Fiat Bravo Service Manual page 109

Bravo-Brava i@t
12v
Engine
Fuel system
10.
Lambda adjustment
The LAMBDA probe controls mixture concentration by measuring the residual oxygen content of ex­
haust emissions.
Because the ceramic is not active below 300° C, the LAMBDA probe is heated electrically by a resistance
(inside the probe) that is continually supplied by a locked positive and protected by a 10 A fuse.
This heating is necessary during cold starts to maintain sensor efficiency if the engine idles for long peri­
ods.
With temperatures in excess of 300° C, the ceramic becomes a conductor of oxygen ions. If the percent­
ages of oxygen present on either side of the sensor differ, a voltage difference is set up across the two
end poles.
The LAMBDA probe sends a voltage difference signal to the control unit through terminals (12) and
(13).
LAMBDA probe measurements of exhaust gas composition allow the control unit to correct the mixture
concentration continually on-line (leaner/richer) in order to maintain it close to a stoichiometric ratio.
Efficient catalytic converter operation and low exhaust gas toxicity are dependent on an exact mixture
composition.
During the above stages, injection pulse changes made by the control unit on the basis of the LAMBDA
probe signal are excluded because they would be at odds with the required driving conditions; the en­
gine thus operates in OPEN LOOP.
With both LAMBDA probe and control unit circuit working properly, the percentages of pollut­
ing exhaust gases are self-regulated by the system without the possibility of external regula­
tion.
TENSIONE
mV
lOOO-
8 0 0 -
6 0 0 -
4 0 0 -
2 0 0 -
n -
, 0,980
MISCELE
GRASSE
— ► ■ -
\
"*
1. '
■ *■ 1,020
MISCELE
MAGRE
I " .
0,7 0,8 0,9 1,0 1,1 1,2 1,3
COEFFICIENTE X
-o+30
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