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Old 09-21-2006, 12:10 PM
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1.21 giga-watts???!!!!
 
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Tach

Hello again. Some of you may remember my posts concerning my OER repro dash tach for '69 Camaro. Ignition system is factory HEI with MSD internal coil and MSD module. Tach has pegged to the right as soon as key turned to "on" position. MSD sent numerous tach filters and none worked except once for a couple of minutes. Started engine again and it didn't work. Sent tach to MSD and they said it is a bad tach. As soon as they put juice to it, it pegs and they can't do anything with it. Chicken and egg type of thing. Was tach bad to begin with or did MSD stuff fry the tach?

My question is this: is there any reason why I could not swap this tach face and needle onto a Autometer tach? Autometer tachs do not need any filters to work with MSD electronics. Do the two tachs need to have the same redline to work? Will accuracy be compromised by going with a different diameter tach, ie a 3.5 inch tach internals with a five inch face? If this can be done this would be the least expensive, least painful way to put away a problem that has gone on for way too long and taken way too much time. Thanks in advance for your replies.

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Old 09-21-2006, 05:16 PM
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Doc here,

If a Stock GM tach of that vintage was run without a Buffer / Shaper, yes..That was the reason it went out..

First , Open Both tachs, Carefully remove one wire from each meter motor winding..With your DVOM measure the resistance across each meter motor..They should be about the same resistance.

IF that's the case..Remove the OEM Calibration card..add extension wires to the + and - meter motor wires, either place a two connector plug at the back of the tach bucket (recommended) or drill a hole and install a grommet here (less recommended)

Now remove the Calibration Card from your Autometer Tach, get a small plastic project box from Rat Shack or the electronics store (be sure it will house the board and an in and out connector.

Next install the calibration board CAREFULLY into the project box, using provided hardware, (If no hardware, you CAN use a "Bed " or RTV on the bottom of the box about 1/2 inch thick, and "Pot" the board in that [less recommended, but will work] ) .

Cut and install a hole for your two connector plug, for the remote tach head..and mechanically install it, then install the wired pins. Make a small two wire harness to go between the remote head and the calibration card box, install the proper connectors..

Finally, you can install a plug from the calibration card box to the Vehicle connections (recommended) or just drill a hole and install a grommet and run the vehicle wires to the car..(less recommended)

Verify the car wiring, install the Stock tach bucket, and start and test the unit..If the magnet motors were within the ballpark or each other (and most are) it should operate just fine..Even if they are different (within range) It may be possible to install a variable resistor to "Calibrate" the motor resistance to the card..

You will end up with a stock ( looking ) Dash Tach, Driven by an new Autometer calibration card. Only you will know the difference..

PARTS:
  • Project plastic box. probably 5 in X 5 in.
  • 2 two pin female connectors
  • 2 two pin male connectors.
  • 1 Three pin Female connector (you won't be using the dim on the calibration board)
  • 1 three pin male connector.
  • Some heat shrink tubing.
  • Misc Hardware
  • 20 gauge wire 50 feet.



(Old tech head) --------- [New Calib card]--------> to vehicle wiring

That should set it right..

Doc
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Last edited by docvette; 09-21-2006 at 09:40 PM. Reason: Added Schematic
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Old 09-22-2006, 11:10 AM
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1.21 giga-watts???!!!!
 
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Thanks Doc. I am going to do this in the next week or two (man, am I swamped between work, familly, etc) and will let lyou know how it turns out.
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Old 09-22-2006, 04:12 PM
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Doc here,



Doc
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