Factory Tach Wire?

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ham1974

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Oct 6, 2012
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To retain the factory tach does it need to have that I guess is a resistor still in line? Mine is mounted on fire wall near distributor I was going to pull it in behind dash just curious about the pod attached to wire. Also the large pink wire that ran to the distributor were does it connect in the fuse block?
 

ssn696

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Pink is switched power from the ignition. Tach has a capacitor in it to damp the unwanted frequencies and give a smooth RPM readout.
 

ssn696

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Here is a post from a timely thread in play on the Forum right now:
Erratic tachometers can be from a number of culprits. The tachometer operates by reading a "pulsed" signal from the ignition coil from the negative terminal. The AC sinewave (thereabouts) is read and the tach reacts. The old style (like yours) operates on this high voltage signal, newer tachs will read a low voltage signal (an old tach will not work on an HEI with a hall effect signal. )
Enough about that....

Several factors can make a tach erratic may of them would require an oscilloscope to verify but here are some places to start:
- 31 culprit is bad ignition wires/distributor cap, this can "bleed" a signal to the tach source wirte on the negative side of the coil (tach output on older HEIs)
- Coil going bad or if the tach has a ballast resistor that might be going bad
- The older tachs usually had a silicone dampening fluid in them, that may have dried out and the tach is operating properly but without the dampening fluid the pointer looks like it is moving erratically.
- #2 reason: The capacitor(s) in the tach are going bad. They are just a battery that charges and discharges quickly. Just liek a battery they have a definite life.

Sometimes a tach filter will clean things up and keep it working well, but that is usually a short term (6 months-5 years).

If you have an oscilloscope, best way to check for a clean tach signal is at the back of the cluster, that way you can "see" what the tach sees. Then if the signal is good you can move on down the line, without a good signal the tach will not operate properly.

Hopefully this helps
 

ham1974

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Oct 6, 2012
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I am not sure if this matters but it will be running from a cdi box. Yes I know what the pink wire is for just trying to see were it connects in the fuse block. There are a couple of pink in the block I am thing it’s the one in the middle that runs under the silver plug in.
 

ssn696

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See if it leads out from behind the 'IGN' fuse. This is the wrong diagram, but it may help.
 

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drogg1

G-Body Guru
Jan 25, 2009
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Are you using something like a MSD 6AL with the factory HEI distributor? You should be able to hook it up like it shows on page 13-15 of the link below. As it says, you want to connect your tach to the the grey output wire from the MSD CDI or connect the white wire to your factory tach wire.

https://www.jegs.com/InstallationInstructions/100/121/121-6425.pdf
 

ham1974

Greasemonkey
Oct 6, 2012
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I am hooking up to msd style(summit racing) box. I was just wanting to put all the wires behind the dash.
 
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