Tachometer 101

[Siia109]

1997 NAS SE7 4.0L with 127K miles - there is a squeal that comes with the rev's at start up and for about 5 minutes until the tach starts moving and the squeal ends. Sounds like a cable needs lube but I am not really finding anything in the workshop manual that speaks to the tach maintenance / troubleshooting.
I assume it runs off the alternator, which I assume is electronically and then is switches to mechanically at some point to move the needle on?
Any help or site as to how the tach runs would be helpful as Im a blank slate with this!

Thanks!

 

[robertf]

serpentine belt problem. take the belt off and make sure nothing is locked up or about to fall off. might need a new idler pulley

 

[Siia109]

serpentine belt problem. take the belt off and make sure nothing is locked up or about to fall off. might need a new idler pulley

The squeal is not a slipping belt kinda squeal, and I am not getting any warning lights about drop in voltage either by the dummy light on the dash or the ultragauge. When the engine rev's drop to idle the squeal stops and then picks up with the increase in revs. A loose belt would squeal all the time and would be a different tone than this.
This is definitely associated with the tach cable as the sound stops in mid rev and the tach needle jumping off the peg. I just don't know how or where this is routed and the workshop manual really doesn't speak to it (or I am blind which I have been accused of before....)

Cheers -
Matt

 

[robertf]

there is no tach cable

 

[ERover82]

The tach is driven by a signal from the alternator. Something is going on with the belt, idler, or a driven component.

 

[Siia109]

The tach is driven by a signal from the alternator. Something is going on with the belt, idler, or a driven component.

This changes everything - I see what you mean now - so this could be an indicator that something is not right with the Alternator or the belt. Is there a way to narrow this down - i.e. if I am not seeing X then it must be Y? Can the interface of how the tach gets its signal with in the Alternator can go bad or cause this squealing?

 

[ERover82]

I'd pull the belt and inspect the belt and everything it touches. Everything should move smoothly and without noise. I'm not aware of anything electronic that could cause the symptoms.

 

[kris812]

WAIT.... did I read that correctly?

It takes 5 minutes for your TACH to start reading anything?

Electronic driven or not, it's still possible to be the TACH itself.. Extremely cold temps, broken gears inside it.. idk

BUT YES, pull the belt off and look it over.. Front main could be leaking oil onto it, idlers shot.. Do like ERover said, check EVERYTHING 30 second job to do once the belt is off... AND just replace the damn belt while its off and toss old in the back for a spare!