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Thread: Tach Quit

  1. #1
    pwdcougar Guest

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    I posted a while ago when I was having trouble hooking up a 6AL and a MSD tach adapter in my 69 Cougar with the stock current style tach. I got everything installed and the car runs great.

    I have two problems now. The stock tach worked when I first hooked everything up. After about 50 miles or so the stock tach has stopped working. All the wires are still connected. Sometimes the tach will jump up a little bit (especially when starting).

    How do I test the 6AL and the tach adapter so I can tell if they are at fault or if the stock in dash tach has quit?

    2nd question is where do I hook up my external tach when doing a tune up and setting the timing? I hooked it up to the usual spot on the coil and got a much higer reading. The tester/tach I am using is an inexpensive Craftsman unit that I have had for years.

    Thanks,

    Paul

  2. #2
    Join Date
    Jul 2003
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    Seattle, WA
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    Paul,
    Most important first...do NOT connect any test equipment to your coil!!! The 6A puts out nearly 500 volts to the coil primary and it will damage the test equipment and/or the 6A. You can get a tach output signal from the tach output terminal on the side of the 6A. This is a standard 12v pulse. Which diagram did you use when hooking up your stock tach?
    -Lazarus

  3. #3
    pwdcougar Guest

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    Lazarus

    I did initially hook up my test equipment to the coil but quickly unhooked it. The equipment still works and the car runs great.

    I used the literature that I downloaded for the 6AL and the MSD Tach Adapter. Both were shipped without instructions!. Luckily MSD has them for download.

    The diagrams were followed exactly. I initially used quick crimp connections and after verifying that the car would run and start I redid all the connections with solder and heat shrink tubing for a smooth look. The car ran great and the tach worked in both set ups. After about 50 miles, the tach quit.

    Which lead do I use for hooking up the test tach? The male lead on the 6AL that the white wire for the tach adapter plugs into?

    Thanks

    Paul

  4. #4
    Join Date
    Jul 2003
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    Seattle, WA
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    Paul,
    I'm assuming you're using the 8920 tach adaptor? The white wire from the tach output of the 6A going to the 8920 has the 12volt tach pulse on it. That will drive voltage style tachs or test equipment needing a tach pulse. Probably the fastest way to check your stock tach is to wire up the stock ignition briefly and see what the results are. You can also use a multimeter accross the stock tachometer terminals (disconnect the tach from everything else) and read resistance (ohms). A good tach will have a fairly low ohms reading. Double check the 8920 connections, especially the ground. With a current style tach, both tach terminals (when everything is working) will have about 12 volts on them.
    -L

  5. #5
    msdtech4 Guest

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    You can check the tach output with a digital meter. The meter should read 1-5 volts DC with nothing hooked to the tach output except the meter. If this reads ok, hook up the tach adapter and supply 12 volts to the red of the 8920 adapter battery or ignition switch. Switch the meter to AC and then hook the red of the meter to the purple of the adapter (was not being used originally but will be for this test) and the black of your meter to the red of the 8920. Start the car back up and read the meter. It should read 2-3 volts AC hooked up like this. This will let you know if the tach output is ok and then if the 8920 is ok.

    Thanks,

    MSD tech4

  6. #6
    pwdcougar Guest

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    I tested as you outlined above. The 6AL had 2.5 volts DC but I go no readings from the 8920 as you directed. The problem is intermittent. Twice I noticed that the in dash tach was working.

    Is there a reading that I can get from the output of the 8920? Not sure which color wire but from the one that goes to the stock tach.

    Thanks

    Paul

  7. #7
    Join Date
    Jul 2003
    Location
    Seattle, WA
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    PW,
    I would temporarily hook up the stock ignition for further troubleshooting. The coil (+) wire in your case routes through the current style tach before it connects to a source of 12 volts. The tach monitors the current pulses that are present in the stock coil. It sounds like there is a loose connection in the tach. You may have to remove the tach and get it bench tested by a specialist. But first, observe the operation with the stock ignition.
    -Lazarus

  8. #8
    pwdcougar Guest

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    Lazarus

    Hooking up the stock ignition is not an option at this point. When I did this installation, I replaced the coil and distributor. All the components are new MSD parts. I have a MSD Billet Ready to run distributor in addition to the 6AL and the 8290 tach adapter. When first installed, I temporarily hooked up the entire set of new parts. When it worked perfectly, I soldered all the connections.

    I have a spare stock tach. I'll temporarily hook it up and see what happens. I may also hook up new Autometer tach I have.

    Is there any way of reading the tach adapters output at the wire that goes to my stock tach? As you said this is the original coil + wire.

    Thanks

    Paul

  9. #9
    Join Date
    Jul 2003
    Location
    Seattle, WA
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    Yes, I would substitute a known good tach into the circuit, temporarily disconnecting the one in your vehicle. I'm assuming you have a spare current driven tach, and not a voltage tach. With the tach disconnected, you should read a very low resistance accross the terminals of the tach - 1 ohm or less. Here is a simple test hookup. Use an automotive tail light as a test light. Connect the tach + terminal to battery 12 volts. Connect the tach (-) terminal to one terminal of the test lamp, and connect a short length of wire to the other terminal of the test lamp. Repeatedly touch the short length of wire to the battery minus terminal whilst observing the tach. The tail light should light up, and the tach should jump slightly each time you touch the wire to the battery and release it. This is just a gross test and we're looking for some slight movement of the tach needle and checking that the lamp lights up. Also, please see your personal messages.
    -Lazarus

  10. #10

    Default

    Did you ever find the answer to this problem. I have the exact same issue in a 1990 Jeep Wrangler 4.2L with the stock tach.

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