Electrical Getting stock tach/speedo to work after holley efi install?

moneypit94

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May 23, 2003
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Tach: I am trying to get my stock gauges to work after a holley dominator efi install. I know my tach guage is supposed to get a DC square wave form. I know my msd box puts out a 12v square wave form. I know on my 94 that the tan and yellow wire is the tach signal for the guage cluster.

My question is what is the voltage that the stock tach signal is supposed to see? 12v? Im wondering if i would be able to luck out and be able to use the tach out signal on msd box to drive stock tach? Dont want to plug it in and it screw something up.

Speedo: from what i found it takes an AC signal and is callibrated at 8000 pulses per mile. Because of the powerglide i put a 8 magnet collar on my pinion yoke and mounted a msd hall effect sensor to pick this up to deliver signal. This im sending to my dominator to calculate driveshaft speed.

I have mounted everything but have not wired it up (3 wire hall effects). So bare with me on some of these next questions. Can i split this signal wire and send one to a dakota digital box to recalibrate the correct pulse rate to send to my tach? Then send the other wire to the holley?
 
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moneypit94

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May 23, 2003
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Ok, found where i had dsconnected a harness off of or around the old ecu that leads to the tach. I tried to hook it up off of the (-) side of coil. Like in the diagram below. Is there a resistor between tach and coil? I found other diagrams that show it hooked up straight to it.

https://goo.gl/images/bhYm1e

I did see the tach kind of move a bit during startup. But it didnt move a bit after car was running.

Because my msd box is literly 15 inches from that connector i decided to plug it into the tach output on the box. It fried EVERYTHING!!... i kid.. the tach actually works great now. Im not going to say how long it will work like this but it is for now. Hopeing to get some input before i soldier in my jumper wire and put moldings back on.
 

moneypit94

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May 23, 2003
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Emailed Dakota Digital tech and they responded by suggesting to use the digital box to power, ground, and take the input from the hall effects sensor. Then send the two signals to the devices i want. The only issue I see with this is that I purposefully bought a 8 magnet collar for more precision on the driveshaft speed. They have taken that 16K ppm and divided into half. I may try this out and change it later if I want more accuracy.

Here is the answer from tech support from dakota digital with a diagram they sent:

8 magnets on the driveshaft would make approx. 16Kppm signal.
The 3-wire HE sensor typically will produce a square wave signal.

Depending on voltage needs of the 3-wire sensor, you may be able to power it directly from the SGI.
If not then just the signal wire would go to the SGI SIGNAL IN. As long as the dominator is flexible for speed in, you could use OUT1 to dominator (16KPPM A/C Sine) and OUT3 to VSS Hi to the mustang speedo. Set SGI-5E to “L-L” Low in – Low Out and OUT3/OUT4 will be 8Kppm. Then you can use the +/- buttons to fine tune calibration of the speed on speedometer
 

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Rcdgl

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Feb 19, 2018
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My MSD Digital 6 operated the stock tach in my 82 Mustang just fine for several years. That tach died, probably the second one I have installed in 15 years.