User 122821

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.



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.





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



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.



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.



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

ltc

If anyone needs electronics help with their prototypes, please feel free to PM me (I've been doing electronics for 40 years)

It would also be a very simple task to design a very quick and inexpensive printed circuit board (probably around $5 + parts) which would make assembly much easier...sort of like an old Heathkit.

ltc

Is there a specific size the PCB should be?
Would an assembled board be better than a bare board/bag of parts? I know some people don't like to solder.

I took a quick look at the schematic and it looks OK, I would perhaps make some minor changes.
(I designed OEM electronics for Porsche, Audi, Mercedes so working on a 12V automotive bus is something you don't forget)

I could probably do something like this on a rainy weekend, send the files out to have boards made (usually 1 week) and assemble a few in my basement electroncis shop.

ltc

I don't mind doing the PCB layout and soldering/assembling the boards.
I am not looking to make any profit from this, so it would just be cost + shipping.

I will double check with the Admin (Bob) to make sure there are no issues.