Testing lambda control
Testing lambda control
The lambda probe compares the oxygen content in the air with the residual oxygen content in the exhaust gas and sends a voltage signal to the control unit.
The voltage signal for "Mixture too rich (low residual oxygen)" is between about 0.7 and 1.1 V (based on reference earth).
The voltage signal for "Mixture too lean (high residual oxygen)" is between about 0.0 and +0.3 V (based on reference earth).
The transition from "rich" to "lean" is accompanied by a voltage jump from between 0.7 and 1.1 V to between 0.0 and +0.3 V (based on reference earth) and vice versa (λ= 1.0).
Because of the sharp voltage jump the lambda control cannot maintain a constant ideal mixture composition of λ = 1.0. The system fluctuates continuously between "slightly too lean" and "slightly too rich".
If the voltage does not change or only changes slowly the following faults are possible:
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◆ Slots or holes in probe head are blocked.
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◆ Lambda probe has been subjected to excessive thermal stress.
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◆ Contact resistance in signal wiring or reference earth wiring.
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◆ Lambda probe too cold; lambda probe heating not working.
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◆ Lambda control switched off (control unit has detected a fault in the injection system);see "Interrogating fault memory" and "Reading measured value block", Display Group 021.
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◆ Lambda probe damaged by contact spray or similar product. (The contact spray is drawn into the probe via the fine cavities in the electrical wiring as a result of temperature fluctuations and capillary effect.)
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◆ Lambda probe damaged by silicone vapours. (The engine draws in traces of any silicone-based sealants that may have been used. The silicone does not burn and damages the lambda probe.)
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