Recap of the SAI circuit
09-16-2011, 02:22 PM
#76
"It would be a simple circuit to design for this (voltage change), assuming the voltage levels were provided."
Probably not that simple, i.e. One would have to characterize the
actual voltage change that was required. For example, the OBDII system
may what 'see' the O2 voltage start at .45 volts and have a time constant
range of 2-3 seconds and have a minimum voltage of 20-30 millivolts.
If the desired change is not 'seen', then SAI monitor is not set and
a fault code (CEL) results.
Probably not that simple, i.e. One would have to characterize the
actual voltage change that was required. For example, the OBDII system
may what 'see' the O2 voltage start at .45 volts and have a time constant
range of 2-3 seconds and have a minimum voltage of 20-30 millivolts.
If the desired change is not 'seen', then SAI monitor is not set and
a fault code (CEL) results.
09-16-2011, 02:55 PM
#77
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"It would be a simple circuit to design for this (voltage change), assuming the voltage levels were provided."
Probably not that simple, i.e. One would have to characterize the
actual voltage change that was required. For example, the OBDII system
may what 'see' the O2 voltage start at .45 volts and have a time constant
range of 2-3 seconds and have a minimum voltage of 20-30 millivolts.
If the desired change is not 'seen', then SAI monitor is not set and
a fault code (CEL) results.
Probably not that simple, i.e. One would have to characterize the
actual voltage change that was required. For example, the OBDII system
may what 'see' the O2 voltage start at .45 volts and have a time constant
range of 2-3 seconds and have a minimum voltage of 20-30 millivolts.
If the desired change is not 'seen', then SAI monitor is not set and
a fault code (CEL) results.
09-16-2011, 03:42 PM
#79
"No! All it does is fool/mis-represent what the O2 sensors 'see' when the
SAI air pump turns 'on', i.e. It basically indicates that an adequate amount
of air is reaching the O2 sensors during a cold start-up. "
The SAI system includes the not only the air pump but:
1. the O2 sensors
2. the SAI monitor diagnostics within the ODBII system
3. the relay which turns the pump 'on' and the by-pass circuit
4. air valves
So one can't really remove the SAI system!!!!!!!!
Potential Problem with the proposed by-pass circuit:
On the '97/'98 DME ECM the SAI monitor may look for a voltage
change on the O2 sensors and NOT an immediate zero volts
from the O2 sensors with the by-pass circuit. So a SAI fault
still may occur.
Bottom line: Before asssuming that the by-pass circuit is the
end-all fix for ALL 993s, thorough testing needs to be done on
all 993s, i.e. Since later/early 993 DME ECMs had different OBDII
firmwares and thus the by-pass circuit may have different effects.
Furthermore, if the by-pass circuit is not grounded in the ideal location
or the circuit is not shielded from 'noise', the circuit may not function
properly and additionally cause the O2 sensors to provide erroneous
voltages, thereby causing additional fault codes.
SAI air pump turns 'on', i.e. It basically indicates that an adequate amount
of air is reaching the O2 sensors during a cold start-up. "
The SAI system includes the not only the air pump but:
1. the O2 sensors
2. the SAI monitor diagnostics within the ODBII system
3. the relay which turns the pump 'on' and the by-pass circuit
4. air valves
So one can't really remove the SAI system!!!!!!!!
Potential Problem with the proposed by-pass circuit:
On the '97/'98 DME ECM the SAI monitor may look for a voltage
change on the O2 sensors and NOT an immediate zero volts
from the O2 sensors with the by-pass circuit. So a SAI fault
still may occur.
Bottom line: Before asssuming that the by-pass circuit is the
end-all fix for ALL 993s, thorough testing needs to be done on
all 993s, i.e. Since later/early 993 DME ECMs had different OBDII
firmwares and thus the by-pass circuit may have different effects.
Furthermore, if the by-pass circuit is not grounded in the ideal location
or the circuit is not shielded from 'noise', the circuit may not function
properly and additionally cause the O2 sensors to provide erroneous
voltages, thereby causing additional fault codes.
09-20-2011, 01:23 AM
#88
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Is there a way to check the function of the circuit before it is installed? If I apply 12 volts to the + and - is there a specific voltage or range I should see at each O2 sensor? Do they go to ground or do they show a small voltage?
09-20-2011, 12:47 PM
#90
The O2 voltage is grounded when 12 volts is applied to the circuit. You could setup a test O2 sensor to the circuit on a testbed with a volt meter across the O2 sensor. I used a previous old O2 sensor to testing.