The sensor cell measur es the differen ce between
the oxygen concent ration in the gas sam ple cha mber
and t he oxygen con centr ation in the out side air fr om
the referen ce air cha nnel. A small volta ge is gen er-
ated across the sensor, which is proportiona l to t he
air-fuel r atio in th e sam ple ch amber. At stoich iomet -
ric ra tio (14.7 lbs. of dry air t o 1 pa rt fu el), th e cor-
respondin g open circuit voltage at the sensor cell is
450 mV. If the stoichiometr ic ratio in the sa mpler
chamber is higher t han 1 (excess a ir) a lower volta ge
is produced. If the st oichom etric ratio is lower than 1
(insufficien t a ir) a higher volt age is produced (Fig.
26).
The ECM u ses this voltage signa l t o det ermine
how and when t o run the oxygen pu mp cell. The goal
of t he ECM is to modu late th e pu mping cu rren t
through the pump cell t o a lways mainta in stoichio-
metr ic air-fuel ratio (14.7 to 1) in the ga s sample
chamber. When stoichiometry is rea ched, t here is no
curren t flowin g to the oxygen pump.
High Excess Air Mode When the exhaust gas is too
lean , the oxygen concentr ation in the gas sample
chamber is high. Th e sen sor cell mea sures the differ-
ence bet ween th e oxygen concentrations in t he sam-
pler cha mber a nd the refer ence air cha nnel. A volt age
lower t han 450mV is generated across the sensor
cell, which is proport iona l to th e a ir-fuel r atio in the
sa mple cha mber. The ECM compa res the sensor cell
volt age wit h the r eference volta ge (V Ref), which cor-
responds to th e stoichiom etric point volta ge. Since
sensor cell volt age is lower th an V Ref, the ECM
determ ines a lean condition exists. Am am plifier
applies a n appropriate voltage to t he pump cell t o
transfer oxygen from the ga s sa mple ch amber
Low Excess Air Mode With low excess air mode,
the oxygen concent ration in the gas sam ple cha mber
is low. The sensor cell measur es th e difference
between oxygen concen tra tions in the ga s sample
chamber an d the refer ence air ch ann el. A voltage
higher th an 450 mV is gener ated across the sensor
cell, which is proport iona l to th e a ir-fuel r atio in the
sa mple cha mber. The E CM determ ines a low excess
air condit ion exist s. The pola rity of the pump cell is
reversed an d so is the direct ion of th e curr ent flow.
Fig. 26 WIDE BAND OPERATION
1 - OXYGEN PUMP CELL CIRCUIT
2 - SENSOR CELL CIRCUIT
VA FUEL INJECTION 14 - 47
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