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Thread: Engine tach pulse division

  1. #21
    Join Date
    Mar 2013
    Chaffee, MO


    Years ago, I checked my Car and saw a 90+ Volt Spike on my O'Scope with the same ringing.

    What about using the signal from the Crankshaft Sensor to trigger the little Digital Tach's Input Pin. That will be a 12 Volt
    signal once per revolution of the Crankshaft. It should be easily available at some plug along the wiring harness.

    The Input looks rather simple as the signal goes through Series R16 to Back to Back Diodes D1 & D2, then through series
    15 to Q1, and from Q1 to series R13 to Pin 13 of the 8051.

    Last edited by ldkraemer; June 12th, 2019 at 03:52 AM.

  2. #22


    I know it is not always that when asked for a piece of information, I am ignored. As a trouble shooter, I often ask for information that does seem relevant but I often do have reasons to ask.
    I'd asked for the sweep rate of the scope. My reason is that with the initial spike, the counter may be getting erratic triggers, making it do a poor job of counting. I'd asked for the sweep rate information of the scope so that I could determine the spark plug firing rate. While it could be putting out 6 pulses per revolution, that is different than my V6 engine ( a Ford car, engine made by Volvo ). It only puts out 3 pulses per revolution, as I'd expected. If 6, it would be firing 2 spark plugs at the same time. Strange but not an issue.
    Please respond with the timing information of the scope image.

  3. #23
    Join Date
    Oct 2008
    Kamloops, BC, Canada
    Blog Entries


    Scope is set at 5msec per division.

    What about using the signal from the Crankshaft Sensor to trigger the little Digital Tach's Input Pin. That will be a 12 Volt
    signal once per revolution of the Crankshaft. It should be easily available at some plug along the wiring harness.
    The vehicle does not have a crank position sensor and I cannot install or wire for one. The harness I can use limits me to the tach and tach only.
    please do not look too far into the waveform. That's what the car generates. That is what I have to work with. The car works and therefor I am NOT modifying that.

    Also I just don't want to work with it because lugging that heavy damn scope around sucks.

    [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
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  4. #24


    Is that for this picture or the original picture?

  5. #25


    The first thing I'd do is put a simple RC circuit in front of that tach module to "debounce" the signal. I think without cleaning up the ringing, your just going to frustrate yourself.

  6. #26


    I can see that you have a heavy scope but I'm really trying to help. I can't see the lines on the scope of this last picture. I believe you are experiencing noisy response on your counter. To solve it might be as simple as adding a small capacitor after the input resistor.
    The first thing to find out is are you getting 6 pulses per revolution or 3. A normal points type 6 cylinder car would be 3, with a typical distributor. That would mean you should be setting the divider to 3, not 6. The reason 6 maybe close to working is that you are experiencing noise that needs to be filtered out.

  7. #27
    Join Date
    Jan 2007
    Pacific Northwest, USA
    Blog Entries


    Looking at the scope trace, I'd hate to connect that waveform to any MCU. You may want to consider an optoisolator setup.

  8. #28


    I got a big screen and magnified this last image, post #23. I can see that the scope was set for 10 ms/div. I measured about 5.2 div/3 cycles.
    Assuming the that 3 cycles is one rotation of the engine:

    60 * ( 1 / ( .010 * 5.2 ) ) = 1150 RPM

    It looks like you have a problem with the extra noise. With the input resistor you have( 2K ), you might try a .001 or .01 uf capacitor across one of the diodes. This should help clean up the noise from the ignition coil.

  9. #29


    You can install a crank position sensor. But it's not worth it.

    I might have a tach adapter that converts to a 5V pulse that I could send you. But unfortunately I don't have time to breathe let alone search for that. I also have a factory tachometer for your dash cluster, but I don't know where that is either. It won't actually match your speedometer because the speedometer has a "square" scale and the tachometer has a round one. But it will fit the cluster and probably no one would know the difference.

    Here's a whole cluster with the matching speedometer:

    You can easily alter the mileage in these speedometers to match the car's mileage.
    Last edited by KC9UDX; June 12th, 2019 at 01:46 PM.

  10. #30


    As Dwight points out there can be very high peak voltages on the ignition coil primary. In addition there are a number of methods by which usual automotive tachometers interface with the ignition system, depending on the design.

    In cases where there is a basic Kettering system with the coil either switched by the contact breaker (or electronically assisted by a contact breaker & switching amplifier) or with a reluctor pickup and amplifier unit (for example GM's HEI module), the signal on the switched side of the coil has the high voltage peak mentioned.

    In some cars though, the wire leading to the coil simply passed through a loop on the rear of the RPM meter, so it was inductively coupled (the current being sensed) so the voltage peaks were of no consequence.

    In other cases, especially aftermarket RPM meters, they would connect directly to the ign coil primary, and have circuitry inside them to snub of (or ignore) the high voltage peak to a lower level and circuitry to ignore the ringing that occurs after the first high voltage peak. The actual ringing waveform is different depending on whether a spark does or does not form due to the secondary loading on the coil. It is due to the coils leakage inductance and total primary capacitance.

    The first peak of the primary voltage is always higher so if the sensitivity of the circuit monitoring the primary voltage is adjusted, the other peaks can be ignored. A monostable can also be used to ignore subsequent peaks, but it must not be set too long.The ringing is not there due to contact bounce, it is a coil resonance.

    It can be difficult to understand the waveforms an an IGN coil, which requires an "ignition coil model" to include most features of the electrical properties of the coil, but it is explained here if you want to know how it comes about, with waveforms on pages 13 to 15 of this document:

    (On page 26 there is a recording of the primary waveform with electronically assisted Kettering, Fig 19).

    Another thing of note, if you plan to put any electronics, especially with IC's in it, into a car that was not designed for automotive use, you have to be careful that the power supply is well filtered , typically with RC filters, zeners and or TVS devices, or else your IC's will likely die from voltage transients which are very severe, especially with a poor connection to the battery.
    Last edited by Hugo Holden; June 12th, 2019 at 04:39 PM.


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