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Twin distr 308 - low coil voltage

Discussion in 'Technical Q&A' started by 4re gt4, Feb 2, 2004.

  1. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    1975 308GT4

    I've got about 10.5V at the "+" side of my coils. A study of the wiring diagram reveals the following current path: Battery --> ign sw --> fuse box --> coil. Note that it doesn't go *thru* the fuse box, but busses with a couple of other connections on the top. These connections have been soldered. However, this is irrelevant, because:

    Voltage is fine where it *leaves* the fusebox, but drops by a couple volts at the coil.

    I see three possibilities:
    1. Corroded wire connector somewhere between here and there. However, such a connector doesn't show on the wiring diagram.
    2. Voltage drop over wires that are of insufficient size.
    3. Designed resistance in the circuit.

    Many American cars have a designed in resistance in the "+" lead, but such resistance is not noted in the GT4 wiring diagram. A resistor IS shown attached to the "-" side of the coil, tho.

    Ideas?
     
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  3. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    572
    NW Rural Nevada
    Full Name:
    Mike Florio
    The resistors aren't shown in the schematic, but they are those big square ceramic (painted red) thingies located under the coils. If I remember correctly they are about 7 Ohms. The parts book shows them as part of the coil assembly - no separate part #.

    10.7 Volts sounds about right to me.
     
  4. ferrarifixer

    ferrarifixer F1 Veteran
    BANNED

    Jul 22, 2003
    8,520
    Melbourne
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    Phil Hughes
    Yep. 10.7 at the coil is actually a bit high, not low. They'll work fine at around 9v after the ballast resistor.

    Check the condition of the wiring there though. The terminals tend to corrode, and the crimpings gets a bit fragile, due to all the heat cycles.
     
  5. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    Thanks, guys, but according to the wiring diagram, the resistors are AFTER the coil - on the "-" side. I'm measuring the voltage from the "+" side - before the coil. According to the wiring diagram, this should be (*if* there are no resistive losses) battery voltage.

    So, a couple of volts are "disappearing" somewhere. But where?
     
  6. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,014
    Are you sure you are measuring to a good ground point? or the other side of the coil?

    Voltage is always across a pair of terminals, so if you are measuring the V+ and the V- at the coil itself, you are reading the 12V from the battery, minus the 1.5V from the ballast resistor == 10.5V at the meter.
     
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  8. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    I'm measuring from the "+" side of the coil to a clamp firmly attached to the engine. The line at the fusebox which *supposedly* goes directly to the "+" side of the coil is measured referencing an unpainted bracket in the glove box.

    I'm wondering if the coil resistor is actually wired to the "+" side, not the "-" side as the diagram says.

    Where all of this came from is I was fishing around the engine compartment looking for a good 12V source for my timing light. I can't seem to find one! Well, if I crawled under the car, I could get to the starter, but that's damned inconvenient just for a quick timing check.
     
  9. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    10,189
    San Carlos, CA
    Full Name:
    Mitchell Le
    The ballast is wired to the negative side of the coil. And it is NOT recommended to use the + side of the coil to connect measurement instruments. I actually ran a jumper cable from the battery back to the back of the car so i can hook up my timing light and my RPM meter.

    Pain in the butt, but correct way.
     
  10. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    Actually, I just got back from Sh!ttio Rack, err, Radio Shack with some big alligator clips, etc. to make a "+" jumper from the battery.
     
  11. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    10,189
    San Carlos, CA
    Full Name:
    Mitchell Le
    BTW,

    I am fishing through the pdf of the wiring diagram and it shows that the + side of the coil goes through numerous connections, a fuse or two, an ignition switch, and a ton or wiring, then gets fed off to numerous +12V points in the car. It probably explains the +9V at the coil.
     
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  13. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    Uh, look at the original post. BTW, the ignition current isn't fused. It goes along the 3 joined terminals at the top of the LH fusebox, but not thru any of the fuses.
     

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