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Wiring a '76 tach to an HEI distributor?

I swapped out the engine in my '76 911 for a water cooled Vortec V6. It has an HEI distributor. I found the black wire with purple dots in the engine bay and hooked it up to the tach connection on the HEI distributor. The tach worked, but I think it's off. It seems very touchy.

What do I need to do to make the tach work correctly?
Old 10-15-2009, 04:58 PM
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Old 10-15-2009, 11:07 PM
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You have the right wire. Problem is the old design of the electronics in your factory tach. If you want it fixed and be done with it, then send it to Horth hollywood Speedometer. They replace the internals completely and put moden electronics and new mechanisms. The needle will no longer bounce or sweep wildly anymore. At the same time you could send them your oil level gauge and have them install a volt meter in it's place or any other reading you want. they also can reburbish them if the are faded or have any blems. Plus replace the incandecent bulbs with LEDs and eliminate the "I CAN'T SEE MY GAUGES AT NIGHT" syndrome.
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Old 10-16-2009, 06:48 AM
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87 911 Carrera platform
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Old 10-16-2009, 06:49 AM
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The GM HEI signal is a sine wave as the tooth passes the variable reluctor. The stock Porsche tach is expecting a square wave signal. You need a converter circuit to make the signal compatible.

See here for some documentation on the signal output.

General Motors 7-pin HEI Ignition Control
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Old 10-16-2009, 07:44 AM
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I have found the megasquirt site has accurate hei info. The tach output from the hei 7 and 8 pin modules is a 12v square wave. I have been using it directly to my tach for years. What hei module do you have? Do you have 4, 7, or 8 pin? Also, do you have anything tee'd into the tach?
Old 10-16-2009, 10:41 AM
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Thanks for everyone's suggestions. I'm thinking that a Dakota Digital SGI-8 adapter will do the trick.

Speedy - I have the stock GM HEI. Where would I find out what HEI module I have to determine 4, 7, or 8 pin?
Old 11-01-2009, 06:06 AM
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Thanks for everyone's input. The response by jpnovak was particularly helpful. He mentioned the HEI puts out a sine wave and the tach is expecting a square wave. That got me to thinking about a converter. I did a little googleing and came up with this: The Banderlog HEI Siganl Converter. I ordered one and am waiting on it to see if it fixes the problem.
Old 11-08-2009, 05:54 AM
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