Building an NBO2 Voltage Divider

A narrow band O2 sensor (NBO2) is normally intended to only understands a 14.6/14.7:1 air fuel ratio (AFR), providing feedback to the ECM to richen/lean the fuel mixture as the AFR drifts off center. There are a few little tricks that can be used that will emulate an AFR of approximately 14.2:1. While this may not be an ideal AFR for idle and cruising for some riders, it is an improvement over the OEM 14.6:1 closed loop AFR used in the Harley-Davidson Delphi ECM's. These can be built with a few parts from Radio Shack or any other electronics parts supply.

In the Harley-Davidson O2 sensor wiring harness, the Blue wire is the (+) or sensor signal wire. The Gray wire is the (-) or ground wire. A basic voltage divider consisting of two resistors can be used to pass part of the voltage down the sensor signal wire and excess voltage into the gray wire. Using the diagram below as a the model for building, the desired location of the voltage divider is identified. Voltage dividers are a basic electronic circuit, common to any electronic device in use today.

The 'A' resistor is 2 times the resistance of the 'B' resistor passing 2/3 voltage or making a 66% Divider.

The "package" can be put together with barrel crimp connectors on the O2 sensor side. After putting it together, it should be placed in something to make it a vibration resistant, weather tight package. The pigtails can be butt jointed or used with quick disconnects. Adding a wire sleeve around the package after installing it on the wiring harness makes it a very neat, nearly invisible package.

Special Notes on the HD NBO2 sensor You should never solder a O2 sensor wire because the sensor "will breathe" through the cable. The acceptable method of splicing or repairing O2 wiring is with crimp connectors. This information was from Bosch Technical Staff in Europe. Here is the technical description of why you do not solder the connections on the O2 sensor side of the updated wiring. The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors insulation and have no vent hole. It's hard to believe, but the tiny amount of space between the insulation/wire and through wire insulation provides enough air to seep into the sensor (for this reason, grease should never be used on O2 sensor leads/connectors because it can block the flow of air). Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to eventually fail. A special thanks to Kristian Heidenfors for all his help and research on the HD Bosch O2 sensor, testing the voltage divider circuit and additional help on SE Race Tuner. Special Notes on the building these devices This device is covered by US Patent Law and WIPO Treaty Laws. Any commercial venture to manufacture, sell or distribute this device without appropriate licensing will be subject to legal action. We have no desire to prevent individual riders from building their own DIY circuits. But commercial ventures must obtain appropriate licensing. All Drawings and Instructions are covered under US and WIPO Copyright Laws. Instructions may only be used in ONLINE Format for personal use only. No commercial use is permitted.

The 66% voltage divider works by taking advantage of the most accurate range of the O2 sensor and some simple electronics to allow .750v output from the O2 sensor normally expected at 14.2 AFR to look like .500v to the fuel injections system. By appropriately changing the percentage voltage drops from other AFR's on the narrow band O2 sensor response chart, various fuel ratios can be emulated at by the ECM.

A 60% divider reduces .850v to .500v which can emulate a 13.8:1 fuel ratio.

A 63% divider reduces .820v to .500v which can emulate a 14.0:1 fuel ratio.

A 66% divider reduces .750v to .500v which can emulate a 14.2:1 fuel ratio.

A 70% divider reduces .775v to .500v which can emulate a 14.1:1 fuel ratio.

By testing various voltage drops from your O2 sensor to your ECM, you can ensure proper operation of your fuel injection system, test operation of the O2 sensor or evaluate the closed loop bias values in the OEM programming.

Evaluating changes to Software Tuning using dividers

Many riders of Harley-Davidson motorcycles use software tuning programs such as SE Race Tuner, SE Super Tuner, Master Tuner TTS and Techno Research to directly update the Delph ECM. The 2007 and later model motorcycles use closed loop EFI operation. When tuning a bike, riders are often faced with the choice of making a change to the closed loop bias tables or making changes to the VE and AFR tables to alter the fuel ratio of an engine. The ease and simplicity of tuning by maintianing a large closed loop operational area is both prudent from a time management perspective and that this method tends to be emissions friendly. In a tuning environment, it can be an advantage to be able to quickly test CLB table changes without loading the tuning software.

The testing of narrow band O2 sensors can also evaluate changes that will occur as the sensor ages. NBO2 sensor tend to produce higher voltages at the same AFR as they age.

This information is applicable to all motorcycles and automobiles using Narrow Band O2 sensors.

Building Temporary Installation H-D Voltage Divider for Test Purposes

If you can locate the appropriate Tyco/Amp weather pak connectors housing (P/N 282080-1, P/N 282104-1) and pins to match the O2 sensor and wiring harness, you can make a temporary voltage divider for ECM testing purposes. There are many companies that advertise precision voltage dividers circuits for sale on the Internet or make your own voltage dividers.

By placing a voltage divider between the appropriate connectors, the temporary testing device can easily be installed or removed on any vehicle.

But you may find that the special tools to crimp the connectors is very expensive or trying to crimp the connectors without the special tool is a very frustrating experience. Properly constructing a set of connectors is not quite as easy as it may seem at first glance.

Nightrider.com's new Modular Component System is one way to create an NBO2 Voltage Divider. These mix and match components are now available. This will allow you to build a variety of your own custom components for whatever purposes you need, including anything described in this technical article.

The advantage to this new modular component system is that there are a variety of interchangeable components that you will be able to add or exchange in the future. You can create your own voltage divider for a 13.8:1 fuel ratio now, but change it out for a set of test clips to test the O2 sensors in the future. Or maybe you want to install an inline AFR guage with bias adjust. It is just a matter of changing a component.

For a preview of the Nightrider Modular Component System go to
http://www.nightrider.com/parts/index_modular.htm