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Old 01-19-2012, 02:48 AM
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is the tachometer directly affected by the crankshaft?

I'm tying to figure out why after I accelerate the speedometer and the tachometer rise like they're supposed to, but if I take my foot off the gas and then reapply, if at that given time the tachometer falls between the 1000 and 2000 rpm by the time I give it gas, it sometimes will stick within that range and won't budge unless I slow down even more or I give it alittle bit more gas with a quick step of the gas pedal, then it will bounce up and read the correct rpms. The engine seems not to be affected because the mph keep rising and engine seems to keep revs fine, although when the tach gets stuck there a feint noise when I give it gas until it gets unstuck then it functions fine.

Does the tachometer have its own motor that operates it? because the Noise it makes is like something is getting stuck and trying to free itself. Also reading on the tachometer, receives impulses from different machines in the car, What are the possible causes that my tachometer sticks like this and makes a feint noise. Is there anything in the tachometer mechanism that would make noise? Or is this a warning sign for an underlying issue.
Also I haven't received any codes, I'd figure if it was something serious that had to do with the engine or transmission that it would throw a code.
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Old 01-19-2012, 03:03 AM
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Re: is the tachometer directly affected by the crankshaft?

It also seems like it can be a quirk I was doing research on the internet and haven't came across anything I described.
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2005 Jeep Grand Cherokee Lauredo
4.7 L V8 Monster under the Hood... At least that's what it sounds like.
Close to 68k, so she's an experienced Virgin.. I think..
No Mods yet, IDK where I stand, but I like *O* Big Shiny Wheels *O*
Now, what the hell was I sayin??
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Old 01-19-2012, 10:49 AM
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Re: is the tachometer directly affected by the crankshaft?

Hopefully someone with more cluster experience can chime in. But, in the interim I'll take a stab at it.

The RPM gauge is powered by its own motor, and receives its drive from the computer, not a direct voltage out from an engine component (I believe).

So, it is conceivable that the the Tach motor itself is having some problems (especially if it sounds like it is getting hung up on something), though I have never heard of this occurring (usually they work or they don't).

That being said, the RPM reading on these machines aren't always as simple as you might think because of how the transmission and torque converter affect engine RPMS. For instance, I believe the 2nd gear has two ratios - one for upshifts and one for downshifts. Throw in the torque converter having the ability to partially lock up or fully lock or not engage in the various gears, the relationship between engine RPM and vehicle speed is quite complicated! So, just because the RPMs aren't doing what you think they should, doesn't mean they aren't doing precisely what they are supposed to.

Easiest way to check the gauge is working properly is to plug in a programmer or OBD monitor and check what the computer RPMs are compared to what is read out on the dash gauge.
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Old 01-20-2012, 03:58 AM
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Re: is the tachometer directly affected by the crankshaft?

Thanx for your input. Dr. Lee Baugh. I did som research and discovered that there's an app for my phone that measures performance through Bluetooth adaptor that connects to the OBD port in my vehicle and they say it's very precise I'll give it a try. Also, I've thought that it could be the torque converter locking up on 2nd gear, could this be a possibility. I really hope it's nothing to worry about. Thanx
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4.7 L V8 Monster under the Hood... At least that's what it sounds like.
Close to 68k, so she's an experienced Virgin.. I think..
No Mods yet, IDK where I stand, but I like *O* Big Shiny Wheels *O*
Now, what the hell was I sayin??
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