anyone know how ignitions work?

Discussion in 'Fox 5.0 Mustang Tech' started by OonDeanisS, Jan 9, 2007.

  1. my 86 with the carbed 351 doesnt spark until i let off the key, so more than half the time it doesnt start. What would cause this? I turn the key to "on" and the fuel pump comes on, then i turn it to "start" and the starter turns the engine over as long as i hold it in start, but then only sparks when i let it back to "on." any ideas why the "start" setting would be grounded? thanks
  2. I can tell you exactly how the EFI system works. With a carb conversation, there is no telling what has been done to the electrical wiring.

    AutoZone wiring diagrams for 79-88 model Mustangs for 89-93 model Mustangs for 94-98 model Mustangs

  3. Disconnect the switch wire to your starter solenoid, the small wire.

    Turn the key to start, and check your coil for 12v to start with. If you don't have 12v there, that means your coil isn't wired in with your start circuit, which it should be. Check those wiring diagrams JR posted up. If you need help reading them give a shout.

    The other issue would be no ground pulse. That could be a whole nasty array of problems not worth getting into unless you need to.
  4. there should be 12 volts to the coil with the key in start position like mentioned above. and in run there should be less than 12 volts because there should be a ballast resistor wired in before the coil so you dont burn it out. most people use a chrysler ballast resistor since they are cheap and readily available
  5. Are you sure about the ballast? I know the 86's and later don't have one, I don't know what the last year for them was though.
  6. Depending on how everything is wired.....
    I had this happen when we had the solenoid for the starter hooked up bass-ackwards.

  7. I am assuiming you are running duraspark, you need to have power to coil at igntion start not just igntion run.

    Check to see if you have power to the coil when cranking, I bet not.
  8. Did you do the carb conversion or did you buy someone's project car?
  9. it was carbed when i bought it, thanks for all the info, im going to test it out tomorrow. Sorry for a dumb question, but what tool should i buy to check the coil for 12v of power?
  10. a cheap multimeter will do the trick, any electronics or parts store should have one for 10 bucks or less, but can be as much as 250-300.00 for top of the line

  11. wow thanks for the quick replies, and which part of the coil should i touch it to when cranking?
  12. Judging from what I have been reading, you're going to need this...

    Step 1.) Find the instruction book that came with your Multimeter.
    Read it and familiarize yourself with how it works and how use it. If
    you lost the book or didn’t get one with it, do a Google search on
    the web to find the manufacturer’s web site & download a copy of
    the manual.

    Step 2. ) Make sure that you know what test lead plugs into which
    jacks on the Multimeter. There are usually several different jacks on
    most Multimeters, and they have different functions. Make sure that
    your battery(s) in the Multimeter are good: if you have any doubts,
    replace the battery(s).

    Step 3.) Once you are sure that the Multimeter is functional and you
    have the leads plugged into the jacks for Ohms ( the upside
    down “U” symbol), do some simple measurements to make sure
    that you know how to use it correctly. Set the switch to the lowest
    range and touch the leads together: you should not see “nothing”
    but you should see 1.0-0.3 ohms. Measure a 60 watt light bulb: cold
    it will measure about 17.5 Ohms. It you measure it while it is hot,
    the reading will be greater.

    Step 4.) Make several test measurements using the ohms function
    and the DC volts function. Remember all resistance
    measurements must be done with the power off the circuit. This
    avoids false readings and possible damage to the ohmmeter.
    Repeat steps 3 & 4 until you are sure that you can do it without
    making any mistakes.

    Step 5.) Then see
    and carefully study ALL the information under the Heading Chassis
    Electrical, Basic Electricity – Understanding & Troubleshooting. There
    are 3 sections with the same heading: read all 3 of them.

    Step 6.) Apply what you have learned and make the test
    measurements using the information in the wiring diagrams.

    Here's the stock 86 ignition wiring diagram. Since it is a carb
    conversion, the diagram may not apply to your car. For some of
    you who have read my negative comments about carb conversions,
    now you know the reason why for some of my remarks.

    if you want to save this diagram in your favorites..

    The red/green wire on the coil should have 12 volts anytime the
    switch is in the Run or Start position. No 12 volts and the fuse link or
    ignition switch are suspect. If there is 12 volts in Run, but not Start,
    the fuse link is good and the ignition switch is bad.
  13. you dont have to run one but all the carb cars had one and all the duraspark cars had one. i think with the tfi there is a built in resistor somewhere or it simply is not needed. i never ran one and dont need one now because of the msd.
  14. If you look closely at the diagram of the 86 wiring, you will see that there is no ballast or resistance wire in the power feed to the ignition coil. Ford engineered it that way to eliminate the extra parts and associated cost.
  15. so is this diagram i drew correct?

    Attached Files:

  16. The wiring to the coil is correct. What is the box labled ignition?
  17. I thought the old Duraspark boxes would burn up if 12 volts were applied to them, not to mention the fact that autozone type duraspark boxes dont seem to hold up as well as motorcraft boxes.
  18. oh i drew that from the wiring diagram posted above, and i meant for that to be the "tfi ignition module" but i figured that was just the stock ignition box. My car has a MSD 6AL, and from looking at my car the power goes from the battery to the 6al, then the 6al has an orange wire and a black wire going to the coil (also msd), and the 6al also has a ground, and a red wire that goes to a blue wire which goes to my fuel pump, and on the opposite side the 6al also has a blue wire leading to a green wire which leads to the dash, im assuming the ignition switch. Can i just use a 12v test light to check each wire?
  19. See the attached diagram for MSD hookup details...

  20. heres a diagram of exactly how mine works (minus the power wire to fuel pump)

    Attached Files: