Wideband lambda control, also known as wideband regulation is when a wideband’s signal is used to control closed loop fueling. M4.3/M4.4 equipped cars, such as the 850 or 1998 V70, C70, XC70, S70, came from the factory equipped with a narrowband oxygen sensor. The purpose of the oxygen sensor is to provide feedback to the ECU about the air to fuel ratio (AFR) that the engine is running. Due to deviations in fuel quality, air leaks, injector flow deviations, etc, the ECU will make real time corrections to the amount of fuel injected, based on feedback from the oxygen sensor.
A narrowband sensor can only tell the ECU if the air to fuel ratio is leaner or richer than 14.7:1. 14.7:1 is what’s called the stoichiometric ratio for gasoline and oxygen and it is the desired air to fuel ratio at idle and cruising, as it results in the least amount of pollutants. However, when you get heavy on the throttle and boost rises, the air to fuel ratio should be much richer (more fuel), at around 11.7:1. When fuel is injected into the cylinder it changes phase from a liquid to a gaseous state. This phase change absorbs heat and cools down the in-cylinder charge. When your engine is heavily loaded and under boost, a cooler in-cylinder charge decreases the chance of spontaneous pre-combustion, otherwise known as knock. Since knock is the #1 killer of boosted engine, we want a richer AFR at full throttle.
With the factory oxygen sensor, the ECU is essentially ‘blind’ at this richer AFR. Normally, the ECU is actively targeting an AFR of 14.7:1, tweaking the amount of fuel injected in small amounts to reach that target. Since the factory O2 sensor can’t read the rich AFR desired at high engine loads, it turns off this active targeting when at full throttle. With wideband lambda regulation the ECU will always know the AFR, and will be able to always make adjustments to the fueling, even when running at a rich AFR. This makes tuning inherently safer, as the ECU can add fuel on the fly to counteract a lean running situation. Furthermore, the factory O2 is quite slow to respond, whereas a wideband is much quicker. So the speed at which the ECU adjusts the fueling can be sped up, leading to a more stable AFR and a better driving car.
Interfacing a wideband sensor with the M4.4 is luckily quite easy. Most every wideband gauge/controller on the market today has a feature called analog voltage output. The wideband gauge/controller interfaces with the wideband sensor, measures the AFR and outputs this signal on its gauge face (if equipped) and also outputs this as a 0-5 volt signal on the analog voltage wire. It just so happens that M4.4 receives the signal from the narrwoband oxygen sensors through a chip that is capable of reading a 0-5V signal. Therefore, the circuitry that was used to interface with the narrowband can be repurposed to interface with the wideband sensor. Since interfacing with a wideband sensor requires a complex control strategy, a wideband gauge/controller must be used to interface with the wideband sensor, before the wideband signal can be sent to the ECU. The AEM X-Series 30-0300 is the best consumer grade wideband gauge/controller on the market and is the wideband gauge we recommend.
For those who don’t fully understand how it’s possible to interface a wideband with an engine management system designed 10 years before a commercially available wideband sensor was ever released, it might be helpful to think of things a little differently. Think of the rear oxygen sensor wire to the ECU not as the rear oxygen sensor wire, but rather as a general purpose analog voltage input, which the ECU was originally programmed to use for interfacing with a narrowband sensor. It is possible to use this input pin for any sensor or device which outputs a 0-5V signal, it’s just a matter of writing the correct code to suit. The rear oxygen sensor input isn’t the only 0-5V input channel on the ECU – the front oxygen sensor, tank pressure sensor, MAF, TPS etc. all send data to the ECU using a 0-5V signal. The code on the ECU could be modified so that any of these inputs could be used to receive the wideband signal. Obviously it wouldn’t be a good idea to use the MAF or TPS input, as those are necessary inputs for the engine to run.
UPDATE 6/25:
Unfortunately the new AEM X-Series gauges ship with an FAE brand oxygen sensor, not Bosch. When the X-Series is used with the FAE sensor, the analog output is rendered inoperable. Since the analog output is how the gauge sends the wideband signal to the ECU, the gauge must be paired with a genuine Bosch LSU4.9 sensor. The X-Series can be purchased without a sensor, part number 30-0300NS.
When sourcing a Bosch LSU4.9 sensor, be wary as there are a lot of Chinese knock off sensors on the market. If the sensor is less than $65, you can be sure it is a fake. The cheapest place to get a genuine Bosch sensor is from Rockauto: https://www.rockauto.com/en/moreinfo.php?pk=11750320
However, you can get the same sensor at a local autoparts store. There are different part numbers LSU4.9 sensors with the correct plug, but different lengths of wires. If a local shop is out of stock, check if they have a different # part in stock.