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XMS5 - Self Tuning Application | Application Note : AN 5


Perfect Power Application Notes.


Application Note - AN 5

This note describes the PRS self-tuning process, which is achieved with CLOSED LOOP LAMBDA operation.


Explanation of Lambda Measuring

The Lambda sensor, sometimes called the oxygen sensor, measures the oxygen level in the burnt gasses.
XMS5 Stand Alone Unit - Air Fuel Ratio Graph
These LAMBDA sensors are used in different configurations :
  • With/Without Heater
  • Single/Two Wire Sensor
  • Narrow/Wide Band Characteristic


The XMS5B unit can only work with the wide band lambda probes, which are wired, directly into it.


Problems with Lambda Sensors

Position in the Manifold
It is advisable to install the sensor as close as possible to the engine. That is the point where the branches meet. If extremely long branches are used, then it may be better to install the probe at a "2 into 1" junction. A temporary installation in the tail pipe is not recommended.
Gas Velocity
Sometimes the gas velocity cools the sensor head, and the result is a "LEAN" reading. This can be adverted by placing the sensor in a "sheltered" position, or by placing a little deflection plate in front of the sensor head. Make sure it does not obstruct the exhaust gas flow.
Electrical Interference
If the sensor element is grounded to the case, then the XMS5 reads engine "noise" from the alternator, or ignition "noise". This makes the reading useless!

Tuning with Lambda

The lambda screen consists of
LAMBDA TARGETS : 384, throttle dependent
LAMBDA LIMITS : 16, the max amount by which the lambda probe can change the fuel
XMS5 Stand Alone Unit - F7 - AFR Loop Target

XMS5 Operation

The XMS5 reads the AFR from the sensor, and compares it with the target value in the map. If it differs, then it adjusts the fuel up or down until the max is reached. It creates a long term adjustment map. The AFR idle loop time, and the AFR loop time determine how fast the loop changes its value. A small number is fast!

Practical Operation

Drive the car and monitor the lambda adjustment. It may fluctuate around zero, which is normal and means that your fueling map is good.
Very often we are asked the question : " Why is long term trim map made?" "Why not let the Lambda automatically tune?". Early software versions had this feature, but it was not practical, for the following reasons :
  • It was difficult to tune yourself our of a total wrong map point
  • There is no difference between a very lean engine, and a "cool" or malfunctioning lambda probe.
  • Some map points should be "rich" and others "lean" in the same column.
  • It removed the human touch from the tuning process.


We at Perfect Power believe that it is the best possible "automated" tuning process, although you may conclude that it is "semi-automatic".