What Voltage For Tic-Toc-Tac Off The Coil On A 70 RR?

Electrical & Ignition

  1. soundhd

    soundhd Well-Known Member

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    Have a 70 Roadrunner (383). Has the Orange Mopar Electronic Ignition Control Module mounted on the firewall. Have a fairly new Tic-Toc-Tac (2 years old) that I purchased from (do not want to say at this point), anyway it stopped working last fall and this past week I decided to try see what the problem is. Tested the voltage coming out of the negative side of the coil (where the tac is connected to) and I get 2.9 volts at idle (get 6-7 volts on the positive side). Contacted the place I got the Tac (assume it has the electronic circuit board in it) from told them what voltage reading I am getting and they told me that the tac needs at least 6 volts to work properly, asked if the car had the Orange Mopar Ignition Module and I replied yes it did and they said owners have been having a lot of problems with those modules putting out out voltage...so I do not know how old the one is that was currently on the car (car was restored approx 14 years ago) so I ordered a new one and installed it....also replaced the coil while I was at it. Well I am still getting the same voltage readings.
    So can anyone tell me what the voltage is supposed to be coming out of each side of the coil at idle? Nothing in the shop manual about that.
    Also was wondering is there a way to that I can test the Tac?

    Thanks
     
  2. soundhd

    soundhd Well-Known Member

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    UPDATE: Took more voltage readings at the Ballast Block (was suggested that maybe that is going bad.....was told that it should have 12+ volts going in and it should only drop the voltage a couple volts so I assume 10 volts going out?) anyway the readings on the old block were 13.3 volts going in and 7.8 going out (what I am getting at the + side of the coil), so I picked up a new Ballast Block from the local NAPA store and installed it and still get the same readings. So either I have something else wrong (I have a volt meter in the car and get 14+ volts when the engine is running) or the guy from where I had the Tac built is wrong with what he told me that the Tac needs at least 6 volts to work. Next step is to borrow a Tac from a Mopar buddy and see if it works if so I have a bad Tac.....
    Oh by the way the car seems to run fine...........
     
  3. mopar 3 B

    mopar 3 B Well-Known Member

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    He is correct the tac needs at least 6 volts. You do have to tac connected to the neg coil post?
     
  4. RJRENTON

    RJRENTON Well-Known Member

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    You need 12v going into the instrument from the switched source (instrument panel harness) to power the electronics. The coil connection furnishes an average voltage pulse to the instrument (as the coil turns on and off generating the spark). As the RPM is increased the voltage pulses increases so the instrument sees a higher AVERAGE pulsed voltage and reads a higher RPM. On the indtrument's tach board there are calibration adjustments to allow the instrument to read the correct RPM at diffrrent pulse levels. REMEMBER the instrument is actually a DC volt meter calibrated to read RPM. The electronics converts the voltage pulses to a steady DC voltage which moves the pointer.
    Bob Renton
     
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    • soundhd

      soundhd Well-Known Member

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      UPDATE #2: sorry guess I should of stated that the Tic-Toc-Tac in question is one of the newer ones that has the solid state electronics in it. Only has 2 connections on the back, #1 connects to the negative side of the coil for the Tac and #2 connects to a constant 12 volt power source for the Clock.
      Did not connect the clock side when I installed it over a year ago, did not want to load on the battery when parked but it Tac part was working fine until late last fall when it stopped.
       
    • soundhd

      soundhd Well-Known Member

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      Thanks for the replies but I am still waiting to fine out what voltage the negative side of the coil is supposed to be putting out on a stock 1970 Roadrunner, 383 with auto trans at idle?

      Thanks
       
    • RJRENTON

      RJRENTON Well-Known Member

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      The instrument's panel lamp is the pigtail connection. The other two connections are for: 12V switched source from the wiring harness to power the instrument and the other wire (brown) is the wire that is connected to the coil's NEGATIVE terminal. This is the wire that supplies the pulse voltage to allow the tach to display the RPM.
      YOU MUST HAVE BOTH 12 VOLTS AND THE COIL WIRES CONNECTED. THE INSTRUMENT PANEL LAMP IS OPTIONAL (if you want to see it at nite). The original tach uses a mechanical clock that is automatically wound about every 5 minutes or so.
      Bob Renton
       
    • mopar 3 B

      mopar 3 B Well-Known Member

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      Voltage at the coil will be 12 volts with the ignition switch on and not running and 6 volts running for a stock ecm system.
      Some ones doing to tell us I am wrong. What for it.
       
    • soundhd

      soundhd Well-Known Member

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      Got the Tac working...discovered that the ACC fuse blew at the bottom of the fuse where I could not see it...found it with a volt meter.
      ANYWAY, the Tac works with 2.9 volts at the Negative side of the coil at idle. Still do not understand where everyone says the Tac needs 6 volts at the Negative side of the coil to work.....

      thanks
       
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