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Help w/intermittent dying and not starting

Discussion in 'General Mustang Discussion' started by Rex66, May 17, 2017.

  1. Rex66

    Rex66 New Member

    Hi, I have a '66 coupe with 302 and C4. I have a Mallory Unilite distributor and a Mallory coil. Recently, the car would shut-off randomly after driving for 5-10 minutes, and then it would be hard to get it started again. Usually, we'd just wait an hour or two, and then it would be good for another 5-10 minutes.

    I replaced the fuel pump and line from the pump to carb. I'm now using a carter pump with integral filter and a hard line. There was a little debris in the old glass filter but the carb is getting fuel. I needed to replace that old rubber line and glass filter anyway.

    Today, I've been testing the electricals in my garage. It started right up and idled for several minutes, then died. I was able to get it started once after that, but right now, it won't start. Battery and starter seem fine - I'm able to crank the engine over easily. I have 12+ volts to the big post on the starter solenoid. On the "I" post, I'm getting 7.0V+ with the key in the "On" position, and ~6.6V at the (+) side of the coil. When I crank it (via a trigger bridging the big posts on the solenoid), it drops to about 6.3V on the "I" post and 5.8-6.0V on the (+) side of the coil. I pulled the coil wire from the distributor and held it with insulated pliers close to some different grounds. I have yet to see a spark while I'm cranking. I figure my coil is bad. I hooked up a Duralast coil that I bought from Autozone yesterday, but I get the same thing - no spark from the coil wire. I'm also getting 1.2V+ from the (-) side of the coil while the key is in the "On" position, and a little higher voltage while cranking - is that acceptable?

    I also tried checking for current in the coil wire and the #1 spark plug wire via an inductive timing light - no light while cranking. I tested continuity in my coil-to-distributor wire with a multimeter and it checked out. Maybe it isn't letting enough current pass?

    So, it seems that I'm not getting a spark. Is it possible that I have 2 bad coils? Where else should I look?

    Thanks in advance! Sorry if this noob is asking stupid questions.
  2. Horseplay

    Horseplay Well-Known Member

    The "I" terminal is a secondary voltage source for the coil used during starting. Mid is our in house electrical expert and will surely chime in here soon but until then I will fake it and try to help. ;)

    Disconnect the power lead on the coil that comes from the "I" post on the solenoid and measure the voltage on the lead while cranking. Positive meter lead on the wire and negative lead on the battery negative post or other good chassis ground to get correct/accurate readings.

    Not much involved here. Let's first make sure the full ignition circuit is working properly. Check the voltage again and tell us what you have.
    Last edited: May 17, 2017
    Rex66 likes this.
  3. Horseplay

    Horseplay Well-Known Member

    Here is a drawing of the wiring. You'll see that there are actually two leads that deliver voltage to the coil (wire 16A which runs through a resistance wire which lowers the voltage and wire 262 which originates at the solenoid). The "I" terminal just takes full battery voltage through a contact in the solenoid to "boost" the voltage to the coil during starting as the starter system draw can cause the normal full time run voltage to drop.
    Rex66 likes this.
  4. Midlife

    Midlife Well-Known Member Staff Member Moderator

    Horseplay is right on. It sounds like either your starter solenoid is not providing juice to the I circuit or the I circuit is interrupted between the starter solenoid and the coil. The joint is at the firewall plug: brown wire and a red/green wire should share an outgoing pin.
    Rex66 likes this.
  5. Rex66

    Rex66 New Member

    Okay, so I just went back out and measured ~12V on the "I" terminal and the red/green wire (#16) that goes to the (+) post on the coil. The car started right up. Let it idle until it died. Then I measured ~6.6-6.8V on the "I" terminal. So, bad solenoid right?
  6. Midlife

    Midlife Well-Known Member Staff Member Moderator

    Maybe, but we need a bit more info as to exactly how you did the measurements, as it makes a difference whether the engine is running or not. With the key in RUN and the engine not running, you should see +12V at the + side of the coil and at the I terminal. With the engine running, you should see ~10V at the coil and the same at the I terminal. Now if you removed the wire going to the I terminal and probed the wire under these conditions, you'd see what I described. However, the I terminal itself should read 0V. The only time when the I terminal itself doesn't read 0V is when the engine is cranking, then it reads 12V.

    Further complicating things is when the engine stops and the points are open or closed. If open, you should see 12V at the coil. When they are closed I believe you'll see 6V or so.

    Based upon what you've provided, it might be that your pink resistor wire has gone bad or your coil has gone bad. I wouldn't be surprised if your Unilite dizzy has gone bad as well.
    Rex66 likes this.
  7. Horseplay

    Horseplay Well-Known Member

    Let's not forget that it starts and runs for a bit and then won't do anything for a while. To me, that means something is getting warm and faulting. There's not much on these old cars that can cause that. It's not the solenoid. Car cranks so it is pulling in. It serves no purpose once you release the key back to "run". Pull a plug wire off a plug, plug a loose spark plug into it and hold the plug's threads again a chassis ground and have someone crank the engine while you watch for a spark. Quick and simple way to see if your coil and dizzy are working.

    If no spark when the "won't start" scenario is happening pull the electrical lead off the coil and measure it to see what voltage is present. Should be 12v when cranking. Slightly less when key in "run". I like to test circuits this way (disconnected) as it helps to isolate the problem quicker. I'm leaning toward your ignition switch just on a hunch, since you've tried a new coil, which would be the first thing to check. How old is the switch?
    Last edited: May 18, 2017
    Rex66 likes this.
  8. Rex66

    Rex66 New Member

    Sorry, I'm an idiot (and I've never claimed otherwise). The problem persists, but I made an error in my testing. I hooked up my remote starter across the 2 big posts on the starter solenoid (I prefer to think of it as a starter relay now). So, I was never getting the increase voltage for starting at the "I" terminal. When I connect my remote starter switch to the large post and the "S" post and pull the trigger, I'm getting 11V+ at the "I" post and at the coil. So, at this point, I'm bypassing the ignition switch, right?

    So before, when I had the remote starter hooked up incorrectly, I was only getting the lower voltage through the pink resist wire to the "I" post. It's interesting to me that my car would still start this way - it didn't necessarily need the extra voltage at "I" to start.

    FWIW, I have 12.2V on the battery when the car is off, and 13.4-13.5V when it is running, so the alternator is charging, though I'd like to see it at 14V.

    Unfortunately, I've had to get back to work, so I can't continue diagnosing until this evening. Horseplay, I'll try your tests next.

    Thanks for the help!
  9. Mach1 Driver

    Mach1 Driver Active Member

    It looks like you've got really good help here, BUT the attached may help simplify how the ignition and starting system works. Hope it helps.

    I don't want to confuse you, but the voltage you read on the coil's BAT terminal when in the "run" position, is dependent on your coil's primary resistance, and the voltage your alternator is charging at. This is because the resistor wire is in series with the coil's primary. My explanation uses a 1.5 ohm coil primary, which I believe to be a stock coil value, and 12v battery voltage. With a "hot coil" the primary resistance may be 0.75 ohms, which seems to be a popular value. Then lets say your alternator charges at 14.25v (it should somewhere between 13.5-15v). Given those variables first you need to calculate the amperage through this combination. 1.66 ohms + 0.75 ohms= 2.41 ohms. Since I=E/R, 14.25v/2.41 ohms = 5.91A. Then E=IxR or 5.91A x 1.66 ohms gives us 9.81 volts at the coil's BAT terminal when running and the alternator charging, using a 0.75 ohm coil primary. I hope that is a little clearer than mud.

    Attached Files:

    Last edited: May 18, 2017
    Rex66 likes this.
  10. Horseplay

    Horseplay Well-Known Member

    If you are getting the voltages you claim at the + coil post when the car is in it's "not gonna start mood", you can rule out the ignition switch. All that leaves is your coil and dizzy (or the wiring to them). Symptoms scream coil...but you say you bought a new one and had same issue.
    Rex66 likes this.
  11. Rex66

    Rex66 New Member

    So, I was able to get away from work and do some more testing. I was getting a good spark form the coil-to-distributor wire when I held it close to a ground while cranking. No such luck when testing a spark plug. That pretty much narrows it down to the Mallory Unilite distributor. I picked up a new distributor, rotor, and cap and will try installing them today and report back.

  12. Rex66

    Rex66 New Member

    Yep, it appears that the problem was in the dizzy. Put the new one on, set the timing, and took it for a nice long drive around the neighborhood and surrounding area.

    Thanks again for all the help!
    tarafied1 likes this.
  13. KBMWRS

    KBMWRS What did the moron say today?

    Yep these guys here know what they are talking about....except for me.:D
  14. coupster

    coupster Member

    Mallory strikes again....
    Rex66 likes this.
  15. Rex66

    Rex66 New Member

    "Mallory Unlit" (instead of "Unilite")

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