From the FSM:

The catalyst monitor uses dual oxygen sensors
(O2S's) to monitor the efficiency of the converter. The
dual O2S's sensor strategy is based on the fact that
as a catalyst deteriorates, its oxygen storage capacity
and its efficiency are both reduced. By monitoring
the oxygen storage capacity of a catalyst, its efficiency
can be indirectly calculated. The upstream
O2S is used to detect the amount of oxygen in the
exhaust gas before the gas enters the catalytic converter.
The PCM calculates the A/F mixture from the
output of the O2S. A low voltage indicates high oxygen
content (lean mixture). A high voltage indicates a
low content of oxygen (rich mixture).
When the upstream O2S detects a lean condition,
there is an abundance of oxygen in the exhaust gas.
A functioning converter would store this oxygen so it
can use it for the oxidation of HC and CO. As the
converter absorbs the oxygen, there will be a lack of
oxygen downstream of the converter. The output of
the downstream O2S will indicate limited activity in
this condition.
As the converter loses the ability to store oxygen,
the condition can be detected from the behavior of
the downstream O2S. When the efficiency drops, no
chemical reaction takes place. This means the concentration
of oxygen will be the same downstream as
upstream. The output voltage of the downstream
O2S copies the voltage of the upstream sensor. The
only difference is a time lag (seen by the PCM)
between the switching of the O2S's.
To monitor the system, the number of lean-to-rich
switches of upstream and downstream O2S's is
counted. The ratio of downstream switches to
upstream switches is used to determine whether the
catalyst is operating properly. An effective catalyst
will have fewer downstream switches than it has
upstream switches i.e., a ratio closer to zero. For a
totally ineffective catalyst, this ratio will be one-toone,
indicating that no oxidation occurs in the device.
The system must be monitored so that when catalyst
efficiency deteriorates and exhaust emissions
increase to over the legal limit, the MIL will be illuminated.


--
Dave Milne, Scotland
'99 TJ 4.0 Sahara

"Jerry Bransford" <jerrypb@mecox.net> wrote in message
news:PS7fb.38240$gv5.27728@fed1read05...
: Why do I seem to recall that my '97 FSM says the downstream sensor is only
: there to monitor the cat, and that the upstream sensor checks and tells
the
: PCM how to adjust the injector's pulse duration for proper fuel mixture?
:
: Jerry
:
: --
: --
: Jerry Bransford
: To email, remove 'me' from my email address
: KC6TAY, PP-ASEL
: See the Geezer Jeep at
: http://members.***.net/jerrypb/
:
: "bllsht" <nospam@invaliddot.net> wrote in message
: news:4n0qnvg1k6p53k6f6r0n3a62i5jaju8d8d@4ax.com...
: > Actually, the PCM will use downstream O2 input to "fine tune" fuel trim
in
: > addition to monitoring the cat.
: >
: > The OP didn't say what engine he had, but there's a TSB for 4.0L engines
: > concerning decarbonizing the combustion chamber and revised valve
springs.
: >
: >
: >
: > In message <bvWeb.37369$gv5.16849@fed1read05>, "Jerry Bransford" wrote:
: >
: > >The downstream 02 sensor, unlike the upstream 02 sensor, has no ability
: to
: > >adjust the fuel mixture or for that matter, anything about how the
engine
: > >runs. Its sole function is to check the cat for proper function and
: display
: > >a Check Engine light if it detects that the cat has gone bad.
: > >
: > >Jerry
: >
:
:

 

Interesting, the catalytic converter converts, carbon monoxide,
hydrocarbons, and nitrogen oxides into less harmful by-products, and the
FSM uses the term "absorbs", if that were true it be as big as a diesel
train.
God Bless America, ßill O|||||||O
mailto:-------------------- http://www.----------.com/

Dave Milne wrote:
>
> From the FSM:
>
> The catalyst monitor uses dual oxygen sensors
> (O2S's) to monitor the efficiency of the converter. The
> dual O2S's sensor strategy is based on the fact that
> as a catalyst deteriorates, its oxygen storage capacity
> and its efficiency are both reduced. By monitoring
> the oxygen storage capacity of a catalyst, its efficiency
> can be indirectly calculated. The upstream
> O2S is used to detect the amount of oxygen in the
> exhaust gas before the gas enters the catalytic converter.
> The PCM calculates the A/F mixture from the
> output of the O2S. A low voltage indicates high oxygen
> content (lean mixture). A high voltage indicates a
> low content of oxygen (rich mixture).
> When the upstream O2S detects a lean condition,
> there is an abundance of oxygen in the exhaust gas.
> A functioning converter would store this oxygen so it
> can use it for the oxidation of HC and CO. As the
> converter absorbs the oxygen, there will be a lack of
> oxygen downstream of the converter. The output of
> the downstream O2S will indicate limited activity in
> this condition.
> As the converter loses the ability to store oxygen,
> the condition can be detected from the behavior of
> the downstream O2S. When the efficiency drops, no
> chemical reaction takes place. This means the concentration
> of oxygen will be the same downstream as
> upstream. The output voltage of the downstream
> O2S copies the voltage of the upstream sensor. The
> only difference is a time lag (seen by the PCM)
> between the switching of the O2S's.
> To monitor the system, the number of lean-to-rich
> switches of upstream and downstream O2S's is
> counted. The ratio of downstream switches to
> upstream switches is used to determine whether the
> catalyst is operating properly. An effective catalyst
> will have fewer downstream switches than it has
> upstream switches i.e., a ratio closer to zero. For a
> totally ineffective catalyst, this ratio will be one-toone,
> indicating that no oxidation occurs in the device.
> The system must be monitored so that when catalyst
> efficiency deteriorates and exhaust emissions
> increase to over the legal limit, the MIL will be illuminated.
>
> --
> Dave Milne, Scotland
> '99 TJ 4.0 Sahara

 

Interesting, the catalytic converter converts, carbon monoxide,
hydrocarbons, and nitrogen oxides into less harmful by-products, and the
FSM uses the term "absorbs", if that were true it be as big as a diesel
train.
God Bless America, ßill O|||||||O
mailto:-------------------- http://www.----------.com/

Dave Milne wrote:
>
> From the FSM:
>
> The catalyst monitor uses dual oxygen sensors
> (O2S's) to monitor the efficiency of the converter. The
> dual O2S's sensor strategy is based on the fact that
> as a catalyst deteriorates, its oxygen storage capacity
> and its efficiency are both reduced. By monitoring
> the oxygen storage capacity of a catalyst, its efficiency
> can be indirectly calculated. The upstream
> O2S is used to detect the amount of oxygen in the
> exhaust gas before the gas enters the catalytic converter.
> The PCM calculates the A/F mixture from the
> output of the O2S. A low voltage indicates high oxygen
> content (lean mixture). A high voltage indicates a
> low content of oxygen (rich mixture).
> When the upstream O2S detects a lean condition,
> there is an abundance of oxygen in the exhaust gas.
> A functioning converter would store this oxygen so it
> can use it for the oxidation of HC and CO. As the
> converter absorbs the oxygen, there will be a lack of
> oxygen downstream of the converter. The output of
> the downstream O2S will indicate limited activity in
> this condition.
> As the converter loses the ability to store oxygen,
> the condition can be detected from the behavior of
> the downstream O2S. When the efficiency drops, no
> chemical reaction takes place. This means the concentration
> of oxygen will be the same downstream as
> upstream. The output voltage of the downstream
> O2S copies the voltage of the upstream sensor. The
> only difference is a time lag (seen by the PCM)
> between the switching of the O2S's.
> To monitor the system, the number of lean-to-rich
> switches of upstream and downstream O2S's is
> counted. The ratio of downstream switches to
> upstream switches is used to determine whether the
> catalyst is operating properly. An effective catalyst
> will have fewer downstream switches than it has
> upstream switches i.e., a ratio closer to zero. For a
> totally ineffective catalyst, this ratio will be one-toone,
> indicating that no oxidation occurs in the device.
> The system must be monitored so that when catalyst
> efficiency deteriorates and exhaust emissions
> increase to over the legal limit, the MIL will be illuminated.
>
> --
> Dave Milne, Scotland
> '99 TJ 4.0 Sahara

 

Interesting, the catalytic converter converts, carbon monoxide,
hydrocarbons, and nitrogen oxides into less harmful by-products, and the
FSM uses the term "absorbs", if that were true it be as big as a diesel
train.
God Bless America, ßill O|||||||O
mailto:-------------------- http://www.----------.com/

Dave Milne wrote:
>
> From the FSM:
>
> The catalyst monitor uses dual oxygen sensors
> (O2S's) to monitor the efficiency of the converter. The
> dual O2S's sensor strategy is based on the fact that
> as a catalyst deteriorates, its oxygen storage capacity
> and its efficiency are both reduced. By monitoring
> the oxygen storage capacity of a catalyst, its efficiency
> can be indirectly calculated. The upstream
> O2S is used to detect the amount of oxygen in the
> exhaust gas before the gas enters the catalytic converter.
> The PCM calculates the A/F mixture from the
> output of the O2S. A low voltage indicates high oxygen
> content (lean mixture). A high voltage indicates a
> low content of oxygen (rich mixture).
> When the upstream O2S detects a lean condition,
> there is an abundance of oxygen in the exhaust gas.
> A functioning converter would store this oxygen so it
> can use it for the oxidation of HC and CO. As the
> converter absorbs the oxygen, there will be a lack of
> oxygen downstream of the converter. The output of
> the downstream O2S will indicate limited activity in
> this condition.
> As the converter loses the ability to store oxygen,
> the condition can be detected from the behavior of
> the downstream O2S. When the efficiency drops, no
> chemical reaction takes place. This means the concentration
> of oxygen will be the same downstream as
> upstream. The output voltage of the downstream
> O2S copies the voltage of the upstream sensor. The
> only difference is a time lag (seen by the PCM)
> between the switching of the O2S's.
> To monitor the system, the number of lean-to-rich
> switches of upstream and downstream O2S's is
> counted. The ratio of downstream switches to
> upstream switches is used to determine whether the
> catalyst is operating properly. An effective catalyst
> will have fewer downstream switches than it has
> upstream switches i.e., a ratio closer to zero. For a
> totally ineffective catalyst, this ratio will be one-toone,
> indicating that no oxidation occurs in the device.
> The system must be monitored so that when catalyst
> efficiency deteriorates and exhaust emissions
> increase to over the legal limit, the MIL will be illuminated.
>
> --
> Dave Milne, Scotland
> '99 TJ 4.0 Sahara