tachometer hookup help

[70challengerrt]

I need help! I was hooking up my tack today and I noticed that the wires were cut and that was why it didn't work. There is a dark blue wire that according to the schematic goes to the tack and a wire goes from the tach to the negitive side of the coil.The original harness had this wire pulled out so I just ran one. When I did this at idle the tach works but reads about 6 grand. It is the 8000 rpm gauge in the original 70 rally dash.  Any suggestions.

[MEK-Dangerfield]

That's a toughie. The blue wire is your power. Do you still have a Mopar ignition system? I'm just wondering if maybe it has been converted to a Mallory, or MSD, or such.

  Mike

Edit: There is always the chance that the tach. just needs taken out and calibrated, as these were cheaply made, but maaan that is FAR off, if that's the case.

[70challengerrt]

Its stock mopar ignition but it was upgaded to eletronic. I used the stock black box not the orange box.

[MEK-Dangerfield]

OK then. I wasn't sure if a non-Mopar ignition would make it act incorrectly or not, but I just wanted to ask anyway. My only guess here is the tach itself needs some attention. Something inside isn't working right.

  Mike

[Chryco Psycho]

www.RT-Eng.com has a fix for Tachs that read incorrectly

[70challengerrt]

I was told that mopars used a resistor wire which had a total of 680 ohms so I used a 680 ohm resistor in a primary wire and the tach was close, but the needle moved slow. Does anyone ever heard of this, is this right?

[71 DEPUTY]

never heard of that one- if the tach needed a 680 ohm resistor- they would of put one in the tach itself as it was cheaper that way instead of using a expensive extra wire for a specific purpose- ie- not worth the trouble!!

tach needs some work to it.

had the same problem years ago- found that by using the old cap on the old points dist. on the neg side of the coil to ground helped to take the bounce out of the tach!

[hemi71]

OK then. I wasn't sure if a non-Mopar ignition would make it act incorrectly or not, but I just wanted to ask anyway. My only guess here is the tach itself needs some attention. Something inside isn't working right.

  Mike

Just a FYI....if you are running a MSD ignition, the factory tach will not work corretly. MSD sells a "tach adaptor" that will make everything work OK.

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[70challengerrt]

It looks stock including the coil, but it is out of a 75 chrsyler newport.