Tach doesn't work, SI lights do

[econti]

So my tach (and by virtue econometer) doesn't work. I thought it was the SI batteries but the SI lights are always lit, albeit on yellow and red.
The previous owner said that he swapped in another cluster and it worked fine. So I'd reckon that the problem is in the cluster itself.
And for a very brief time when I was last driving, the tach came up to where it should be, then died again.
I've got the cluster out and in pieces in front of me.
What should I check?

[KIRIEIW]

Wow my tach broke today. Interested to see your results.

[jlevie]

The first action would be to swap in a known good SI board, as that is likely to be the cause.

[econti]

It seems odd that the SI lights still work, and that the tach did very briefly work. Bad contacts inside the board? Is it worth my time taking it out and looking for broken connections?

-edit-
Also, what's the yellow connector for? Mine doesn't have a plug for it, but there's one on the dash. Some of the guides have the plug and others don't. Is it an early model thing to have it?

[jlevie]

The SI lights being on doesn't mean that the SI board is good. While cracked solder joints or bad connections are possible, other faults are more likely.

I'm not sure what you mean by a yellow connector. Nor do I know what car you have.

[econti]

Oh, right.
On the back of the cluster, I had two plugs that connected it to the car. One blue, and one white. They are on the far left and far right of the cluster.
I also have a yellow plug, that's right in the middle of the cluster. This didn't have anything connected to it.
In a guide I was following, the guy said that he had something connected to the yellow one, unlike me.
Why are some different?
Mine's a 1990 318is. Is it an early model thing to have a yellow plug?
-edit-
Hopefully picking up a new SI board soon.

-edit again-
Checked the batteries, both on 3.17V. So a connection on the board is most likely the problem.
My batteries aren't AA size though, is that also an early/late model thing?

[econti]

Okay, so I got a known working SI board and installed it, still no tach.
So the problem is in the actual tach part of the cluster? The mechanism itself?

[Liquidity]

Mine has the yellow socket but it there is a matching yellow plug in it. When my milometer gears get here I'll try to see what it's for.

[uturn]

Fish around for that yellow connector, every e30 I've seen from 1986 to 1992 has it connected to the back of the cluster.

[econti]

Any idea what it does?

[nando]

pretty sure the yellow connector is for the OBC, it's not required for the tachometer. the white and blue plugs are however.

[econti]

That would make sense, as I've got the swanky clock instead. You got me exited though.

So can the tach unit itself be removed, rather than just replacing the whole cluster? Changing the odo reading is something that's frowned upon in rego inspections here.
Is there something in the tach unit that's a known failure point?

[uturn]

Sorry about that, just looked a a spare dash I have in my garage and it's like what Nando says... yellow plug goes to the OBC.
Hesitant yes on swapping the tach, it is a separate unit when you open up the cluster. Just not sure if the two are interchangeable (eta motor tach vs "i" motor tach). Eta tachs are found in US cars, unless one was imported, whatever you find near you should work.

[Gregs///M]

Okay, so I got a known working SI board and installed it, still no tach.
So the problem is in the actual tach part of the cluster? The mechanism itself?

How do you know the SI board is good? Did you recently purchase the board from a rebuilder like myself or other company?

Of the many clusters that I have repaired, rarely does the tach itself fail. It is almost 100% of the time the SI board batteries. Also, Si board can easily get damaged during installation.

http://www.r3vlimited.com/board/showthread.php?p=3676570

Also make sure there is a coding plug installed or the tach will never work. With the red and yellow lights remaining on and not resetting, is a sure sign the SI batteries are dead.

[Gregs///M]

Oh, right.
On the back of the cluster, I had two plugs that connected it to the car. One blue, and one white. They are on the far left and far right of the cluster.
I also have a yellow plug, that's right in the middle of the cluster. This didn't have anything connected to it.
In a guide I was following, the guy said that he had something connected to the yellow one, unlike me.
Why are some different?
Mine's a 1990 318is. Is it an early model thing to have a yellow plug?
-edit-
Hopefully picking up a new SI board soon.

-edit again-
Checked the batteries, both on 3.17V. So a connection on the board is most likely the problem.
My batteries aren't AA size though, is that also an early/late model thing?

Also measuring voltage is not a conclusive way to test the batteries. You can read my technical article about testing the batteries at my website.