Tac. in 80 reading high. Need some input

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Feb 13, 2008 | 10:05 PM

bliehr

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Instructor

Joined: Jul 2005

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Tac. in 80 reading high. Need some input

Hey Guys
The tac. in my 1980 is reading higher then what the engine is really running at.
What causes this and what is the fix.
Thank

Feb 13, 2008 | 10:27 PM

ram100987

Instructor

Joined: Sep 2006

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I have this issue from time to time, especially when getting off a highway. The tac may show i'm idling at 1k or more but I know from the sound that it's idle at it's normal 600ish

Haven't fixed the issue though, sorry

Feb 13, 2008 | 10:45 PM

65GGvert

Team Owner

Joined: Jul 2006

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From: Kannapolis NC

A likely suspect is the tach board mounted to the back of the tach. If it has been changed and not calibrated for the 80 model, it will be off. Also the needle may have slipped. Does it read 0 with the switch on and engine off? Has the distributor been changed to a different type?

Feb 14, 2008 | 01:54 AM

thrilher

Racer

Joined: Sep 2006

Posts: 484

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From: McComb Ms.

i agree with the tach board behind the dash, the tach board for that year is '80 specific in other words '79 for instance won't show proper rpms in an '80 vette...also when replacing those boards there were alot sold that are made in china and were and are total crap, never correct calibration..make sure when buying if you choose to do so to make sure yours is made in US or somewhere besides China. i forget where i ordered mine now but ask the vendor.i ordered mine frmo someone that strictly does those boards and no other products.make sure your salesman is aware that you know the issues if you don't speak directly to the professional who calibrates them.returned items on vettes net the vendor huge annual profits thru ridiculous shipping charges, especially when they are sent twice and returned once...goodluck

Feb 14, 2008 | 01:46 PM

C3 4ME

Le Mans Master

Joined: Oct 2005

Posts: 6,247

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From: Glen Allen, VA

Get in touch with board member Aceco, he fixed the tach on my 79 that was also readign high. I took mine apart and sent it to him, and he fixed it for less than the cost of a tach board.

Feb 14, 2008 | 05:56 PM

1Z87L8

Drifting

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he fixed mine in my 78 too

Feb 15, 2008 | 09:02 PM

bliehr

Thread Starter

Instructor

Joined: Jul 2005

Posts: 103

Likes: 1

Thanks for the replys
I've had this car for years and nothing has been changed.
The tac did work at one time.
What is the small plastic box close to the distributor which the distributor tach wire runs through ?

Feb 15, 2008 | 09:26 PM

65GGvert

Team Owner

Joined: Jul 2006

Posts: 22,190

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From: Kannapolis NC

Don't know for sure about the 80, but on the 78 the wire runs through a tach filter. It's a metal cylinder shape that's bolted to the intake manifold. It looks like the old condensors from the distributor, but has longer wires. The filters can short or open too. You can bypass the filter to troubleshoot, but I don't really see how it could make it read consistently high. It sounds like the tach board. I'm betting that if you pull it out and take a look, there will be a burned spot around one of the components on the board. Again, if you get a tach board, make sure you get one calibrated for the 80. Let us know what you find.