Tach staying under 1000rpm

[TSI]

My gf's vert is acting funny. The tachometer stays between 300 and 1000 rpm all the time, even though the engine revs fine.
The needle moves proportionally to the engine's revs, it's almost like the whole normal range of the tach was compressed into this 700rpm range.
SI batteries were replaced 6 months ago, the tach was working fine after that. What gives?

[jlevie]

When the SI batteries were replaced, had the original batteries leaked? If so the fluid and subsequent corrosion has probably damaged the SI board beyond repair. That could cause the tachometer problem. This pretty much has to be the SI board or the tachometer. Swapping in known good parts will be the easiest diagnostic.

[Gregs///M]

My gf's vert is acting funny. The tachometer stays between 300 and 1000 rpm all the time, even though the engine revs fine.
The needle moves proportionally to the engine's revs, it's almost like the whole normal range of the tach was compressed into this 700rpm range.
SI batteries were replaced 6 months ago, the tach was working fine after that. What gives?

Not all Si boards are refurbished the same. I've been refurbishing Si board for two years now and have done hundreds. I have seen a bunch of SI boards that have physical and/or electrical damage yet only the batteries are replaced.

Many companies that repair SI boards have sub-standard quality, do not test the SI board before shipment, and often miss physical damage that will render the SI board not fully functional. Most of the companies replace the batteries and send it back to the customer without testing it first.

Please see my thread on SI board damage here, you might find similar damage on yours.
http://www.r3vlimited.com/board/show....php?p=3676570

Also try replacing fuse 10, 12, and 21. Old fuses can cause intermittent electrical problems.

[Steve Corbett]

Bump from a very satisfied customer of Gregs work. I am running his rebuilt boards in my 84 325, 89 325, 83 528E and my 84 745I. The first board I ever swapped was out of my 83 528E. I bought off EBay, Piece of crap. Then found Greg.

[TSI]

When the SI batteries were replaced, had the original batteries leaked? If so the fluid and subsequent corrosion has probably damaged the SI board beyond repair. That could cause the tachometer problem. This pretty much has to be the SI board or the tachometer. Swapping in known good parts will be the easiest diagnostic.

This may be the case. I might get a chance to open it up this week. Strange that it should happen months after the replacement though...

[TSI]

Interestingly, about a week ago it started working fully again of its own accord. I am thoroughly perplexed.