Common ground between heater fan and tach | FerrariChat
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Common ground between heater fan and tach

Discussion in '308/328' started by Doug, Dec 5, 2009.

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  1. Doug

    Doug Formula 3

    Nov 13, 2003
    1,473
    Louisville KY
    Full Name:
    Doug
    #1 Doug, Dec 5, 2009
    On my '77 308 I have had an issue with the tach cutting out to 0 at @5200 rpm and then bouncing when the RPMs dropped before settling back in. Up to that point it functions perfectly. I redid all my main grounds as well as those to my electromotive unit. I figured it was the tach itself and would send it off at a later date to be repaired.
    I noticed yesterday when driving that the tach bounced when I turned on the heater fan and would do it every time I turned it on. The car has Birdmans fuse blocks and I also have Pauls wiring diagrams but I can't see where there would be a common connection for the 2 areas.
    Any suggestions?
     
  2. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #2 Paul_308, Dec 6, 2009
    Strange because the tach signals are at the rear and the heater motors are at the front. One thing I found on my car and notice many diagrams show it on others, is the heater fan grounding is not near the fan but at the sidemarker lights! IIRC, you need to pull the grates behind the headlight buckets to get at them.

    Just thinking about what would cause a tach to cutout at some high speed (frequency). The tach would expect a nice rectangular wave and if it's become distorted at high rpm (too narrow or reduced amplitude), the chip which drives the meter will no longer give an indication.

    The signal from the tach (brown wire) comes from the R1 terminal (main breaker points) of the distributor, passes through the coil to the tach. I.e. the tach reads the breaker point signal. You didn't say anything about the engine starting to miss at higher revs. Also, there are capacitors between R1 and R2 of the distributors. A bad cap or something foul at secondary points, R2 maybe ?? Interesting problem. Hope I've given some thoughts toward the right direction.


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  3. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #3 Paul_308, Dec 6, 2009
    Just looked at the diagram and it shows the tach is grounded at the relay panel ground with grounds for 5 things at that one point. Nothing suggesting a connection with the heater fan itself though except the heater fan indicator light is in the same area.
     
  4. Doug

    Doug Formula 3

    Nov 13, 2003
    1,473
    Louisville KY
    Full Name:
    Doug
    #4 Doug, Dec 6, 2009
    Paul
    I am running Electromotive ignition on the car so no caps, points, rotors etc....
    The car itself revs nicely all the way to redline with no stumble or falling off. I generally use MPH as my shift points. Basically, the tach falls to 0 @5200 RPM and stays there until the car drops back down in RPM either by shifting or letting off the gas. At that point it bounces a couple of times until it settles back into the correct RPM. It may still be a tach problem in itself, but I though it was interesting that turning on the heater fan switch causes the same bounce in the tach regardless of the current RPM. Turn signals, lights, brights, radio, windows do not cause the same anomaly.
    I guess I will try out the hazards, wipers and some other seldom used electrical items to see if they cause it to happen.
     
  5. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #5 Paul_308, Dec 6, 2009
    Since your Electromotive ignition completly rewires/replaces the disty/coil circuitry, where does your tach signal come from now? I'd concentrate there and consider the heater fan glitch an unrelated oddity.
     

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