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

flying_low 09-08-2011 04:52 PM

Recap of the SAI circuit
 
I've received several PMs with questions on SAI bypass circuit I have used so I
thought I would just summarize the information I have gathered.

First of all, I did not invent this circuit. It's a circuit that was posted here by another rennlister (I don't remember who) but I have also seen similar circuits on BMW forums. All the parts were purchased at Radio Shack or Jameco.com. If you are comfortable with a soldering iron, its easy to build the circuit.

The pictures are click-able for larger views.

The circuit below is the circuit I'm currently using. I have previously used the one transistor version which had intermittent issues. This one has been working reliably for a few months now. It can be installed without any modification to the wiring harness and is completely reversible.

http://www.vintage911.com/porsche/sa...ai-circuit.jpg

Attaching the leads to the O2 sensor can be done by sliding one of the four leads into the hole already in the back of the O2 connectors on the side
of the engine compartment. Put the bare wire into the female connector of the black O2 wire and then slide the O2 connectors back together as they were. The new wire is sandwiched between.

http://www.vintage911.com/porsche/sa...m/dsc00259.jpg

http://www.vintage911.com/porsche/sa...m/dsc00261.jpg

The entire circuit box from Radio Shack fits in the electrical panel in the engine component hidden from view.

http://www.vintage911.com/porsche/sa...m/dsc00262.jpg

The trigger voltage for the circuit comes from post 87 of the SAI pump relay. Just press the bare wire into the prong and reinsert the relay over it.

http://www.vintage911.com/porsche/sa...m/dsc00263.jpg

When the ECU activates the SAI pump, the circuit grounds the O2 voltage. Here is a snapshot of the O2 voltage while the pump is running.

http://www.vintage911.com/porsche/sa...m/dsc00264.jpg

When the ECU deactivates the SAI pump, the voltage is the same as before the circuit was installed.

http://www.vintage911.com/porsche/sa...m/dsc00265.jpg

vincer77 09-08-2011 04:59 PM

And the purpose of the circuit is?

flying_low 09-08-2011 05:04 PM

If you have issues with Secondary Air Injection ports being blocked or you want to remove the entire SAI system from your '96 - '98. '95s have the system but the ECU does not check if the system is working correctly.

The blocked ports issue shows up as OBD errors:

P0410 - Secondary Air Injection System: Malfunction
P1411 - Manufacturer specific code

vincer77 09-08-2011 05:12 PM

This one is a keeper! Thanks.

Rick Lee 09-08-2011 06:01 PM

This looks interesting. I'm curious if you need to do this before a CEL is tripped or if it can be done afterwards, clear the CEL with my OBDII code reader and then still get through emissions.

flying_low 09-08-2011 06:47 PM

It can be done before or after. Of course if done after tripping the CEL, the error must be cleared and the registers retested with the drive procedure to pass smog.

Aircooled Dave 09-08-2011 07:25 PM

Interesting solution.

Is there a way through SW to just 'ignore' this fault code so it doesn't register at all? There is no real drivability concern, just your CATs are not getting as hot as they could if they were getting more fresh air at startup. I vaguely remember back from my BMW days that there seemed to be a SW fix for some of those guys who also suffered from clogged SAI issues. Some companies offered it as part of a chip tune. I think they called it a "Carbon Ignore" or something like that. Granted, my coding knowledge is very limited, but it seems reasonable that you could edit it to just ignore a particular code. Of course there is the whole 'legality' of such a solution, but if it is for 'off road use only', maybe it will be OK. :)

flying_low 09-08-2011 08:08 PM

I have yet to see a software solution in person. There was a rennlister who posted a friend paid around $1000 to reprogram the ECU to ignore the fault. There is a company called TurboWerx saying for a year that they were coming out with a ECU add-on to ignore the error for $400. This circuit was $10 which is for "testing and educational only". ;)

Aircooled Dave 09-08-2011 08:41 PM

I certainly think this HW solution has merit. Hard to say no to $10.

Still, it would seem there should be some kind of SW fix possible. Granted, I doubt it would be $10, but it would be totally stealth as long as it didn't throw anything else off. Given a lot of folks go the chip route anyway, adding this 'SAI ignore program' while you're at it seems reasonable.

Please forgive my ignorance here, but don't ROW cars ignore the SAI since it is not used? What if someone swapped in one of those ECUs? A lot of the OBDII codes are generic so it seems plausible on the surface although i'm sure there is some devil in the details. On that note, what are the major differences between a ROW and US ECU? Perhaps some different level of tune due to the octane ratings? What else constitutes the difference? Would a US car run if a ROW ECU was swapped in and nothing else changed?

JM993 09-09-2011 12:39 AM


Originally Posted by Aircooled Dave (Post 8854501)
I certainly think this HW solution has merit. Hard to say no to $10.

Still, it would seem there should be some kind of SW fix possible. Granted, I doubt it would be $10, but it would be totally stealth as long as it didn't throw anything else off. Given a lot of folks go the chip route anyway, adding this 'SAI ignore program' while you're at it seems reasonable.

Please forgive my ignorance here, but don't ROW cars ignore the SAI since it is not used? What if someone swapped in one of those ECUs? A lot of the OBDII codes are generic so it seems plausible on the surface although i'm sure there is some devil in the details. On that note, what are the major differences between a ROW and US ECU? Perhaps some different level of tune due to the octane ratings? What else constitutes the difference? Would a US car run if a ROW ECU was swapped in and nothing else changed?

No SAI on ROW cars. No OBDII either :)

Check out the link in my sig if you're interested in the ECU difference between ROW and US.

Cheers,
Joe

MDamen 09-09-2011 11:39 AM

Where does the gray wire go? Is it just grounded or does it have to go to wire 3 of one of the O2 sensors?

flying_low 09-09-2011 11:58 AM

I grounded the gray wire to the chassis.

Lorenfb 09-09-2011 12:38 PM

"Still, it would seem there should be some kind of SW fix possible."

Not really, without a MAJOR redesign (patch) in the firmware coding,
and not just changing cell entries like the Mickey Mouse so-called tuners
do! This would require a lengthly dis-assembly of the 993 DME ECM
firmware and redesign of OBDII firmware.

Remember, besides the actual test, the cycle flag for the SAI test must
be set or in the end none of the readiness codes (monitors) will be set
to allow an emissions test to begin at the smog station, i.e. readiness
codes ONLY show full completion when all tests have fully completed
successfully.

Read here for more info: http://www.systemsc.com/codes.htm

MDamen 09-14-2011 02:01 AM

I got my parts from Jameco and I'm going to see if I can assemble the circuit board. Looks pretty simple. I have had the CEL for about 2 years due to the SAI. Tried cleaning the ports but ran into a corroded manifold. If this works, it will be nice to get rid of that yellow light on the dash. I'll try to remember to post some pictures and updates. I'm a mechanical engineer, not electrical, so I will trust you spark chasers you the components all worked out.

Amfab 09-14-2011 09:12 AM

Good luck Mike, let us know how it turns out, Im thinking about doing this myself


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