resistor box to the main +12V switched power source wire coming from the vehicle harness.
5. Connect Outputs: Connect the four Black wires of the resistor box to the injector-side of the cut Yellow/Black power wires (one Black wire to each injector).
6. Solder and Insulate: Solder all connections and insulate them with marine-grade adhesive-lined heat shrink tubing to protect them from engine bay moisture.

Related#

• High impedance injectors
• Injector sizing
• Introduction to ECU chipping