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Discussion Starter #1
The tach on my 81 Corvette reads slower than the engine rpm unless I rev the engine up around 3,500 or more. Can anyone suggest what the cause is and a quick fix?
Thanks
 

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Tach

Doc here:pimp:

Check the Filter on the manifold that comes off the tach wire...If fact, if it's stock just replace it...It's a cap that wave shapes the pulses to the tach and the computer.

Looks like a little round dude about 3 inches long with a tan wire out each end should be mounted on the right rear of the manifold.

ALSO double check that your tach is on the Filter side, not the Distributor side...The other way around will damage or destroy the tach.

If that dosen't Fix it, you need a Tach R&R calibration board, You can get one on EBAY (ongoing add) For 50 bucks.Listed under 1980 Corvette...or maybe look for a whole Tach Cheaper (good luck on that one...)Than 50 bucks...

The Tach's on C3-C4 Corvettes are always a matter of contention for calibration .. If you are handy and careful, you might try a Corrosion Control program on the circuit card, and plug, and the mechanical movement (may have a sticky movement) and bear in mind these aren't always instanious reporting tach's either..they take a second to get up to speed, I have a Monster Comp tach in mine, and it bunches quicker than stock, and about 250 rpm higher...

Doc
 

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Discussion Starter #5 (Edited)
Thanks and more questions

Thanks for the info. I looked last night when I got home and found round plastic devise mounted on a short metal bracket at the right-rear of the intake manifold. It is only about an inch (or less wide) and about the diameter of a 50 cent piece. I did not find tan wires in and out of it. Could this be the filter to which you refer or should I be looking for something that looks different? Do you know if this is a dealer-item or available at parts stores and about what I might expect to pay?

Thanks again.

I found a picture of the filter at the following link, so now I know what to look for and the cost.
Again, thanks.

http://www.madvet.com/shop?frame=1.132.5902
 

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Filter

Doc here:pimp:

Yup, That's the Tach Filter...

Spendy little dog, isn't it? 50 bucks for nothing more than an electrolytic capacitor, that you could probably get at rat shack for $2.49 .....

It's Sole purpose in life is to Clean the spiky Wave coming off the Tach output of the HEI and Shape it to an "Nice Clean" Square wave (or facsimile) for the Computer reference pulse and the Tach.

If I knew the Value, in MFD's and WVDC, I'd try an Electrolytic Cap on mine, and let you know the results, however It's another General Mystery Secret...Nobody seems to know the value...and, Caps can be nasty little bullets when you explode them from over WVDC...or reversing the polarity...

I don't think your local NAPA auto parts is gonna carry one either, although I didn't try..did a check on CSK, Auto zone, and Kragens, and they don't carry them..so It's probably a Corvette specialty item, Corvette Central carries them at $50 plus change.

If I had a Decade Box, I think I'd Find out the value of the darned thing, buy a box of $2.49 Caps, Vulcanize them to a metal manifold mount, and sell them on EBAY for $25..and Re~retire..Fat , dumb , and Real, Real happy..

Doc :pimp:
 

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Discussion Starter #7
Well, guess I don't have to tell why my user name is Corvettepoor. Even though I know virtually nothing about electronics and only understand a little of your technical explanations, I understand basically what it does, thanks to you.

I do appreciate the info. and the extra effort you went to in looking for the item at parts house websites.

Thanks again.
 

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Is'nt it amazing that the minute you add the word "Corvette" to a part the price doubles. Called the local parts house for a heater blower motor for a 74 Corvette and then the next day for a 74 Chevy and the price was double for the exact same motor.:thumbup:
 

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Price

Doc here:pimp:

Pony,

EXACTLY!!...The Word "Corvette" is synonymous with upping the Company Bottom line where parts and service is concerned...

This is why, I spend a lot of time in the Boneyards looking for the same year/passenger car parts...then if unchangeable, ordering the new passenger car part at like half the "Corvette" price..

The Downside is some of Corvette parts are "Almost" the same..but have been modified to "Fit" in a Vette's Cramped spaces...

Brake, Driveline and suspension parts are a "One Of" too.. So you just have to bite the Bullet on those parts and dig a bit deeper in the pockets.

I get Nervous when the parts book say's "ALL EXC CORVETTE" I know I'm gonna take a major hit in the Checking account..

Doc :pimp:
 

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tachometer resistor size

I have a 1981 Corvette . It's tachometer is blown. It needs a new resistor . The color code is melted so I can't be sure of resistance. Does anybody know what size resistor I need ?


:smash:
 
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