Late production tachometer wiring schematics question

[ZeKahr]

I have a 1986 325e with the late production gauge cluster/SI board non-functioning tachometer/econometer. I suspect the SI board batteries are dead (fuses 10, 12, 21 look good), so I ordered new ones. While waiting on the batteries to arrive, I did some research on why the batteries are necessary for the tach/econometer to work and found out that the tach/econometer signal and power to the tach and econometer go through the SI microprocessor on the late production gauge clusters, which needs the batteries to work. Seeing that this was one of BMW's more idiotic ideas back in the 80s, I researched potential bypasses for this and found that there are none yet, so I am trying to see if I can come up with a way to bypass the SI board. I looked up wiring schematics for my car to see if there was a way to bypass the SI board in order to have a at least working tachometer. The schematic for the late production tachometer labels the inputs to the tach as +1, -1, +2,and -2. Does anybody know what these numbers mean? I'm a noob at reading wiring diagrams.

[Gregs///M]

Those are the wire connectors to the tach motor. There is no way to bypass the SI board. The SI board is more complicated than you think, it powers and controls the tach, temp, MPG, SI lights and OBC. I recommend you just have your SI board refurbished.

When the batteries die on the SI board, they cause a condition that will make the SI board power on and off very rapidly and constantly. This constant switching will kill off those 20+ transistors. So only replacing the batteries will most likely not repair your SI board. Have you ever heard of people smacking their dash to get the tach or temp gauge to work? That smacking is jogging the transistors from their stuck state.

I know this to be fact because 10 years ago when we first starting refurbishing SI board, we would only change the batteries and capacitors. Many customers were sending their SI boards back because they would fail prematurely. After we started replacing the transistors and voltage regulators, we have refurbished several hundred with zero failures, 10 years later.

If you are noob at reading wiring diagrams, you probably do not have the technical expertise to bypass the SI board. Even myself, having written most of the articles about SI board technology and having refurbished several hundred and building our custom made cluster tester, there is no easy and cheap way to bypass it. Best to have it repaired and retain it.

[ZeKahr]

Those are the wire connectors to the tach motor. There is no way to bypass the SI board. The SI board is more complicated than you think, it powers and controls the tach, temp, MPG, SI lights and OBC. I recommend you just have your SI board refurbished.

When the batteries die on the SI board, they cause a condition that will make the SI board power on and off very rapidly and constantly. This constant switching will kill off those 20+ transistors. So only replacing the batteries will most likely not repair your SI board. Have you ever heard of people smacking their dash to get the tach or temp gauge to work? That smacking is jogging the transistors from their stuck state.

I know this to be fact because 10 years ago when we first starting refurbishing SI board, we would only change the batteries and capacitors. Many customers were sending their SI boards back because they would fail prematurely. After we started replacing the transistors and voltage regulators, we have refurbished several hundred with zero failures, 10 years later.

If you are noob at reading wiring diagrams, you probably do not have the technical expertise to bypass the SI board. Even myself, having written most of the articles about SI board technology and having refurbished several hundred and building our custom made cluster tester, there is no easy and cheap way to bypass it. Best to have it repaired and retain it.

I replaced the batteries and the tachometer and econometer now works but are a little choppy. From the multiple times I started up and drove the car I haven't yet had to smack the dash to get the tachometer/econometer working. Fuel and temp gauge still seem to give funny readings sometimes, even after I tightened the back nuts on both. You're probably right that some of the transistors on my board are bad, but I can live with it for now and will refurbish my board later.