intermittent tach

[Kershaw]

tach has been working for the last 3-4 weeks i've been running my lachs car. (was sitting since march before that.)

today i turn on my car and the tach doesnt work. on my commute to work it will sporadically jump to 1k and if i gave gas it would rise to 1100-1200 but no further. (not my actual rpms. i was going 75-85mph.)

on my errands around town for work today, i'd start up my car and the tach would work for the first 20 or so seconds. then it would drop dead.

SI battery? loose connection? anyone experience this before?

[phenobarb1]

I had an issue like that, but it sort of cleared itself up for the most part. Every now and then my tach will still briefly jump by maybe 300-500 rpms, before settling back down, but it doesn't play dead anymore. Considered the SI battery, but also noticed that the cable on my CPS is a bit frayed, and when I tested the resistance it was way off. Won't know if that sorts things out till the new sensor arrives though, but I thought I would put it out there, since I think the tach is controlled by the CPS, at least on the later 325i's.

[Kershaw]

huh, i hope it's not chewed up by the fan.

[neuroboy]

I was having a problem like that (also combined with a battery issue) where the tach worked intermittently. It turned out to be a bad connection behind the instrument cluster. My mechanic said he pulled it out and re-soldered the connection. The "mpg gauge" under the tach and the engine temp gauge were affected, too, and everything's now back online.

[Kershaw]

hitting my dash has no effect on it whatsoever. but i'll pull the cluster to check.

[dnguyen1963]

There is a small transistor (next to the battery terminal) on the SI board controlling the tach and fuel econo gauges. If this transistor gets corroded from battery leakage then the gauges will no longer work. It might be time for you to replace the SI board if you see a lot of corrosion. If you are really good with soldering, you can try to replace the transistor. It is tiny.

[rares]

There is a small transistor (next to the battery terminal) on the SI board controlling the tach and fuel econo gauges. If this transistor gets corroded from battery leakage then the gauges will no longer work. It might be time for you to replace the SI board if you see a lot of corrosion. If you are really good with soldering, you can try to replace the transistor. It is tiny.

i agree, had the same issue happen as the OP, took the board out and it looked like white power was coming out of the batteries... took my board to the store (batteries plus) and the guy soldered two new batteries.. quick, easy, cheap and kabooom!!!

good luck