SI Board Bypass

[ialevy]

I have spent hours searching threads on multiple boards on bypassing the SI board. It looks like all of the threads end on a cliffhanger.

I have an 83 633 and the SI board is toast.

There is information on what pins do what and some sketchy info on connecting the econometer.

I have also found "dummy" boards for sale on eBay that look like they have a few capacitors and a diode in the mix. They look interesting and I would love to know the purpose of the additional components.

Does anyone have a complete schematic of what is needed to successfully bypass the board and retain all functions (excluding the SI function, of course)?

[DesktopDave]

I've always been curious about those too, but I haven't installed one of the 'battery-less' units yet. I'd assume the SI battery is only required to save the mileage counter for the SI lights in case of starter battery problems, but it seems like a lot of other functions run through that board.

There are diagrams in the ETM showing pin assignments but I'm not sure if every connection is documented. It does seem sketchy, like you said. For my '86 that info is on pages 87-90, section 6210. To make things more interesting...there are several different versions.

The ETM does specifically show the econ gauge is directly driven by the SI processor, although I'd assume the processor sits on the primary gauge cluster board, and not on the SI battery board. I was surprised to see that some cars have the coding chip on that board.

[BMW Performance]

I am having the SI boards reverse engineered and reproduced currently. Should have a cheap replacement board soon.

[DesktopDave]

How's that for service? I'm continually amazed by the services offered here!

@BMW Performance: Be sure to let us know how that's coming along...

[Cmcgettrick]

This is great news! I just had my cluster restored and one of the unknown risks in this process is the condition of the batteries and SI board. If there is irreparable damage then you have the begin the process of finding donor clusters with the hope of finding a good SI board. Needless to say this can get extremely expensive. Hopefully these new boards come to market!
Thanks for taking on this project!

[bigperm206]

Subscribed. And thanks for taking this on.

Sent from my SM-N970U using Tapatalk

[ialevy]

Update on bypassing SI board! 99% success!!

I used some of the information here:https://www.mye28.com/viewtopic.php?t=94937

It was not complete since it was from a 524td. I also took a look at a couple of boards sold on eBay from eastern Europe and Australia.

I toned out several circuits and came up with the following diagram (please excuse my terrible drawing):
diagram.jpg
I bought a set of header pins from Amazon and used some pins off of my dead board. This is my bypass harness:
Harness.jpg

EVERYTHING WORKS! All gauges - even the econometer. The only issue is my tachometer seems to vibrate. I suspect there is a capacitor in the circuit on the board to smooth the signal. I can live with it until the board being developed is done.

[BMW Performance]

I got some bad news.

The guy I had doing the boards got remarried and got a new job and does not have time to finish the project.

Anybody here deal with PCB for a living?

[DesktopDave]

That's a little disappointing, but all-in-all you've still made great progress. I wish I knew someone offhand who did this, I'll check around. I've noodled around with prototyping but found I was incredibly bad at it.

If I find someone locally I'll PM you.