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Fuel Injector and Resistor Box Basics - Wrenchin'

Tell 'Em Apart, Hook 'Em Up And Look Smart Doing It.

Staff Report
Apr 1, 2008
Htup_0804_00_z+honda_injectors_and_resistor_box_guide+peak_hold_and_saturated_types Photo 1/7   |   Fuel Injector and Resistor Box Basics - Wrenchin'

For simplicity's sake, let's just say there are two kinds of fuel injectors: peak and hold and saturated. The difference between the two has to do with resistance. Peak and hold (aka low impedance) injector coils exhibit less electrical resistance than saturated (aka high impedance) injector coils. Typically, peak and hold Honda injectors will exhibit between 2 to 3 ohms resistance while saturated ones will measure somewhere around 12 ohms. This is all pretty useless and boring information until you do something like an engine transplant or go out and swap your stock fuel injectors for a high-flow set.

Injectors are really just valves controlled by the ECU by timed electrical signals called pulse widths. Honda fuel injectors rely on constant power for their pulse widths-the ecU supplies the ground-therefore, the injectors open and close based upon ground signals dictated by the computer. When ground is applied, they close and when it's not, they open. Some Honda ECUs are not designed to work with low impedance injectors. An injector or ECU swap or engine transplant can be all it takes to fry an otherwise perfectly good ECU-and that's exactly what can happen given the right circumstances.

2019 Honda Civic
$21,450 Base Model (MSRP) MPG Fuel Economy
Htup_0804_02_z+honda_injectors_and_resistor_box_guide+digital_volt and_ohm_meter Photo 2/7   |   Fuel Injector and Resistor Box Basics - Wrenchin'

The problem has to do with heat, which can be correlated to resistance in this case. For those Honda ECUs not designed for low impedance injectors, their injector drivers can heat up and fail because of the added current. Even the injectors themselves can heat up. With peak and hold drivers, the 12V is still delivered to the injector but current rises. Testing has revealed nearly double the temperature when using low impedance injectors on the wrong application. It's important to note that resistance decreases current and that current creates heat. If current can't be reduced then the chances of the drivers heating up and failing is increased. Of course, some of this depends on driving habits as well. High engine speeds and injector duty cycles can also create added heat, like when driving up a hill or at the track.

Determining which fuel injectors you have is simple. All you need is some sort of digital volt-ohm meter (DVOM)-even an analog one will work. Set the tool to read ohms and connect both of the DVOM's probes to the injector's two pins. The injector doesn't need to be installed to do this so just leave it on your workbench.

What do you do when you find out those low impedance injectors you ended up with aren't compatible with your ECU? This isn't really uncommon, by the way, since most aftermarket high-flow injectors are the low impedance type. The easiest fix is to add an injector resistor box. They aren't all that hard to find and can be sourced from many older Hondas or even Mitsubishis, like the Eclipse. A resistor box is nothing more than a simple device that steps down current between the ECU and fuel injectors. It's also not terribly difficult to wire up. For OBD-I vehicles, begin by locating the driver-side harness connectors near the brake master cylinder. Using your multimeter, locate the four wires that go to the injectors. Cut these four wires and connect the injector-side wires to the resistor box's black wires in any order. It's easiest if you score the resistor assembly's opposing plug to make the engine removal easier later on. Connect the resistor box's remaining power wire to all four of the remaining ends of the cut injector signal wires, which will give them their 12V power signal. That's it.

Resistor Boxes...Find Yours

1990-1997 ACCORD
1997 2.2CL
1986-1993 INTEGRA
1991-1995 LEGEND
1995-1997 ODYSSEY
1992-1996 PRELUDE

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