Rennlist - Porsche Discussion Forums - Recap of the SAI circuit

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- - Recap of the SAI circuit (https://rennlist.com/forums/993-forum/653881-recap-of-the-sai-circuit.html)

"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.

Quote:

Originally Posted by Lorenfb (Post 8873884)

I thought about it for 30 seconds ... I'm pretty sure it's not rocket science.

I don't need it right now, but good to have just in case. Please add me to the list!

Quote:

Originally Posted by Lorenfb (Post 8872986)

"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.

Thanks for the technical explanation. I do agree this is not a fix all for all cars in this situation but it is working on two 993s as data points. I will be interested in getting feedback from '97 and '98 993s.

Add me to the list for an assembled board please!

Please add me to the list. I'd like (2) of the assembled boards.

Thanks for doing this!

Please put me down for 1 of the assembled boards also.

Great work!

Lewis, Please also put me down for two assembled kits! Thanks for the contribution to Rennlist! We all appreciate it!

PLZ put me down for one assembled kits.......we can't thank you enough!!!!!!!

Sign me up for one assembled board. Thx Lewis.
_____________
'97 Blk/Blk C2S

Please count me in for one. Thx

Great idea!!! Please add me to the list for one assembled and on un-assembled unit.

Thanks

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?

I would definitely like to be able to test my system. Put me down for one assembled.

Thanks!

-Andrew

Quote:

Originally Posted by MDamen (Post 8881670)

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.