Tach doesn't work, SI lights do

[econti]

The board that was in the car when I bought it was apparently fine in another cluster, as told by the previous owner. He also said that when he put said different cluster with the same SI board in, the tach worked great.

I have since bought a board that was almost unused, had the sticker telling you to switch the jumper and everything. The seller of that board said that it worked 100% in another cluster.

Both boards have batteries that are 3.1v or thereabouts.

Adding to this, the tach sometimes working is odd as well. That would indicate that there's a bad connection or something somewhere inside the tach unit itself.

[Gregs///M]

3.1 is low. These batteries have 3.25-3.3 volts when new.

My point is this. Do not trust what previous owners tell you when buying SI boards. People will do and say anything to sell their items. If the board has not been refurbished, then it has the 20+ year old batteries in it. Would you trust a 20 year old battery?

[econti]

3.1 is low. These batteries have 3.25-3.3 volts when new.

My point is this. Do not trust what previous owners tell you when buying SI boards. People will do and say anything to sell their items. If the board has not been refurbished, then it has the 20+ year old batteries in it. Would you trust a 20 year old battery?

The guy I bought the car off, I knew him from before I bought it.
The other guy I bought the second board off, is from a BMW enthusiast facebook page. He offered to show me that it worked, which I had to decline as I didn't pick the board up.

I severely doubt that two boards which have both been confirmed as working, and both have good voltage (3.2V, as just tested by me) would somehow both have working SI lights but a dud tacho.
Not just that, but the tach start working, and then stop again.

So, again. Is there a way to separate the tach from the rest of the dash? Or will I just have to get an entire new cluster and deal with not having a correct odo?