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Old 11-11-2001, 11:05 PM   #1
Joelk
Scooby Newbie
 
Member#: 7790
Join Date: Jun 2001
Chapter/Region: MWSOC
Location: Minneapolis
Vehicle:
2003 wrx
sy

Default Noisy tachometer??

I have a strange clattering noise from what sounds like inside the dash of my car. I can only hear it when I'm inside the car. Under the hood I can hear the valves ticking away, but that's a different sound.

The valves are a very steady, consistent ticking noise. This noise goes up and down with RPM, but it's not a steady tick, the only word I have for it is a clattering sound.

My old S-10 used to have a squeaky speedometer in cold weather, and this sounds kinda like that, except it is RPM dependant, not speed dependant. It gets louder when the weather is colder, but it doesn't seem to be affected by engine temperature.

So, is there anything spinning inside the tachometer? I don't think it's cable driven, but does it have a magnet spinning past a coil to move the needle up?

Otherwise, what else is in the firewall / dash region that could make a clattering noise like that?

Is it maybe my starter gear not dis-engaging (I just thought of this...) The starter is in about the right area that I'm hearing this sound, the gear spins out and makes contact with the flywheel when you hit the starter, then it should spin back after the car starts and stop spinning. Hmm... now that I think about it, that might be it, it sounds about right...

Anyone have ideas?

Thanks

Joel
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Old 11-13-2001, 01:12 PM   #2
Richard L.
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Member#: 8465
Join Date: Jul 2001
Chapter/Region: NWIC
Location: Puget Sound, WA
Vehicle:
'00 2.5RS Coupe, AW
'93 GMC Typhoon AWD

Default

Joelk, let me know what you find out. I'm hearing the same noise from my car. It's more of a random clattering sound. In my case, I don't think it's the starter. If it was the starter, the noise would be at the same loudness level whether it's cold or hot. My noise completely goes away after five miles of driving.

--
Richard
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Old 11-13-2001, 01:31 PM   #3
Joelk
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Member#: 7790
Join Date: Jun 2001
Chapter/Region: MWSOC
Location: Minneapolis
Vehicle:
2003 wrx
sy

Default

well, it's possible that the throw-out gear on the starter would only stick when it is colder. But, if it were the starter, I think it would die within a few hundred miles. So I have to agree that it's probably not the starter.



I just found this on http://rabi.phys.virginia.edu/HTW/el...eneration.html


Quote:
How does the tachometer in a new car work? It looks like a magnet wrapped with wire that's located very near a saw-toothed wheel that spins as the engine turns. -- TR, Provo, UT

The device you describe is essentially an electric generator. The toothed wheel is made of pure iron so that its teeth can become temporarily magnetized while they are close to the permanent magnet. When a tooth becomes magnetized as it approaches the permanent magnet, or demagnetized as it moves away from the permanent magnet, it changes the shape and strength of the magnetic field around the permanent magnet. Since changing magnetic fields produce electric fields, the tooth's movement causes an electric field to appear around the magnet. This electric field pushes on mobile electric charges in the wire coil wrapped around the magnet and generates electricity. The current in the coil flows one way as a tooth approaches the magnet and reverses when that tooth moves away from the magnet. Also, the faster the tooth moves, the stronger the change in the magnetic field and the higher the voltage generated in the coil. The tachometer can tell how fast the engine is turning by how frequently the current in the coil reverses directions or by how much voltage the coil generates.
If there's something like this spinning behind the tachometer, that could explain the noise...
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