Wiring the Wideband Sensor

Bosch LSU4 sensors have 6 wire connectors. The wire colors are shown below. The table lists the pin-outs for the LSU4 sensor (Bosch part number is 0 258 007 057). The pin numbers are imprinted in the connector shell.


The Bosch LSU4 Wide Band Sensor has 6 wires into the connector. These are connected to the 20-pin MIni-Fit Jr. connector on the Precision Wideband Controller as follows:

The green lead is connected to the calibration resistor (Rcal), and is not used with the Precision Wideband Controller. Most of the UEGO sensors, most notably the NGK and Bosch, include a calibration resistor in the wiring harness. The resistor is used in OEM applications for determining calibration, such that the sensor can be installed in mass production without interaction.

The “problem” with the resistor lies mainly in the fact that it is buried in the wiring connector, so the mating connector is a requirement in order to use calibration resistor. With all of the different target vehicles that use the UEGO sensor, the result is a plethora of connector versions, and each is unique.

In addition, in the configuration used by Bosch, the calibration resistor is connected in parallel to a known internal resistor (61.9 ohms), forming a new resistance in the 30 to 100 ohm range. This resistance is used to determine pump current in a differential amplifier mode. But the resistor is buried in the connector harness, and aging of the resistance will change the calibration. In addition, with the extra wiring to include the resistor into the circuit, there is more chance of inducing common-mode noise.

In order to eliminate the requirement for a mating connector for the particular sensor in use, we chose instead to use a fixed pump current measurement resistor (61.9 ohms). Using this known value, and using a free air measurement (that has been corrected for altitude/vapor pressure effects) the conversion required from pump current to lambda can be determined in software. For the rich side, representative numbers can be used (scaled by this measurement), but (of course) the best method is to determine the diffusion coefficients for carbon monoxide (CO), hydrogen (H2), and hydrocarbons (HC) by the use of known gas standards on a measurement test bench.

Accurate temperature control of the wideband UEGO probe is an absolute requirement during operation. Changes in UEGO probe temperature will result in a change in required pump current (from the difference in diffusion in and out of the measurement cavity), so monitoring the temperature allow for corrections to be applied to the measurements. The LSU probe does not have any form of direct temperature measurement (i.e. thermistor, etc.). However, monitoring the resistance of the reference cell yields a close representation of the probe temperature - the resistance of the reference cell varies with temperature. The Nernst reference cell has a high resistance at low temperatures (i.e. ambient temperatures) and a resistance of approximately 80-100 ohms at normal operating temperature. By monitoring the internal resistance of the reference cell, it is possible to determine an accurate UEGO probe temperature without the need of an external temperature sensor element.

There are several methods available to measure the resistance of the reference cell, including disabling the pump circuit and applying a known constant current across the reference cell and measuring the resultant voltage, finally re-enabling the pump circuit. This method requires several analog switches to apply the current and re-establish the pump servo circuit when done. Also, if a bias is applied to the Nernst cell, then an opposite polarity current with the same duration needs to be applied in order to “reset” the polarization on the cell. The one problem with this method is that it is “intrusive” to the feedback loop of the Nernst/pump.

Another method is to apply a high-frequency waveform to the pump circuit and measure the resultant deviation in EMF. The reference cell's resistance is determined by AC-coupling a square wave of known amplitude and frequency via a series resistance, and measuring the resultant AC waveform's amplitude. This waveform is always present, and since it is at a high frequency with respect to the response of the Nernst/pump feedback loop, it essentially averages out. This is the method employed in the PWB.

Circuit operation is very simple. A known square-wave source of 5 volts peak-to-peak and at a frequency of 1 to 3 KHz (generated by the DSP) is capacitively coupled to the reference cell positive terminal. Overall current is limited by a series resistance (plus Ri internal resistance) to 500 microamps peak to peak, or 250 microamps around the Vbias point (Vbias is set to 2.5 volts to allow for bi-polar pump operation) - this value meets the specification outlined in the Bosch LSU 4.2 data sheet. The alternating current signal generates a corresponding alternating voltage with value based on the internal resistance Ri. For example, if Ri = 100 ohms, then 500 microamps (P-P) multiplied by 100 ohms yields 50 millivolts p-p, or 25 mv around the Vbias point. Actually, the series current limit resistance and Ri form a resistor divider circuit driven by a voltage potential.

To measure the voltage, a capacitor is used to block the DC offset (i.e. reference cell voltage) and pass the alternating signal. A gain stage is introduced and the voltage is fed into a A/D port on a processor. Note that this signal is an AC signal, so ADC sampling needs to correlate with the polarity of applied square wave signal – this is known as synchronous rectification. An alternative method would be to use a bridge rectifier circuit to recover the positive/negative swings and then filter before application to the ADC channel.

A picture is worth a ton of words:

The remaining terminals on the 20-pin Mini-Fit Jr. connector are as shown below:

Sensor Installation:

Installation of the sensor in the exhaust flow must be at a point where representative exhaust gas composition is sampled, while also satisfying the specified temperature limits. Installation angle should be inclined at least 10° upwards from horizontal (electrical connection upwards). See the illustration below. This prevents liquids from collecting between sensor housing and sensor element during the cold start phase.

High exhaust pressures can cause the sensor to read incorrectly. In turbocharged applications, place the sensor downstream of the exhaust turbine where exhaust pressures are lower.

To operate properly, the sensor needs ambient air at its reference gas side. Replacement of the air volume inside the sensor must be guaranteed by a sufficient air permeability of the wires and the connectors between sensor and ECU. The breathability should be higher than 1 ml/minute at a test pressure of 100mbar. The sensor must be covered prior to applying any underseal (wax, tar etc.) or spray oil to the vehicle.

Under floor installation of the sensor remote from the engine requires an additional check of the following points:

- positioning of the sensor with respect to stone impact hazard
- positioning and fixing of cable and connector with respect to mechanical damage, cable bending stress and thermal stress.

Avoid excessive heating of sensor cable grommet, particularly when the engine has been switched off after running under max. load conditions. The use of cleaning/degreasing fluids (thinners or VarSol) or evaporating solids at the sensor plug connection is not permitted. Assemble with special high temperature resistant grease (i.e. anti-seize) on the screw-in thread (e.g. Bosch-No. 5 964 080 112 for the 120g tin). The LSU4 uses the same metric 18mm x 1.5mm threaded bung as virtually all other oxygen sensors, including narrow band sensors. The sensor should be torqued to 30-45 ft·lb (40-60 Nm).

If your car did not come with an oxygen sensor, you can add one. The thread for all oxygen sensors [including wide-band] is: M18mm x1.5mm, the same as 18mm spark plugs. So you can go to your local automotive parts store and in the section with all the HELP products, pick up a package of "18mm Spark plug Anti-foulers". Cut off the externally threaded part, and weld the rest onto your manifold or down pipe. This works well and you can do 2 cars for 4 bucks. Or you can go to muffler shop and ask for an O2 bung. They may have inexpensive machined 02 sensor bungs, and they can weld them in for you too!

The heater power must always be power controlled (i.e not connected directly to a 12 volt source), starting with a maximum ramp-up duty cycle. This reduces the thermal stress on the sensor element at cold start due to high peak power in the first seconds of engine running.

The active sensor ceramic element is heated up quickly when the heater power is switched on. This means that the sensor installation location must be selected to minimize exhaust side stressing with condensation water in order to prevent ceramic element crack.

Design measures:

- Locate sensor as close to the engine as possible, respecting maximum allowed temperature range
- The exhaust pipe in front of the sensor should not contain any pockets, projections, protusions, edges, flex-tubes etc. to avoid accumulation of condensed water. A downward slope of the pipe is recommended.
- Make sure that the front hole of the double protection tube does not point towards exhaust gas stream.
- Never switch on sensor heating control unit before engine start.

Before the sensor is operated, make sure the connections to the Precision Wideband Controller are good. If the signal of the reference cell is missing (e.g. connection failure), the internal control circuit can not operate correctly, so that:

The sensor cables must never be connected in the wrong way or wrong polarity, otherwise the sensor may be destroyed. The sensor can stay in the exhaust gas stream for a short time if the Precision Wideband Controller is not connected. Only after the sensor is connected completely should power be applied to the Precision Wideband Controller. Connect and disconnect the sensor only when the Precision Wideband Controller is powered down.

Note that the wide band sensor can be fooled in the same way as a conventional oxygen sensor by air leaks in the exhaust upstream of the sensor, as well as by misfires that allow unburned oxygen to pass into the exhaust. Either of these conditions will cause the sensor to indicate a false lean condition which, in turn, will cause the Precision Wideband Controller to adjust the fuel injection to run richer (usually way too rich!).

* We dedicate the Precision Wideband Controller to the memory of Garfield Willis. Garfield was instrumental in early research and development of the EGOR wideband controller.

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MegaSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a MegaSquirt® or MicroSquirt® controller is legal for your application.
©2004, 2010 Bruce Bowling and Al Grippo and Lance Gardiner. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of MegaSquirt® boards from Bowling and Grippo.