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Old 06-09-2007, 03:01 PM
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Electric Tach 68 396 Rs Ss Conv Help

I'm having problems with my tach being to high. I've checked all the ground and seem that the tach is running about 2000 rmp high. everything orignal but i took the point out and add a electronic ingnition. any idea's.

steven
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Old 06-09-2007, 07:35 PM
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Doc here,

You may need a Tach filter..

If you upgraded to an HEI, the feed to the Tach is too High and not the proper waveform.

Running it like that will destroy the Calibration Card (the guts) of your stock tach.

A new aftermarket Tach will operate just fine without a filter, But a Pre HEI tach won't.

If you can't find a filter, go to Corvette central, they sell them $56.00..but Add another $50 or $60 Fazools, and you can get a Monster Tach with shift lamp..and save the Stocker for resale of the car.

Doc
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Old 06-22-2007, 11:36 AM
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Doc,

Do you have any idea if the circuit board for the '75 Corvette tach would be the same type as the factory tach on in my '75 GMC truck (with 350 and stock HEI)? GM truck tachs of that vintage are only 5000 rpm maximum.

My tach works, but has either a loose connection or bad solder joint that makes it unreliable. It usually only works about 2/3rds of the time, and even less in colder weather. I don't want to spend the money on a complete rebuild, but $60 for a new circuit board would be manageable.

Thanks,

Bruce
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Old 06-22-2007, 01:48 PM
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Quote:
Originally Posted by 75gmck25
Doc,

Do you have any idea if the circuit board for the '75 Corvette tach would be the same type as the factory tach on in my '75 GMC truck (with 350 and stock HEI)? GM truck tachs of that vintage are only 5000 rpm maximum.

My tach works, but has either a loose connection or bad solder joint that makes it unreliable. It usually only works about 2/3rds of the time, and even less in colder weather. I don't want to spend the money on a complete rebuild, but $60 for a new circuit board would be manageable.

Thanks,

Bruce
Doc here,

The card should be the same..check the card for a run number to be sure..But provided it came from the same type Engine / Vehicle, It should work..

More important, Have you been running this on an HEI from points type, OR older Tach to HEI, without the STOCK tach filter/Buffer installed ?

If so, that's why your Tach is dying..AND your new card will do the same without one..

A Stock Points type tach was not compatible with an HEI system, Nor were the early Tach's without the Buffer / Filter..and the higher voltage / waveform will damage the calibration card.

It looks like a "Noise Cap" on a coil, but It Isn't (round metal canister with 2 wires on it.) ... it goes between the TACH wire (white wire) and the TACH terminal on the Dizzy, usually mounted on the intake manifold.

Doc
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Old 06-22-2007, 05:21 PM
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My '75 truck had stock HEI, but the instrument cluster I'm using is from a '74, which was the last year of points. When I pulled the cluster at the junkyard the engine was already gone, so I don't know if there was tach filter/buffer on the donor truck.

There are several articles on the net about circuit board wiring problems with the old tachs and I assumed that was the problem I had. However, maybe its the missing filter.

Where do I find and buy the tach filter/buffer to use this old tach with HEI? Is it a standard GM part or electronics store item?

Thanks,

Bruce
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Old 06-22-2007, 06:11 PM
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Quote:
Originally Posted by 75gmck25
My '75 truck had stock HEI, but the instrument cluster I'm using is from a '74, which was the last year of points. When I pulled the cluster at the junkyard the engine was already gone, so I don't know if there was tach filter/buffer on the donor truck.

There are several articles on the net about circuit board wiring problems with the old tachs and I assumed that was the problem I had. However, maybe its the missing filter.

Where do I find and buy the tach filter/buffer to use this old tach with HEI? Is it a standard GM part or electronics store item?

Thanks,

Bruce
Doc here,

If you can't find one local, Corvette central has them for $56.00.

Doc
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