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Coil effecting speedometer

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  • Coil effecting speedometer

    Today I changed out my coil to the flame thrower 40111. My electric tachometer is hooked up to the coil. Now my tac is showing fast for some reason. Any idea why.

  • #2
    got the tach lead on the correct side of the coil? Double check for your brand of tach but usually it would be the dizzy side of the coil.

    Coil same voltage as the old one?

    Was it known that the old reading was in fact accurate? Meaning was a shop tach ever used to calibrate the tach when first installed?


    • #3
      All leads are hooked up the same. New coil same voltage, 6 volts. The old reading was accurate checked mathematically and GPS for the speed. It revs to 2500 now without sounding wound out at all.


      • #4
        What type of readings were you getting before? upwards of about 2200-2400 when at top speed? i assume?

        I have to wonder if the new coil secondary voltage is leaking by ....
        make sure the coil is well grounded.

        Tom W may also have some ideas......


        • #5
          3~ Tudor's & 1~ Coupe
          Henry Ford said,
          "It's all nuts and bolts"
          "Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."

          Mitch's Auto Service ctr


          • #6
            One wire from the tac goes to the + side of the coil and the other wire goes to the -- side. Before with my overdrive, at 1500 I was going 46 MPH. Now at 1500 I am only going 35 MPH.


            • Mitch
              Mitch commented
              Editing a comment
              Make sure your clutch is not slipping!

          • #7
            If the wires are not switched, check Mitch's link above
            looks like you need 2 diodes

            needing a diode tells me there is leakage


            • #8
              I will check for a good ground. The coil case has new paint on it. By leakage do you mean there is something wrong with the unit.


              • #9
                no, some coils have such a high secondary voltage (the high voltage side) that some of it spills into the primary (low voltage ) side of the windings. I am sure there is a better more technical explanation of this. We have an engineer on here that can do better than I (Mike K)

                On a Model A which has relatively low compression (even our Snyder 6.0 CR heads are still nonetheless relatively low compression compared to high performance cars, race cars etc), it is not clear why there is a need for a coil which puts out more than 10K volts on the secondary side on a Model A. The system cannot use any voltage over that. The waay higher voltages are usually only needed for 10:1 CR etc, where the higher compression is trying to blow out the spark. Also, we run our cars way richer than say a modern. The EPA has the car manufacturers by the short hairs, and they have leaned the mixture out so much it doesn't want to ignite, thus another reason for a super high voltage coil. Anything over 40K on a performance car is wasted.

                Here is an Amazon link to the diode specified (1N5404)


                use alligator clips on the leads as heat sinks when you solder them, to avoid damaging them.
                If a diode is 'blown' it becomes a 2-way 'wire' meaning it passes electricity in both directions, instead of only one-way which was the intended function

                Also, try to make sure each spark plug lead has a path to ground, else the insulation of the coil windings can be damaged. ie avoid pulling a plug wire off a plug while it is running. If you want to check for cylinder balance, use a screwdriver and short that plug to ground

                I have to wonder if you have a VDO brand tach.
                We could never get VDOs to read properly even with a diode. They used to be the best, but I think they sold out to that 5-letter country. We now use AutoMeter with very good results
                Last edited by tbirdtbird; 09-10-2017, 10:16 AM.


                • Dennis
                  Dennis commented
                  Editing a comment
                  I have a pertronix ignition and a flame thrower in my pickup along with a AutoMeter tach. No problems with accuracy. I also have a ballast resistor in the circuit because it was changed to 12v from 6v later in the game.

                  I also have a flame thrower coil in my A when I switched to 12 volts. It also has a tachometer made by Westach, not the one sold by Snyder's, and have no accuracy problem with it.
                  Last edited by Dennis; 09-11-2017, 08:26 PM.

              • #10
                Thanks tbirdtbird for the explanation. No I have a Westec tachometer. It always worked fine with the old coil I got, I think, from Brattons. Just thought I would try one of these Petronix Flame Throwers everyone talks about being so great. I should probably just go back to my old coil.


                • Mitch
                  Mitch commented
                  Editing a comment
                  Did it run any better with the flame thrower?

              • #11
                WMWS, The only reason I bought the flamethrower coil for my A is Pertronix recommends epoxy type coils when they hang with the terminals to the bottom as our A's do. So I have the epoxy filled coil, but I've never seen an oil filled coil leak. I have a Westach in my A also, but it's an old one I found on ebay that had never been installed.


                • #12
                  some say the oil filled coil upside down leaves the windings not covered in an oil bath, and they burn out sooner. Evidently they are not totally filled with oil


                  • #13
                    OK, I see two separate items in this thread. First, the O.P. (WMWS) mentioned a coil change. Second, Mitch posted links to fixes for conversions to Pertronix (MOSFET) electronic switching to eliminate mechanical points.

                    Electronic tachs work by counting changes in a waveform (a.k.a. oscilloscope) and comparing that to some reference. They are made to recognize common Kettering ignition wave-forms using circuitry called a Schmitt trigger that converts sloppy looking input signals to the sharp ON/OFF signals that digital switches and counters (tachometers) need to function properly.

                    Most people think that resistance/ Ohms and turns ratio are the only properties an ignition coil has. Magnetic core design and other factors influence a property called inductance. When the magnetic field is moving yet another electrical property called impedance also comes into play. Changing to a coil with a different inductance and/or impedance changes the waveform and generally requires a different capacitor rating (Farads) for best performance in a Kettering ignition.

                    The result of any coil change is a different waveform. Changing from points/capacitor to transistor switching also gives you a very different waveform. Your Tach may or may not have the Schmitt trigger hysteresis calibrations (don't ask) set to read one clean on/off from each spark waveform input. Some signals may be skipped, or double counted. Result- wrong tach readings.

                    The two posted Pertronix 'fixes' are dirty ways to change the waveform your tach 'sees'. They may or may not work. If the only change was coil inductance causing a high reading another 'dirty' fix may be wrapping the tach feed wire half a dozen times around/through a heavy 1/2" steel washer (a torroid inductor).

                    Mechanical engineering 101: If you put an adjustment knob, screw, bolt, or tolerance specs on something, some people will immediately fiddle with it. If you mark it DO NOT TOUCH everyone will mess with it.


                    • #14
                      Thanks for stopping by, Mike, I knew you could help out

                      BTW, Mike, could you explain the reasoning behind Pertronix showing 2 diodes in series?? I have never seen that on a schematic
                      Last edited by tbirdtbird; 09-12-2017, 11:20 AM.


                      • MikeK
                        MikeK commented
                        Editing a comment
                        They are 400V diodes. Two in series will block 800V before breakdown. A normal Kettering ignition gives about 80V reverse EMF. If you have the pertronix triggering a CD ignition you can easily get 350V reverse EMF. Of course, this seems rather moot to me. If you have already subjected a normal tach to that kind of reverse EMF and it doesn't work you have probably already toasted it.

                    • #15
                      The original post sed SPEEDOMETER, IF a coil made my SPEEDO faster, I'd wire 3 coils, in SERIES, I know MikeK will say it WON'T WURK!
                      One would be for RETARD.
                      One for half assed ADVANCE.
                      And one for BALLS OUT.
                      SA KDad


                      • #16
                        Originally posted by BILL WILLIAMSON View Post
                        The original post sed SPEEDOMETER, IF a coil made my SPEEDO faster, I'd wire 3 coils, in SERIES, I know MikeK will say it WON'T WURK!
                        One would be for RETARD.
                        One for half assed ADVANCE.
                        And one for BALLS OUT.
                        SA KDad
                        Bill, Who sez it won't work? I dunno about RETARD and 1/2Azz ADVANCE but if you wire that third coil to the seat springs your third option will definitely work!
                        Mechanical engineering 101: If you put an adjustment knob, screw, bolt, or tolerance specs on something, some people will immediately fiddle with it. If you mark it DO NOT TOUCH everyone will mess with it.


                        • BILL WILLIAMSON
                          BILL WILLIAMSON commented
                          Editing a comment
                          That might be GREAT FUN, Mike!!!
                          Bill Whoopie!

                      • #17
                        I may try what Mike K suggested wrapping the wires to the tac around a steel washer. Which wire should I wrap or should I do both wires. Just to clear up any misunderstanding it is the tac that is off not the speedometer. Also I have an original 6 volt point system no electronic.


                        • #18
                          The tach has plus, ground, and a sensor lead. I think he means to wrap the sensor lead around the washer. Be sure to let us know how you make out!!! You have nothing to lose!


                          • #19
                            Well I tried Mike K's suggestion to wrap the wire around a steel washer. That did make some difference but only some. The tach is still off, indicating high. So I put the old coil back on and the tach is fine now. Thanks for your suggestions.


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