Could be bad alternator (this is a sign), or like mine, there's a single wire that plugs into the top rear of the alternator that is for the tach. Mine was loose, I just spread the connector some on the end of the wire and pluged it back in and it is now fine.
Hey, thanks for posting about this, I finally got my tach to work correctly, after it hadn't functioned for almost a year. The old sender wire connector had lost connectivity at the alternator, so I crimped on a new blade connector, cleaned the alternator-side contact, and voila! Functional tachometer!
Same thing starts happening to me about 500 km before the alternator fails (I've had it rebuilt several times already). First couple of times it was the diodes, then the bearings seized. I'm buying a new one soon...
Mine is on its way out also. It started out being really intermittant and it omnly lasted a few seconds, now it takes 5 minutes to come up, there's a whineing sound when it comes up so i think its the diode, although it may be a bearing because it shows some revs, just doesnt move when i step on it. i usually let the car warm up before i drive it anyway and it works after sitting for a bit. I sprayed some 50/50 diesel/90wt on the front bearing and it freed up a bit.
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