Wednesday, April 2, 2008

Starting system


Here is the system responsible for starting your car for you. At a glance, you will immediately see that this is a veeeeery simple system. How this works is, when you turn the ignition key to the start position, that sends power to the ' S' post on the starter solenoid. It helps to think of the starter solenoid as a switch. When power goes to that 'S' post, that activates the switch and completes the circuit running from the positive post on the battery to the starter. When the key is released and it returns to the 'on', that turns off the switch and the starter quits turning because it has no power going to it. Cars equipped with an automatic transmission have a thing called the neutral safety switch. The wire going from the ignition switch to the 'S' post on the starter solenoid makes a trip through the neutral safety switch. This works as a kill switch so that the car can't be started while it's in gear. The transmission shift lever must be in either Park or Neutral. Manual transmission equipped cars don't have a neutral safety switch.
A starting system problem that is frequently mis-diagnosed is what is sometimes referred to as ' the starter hanging up'. That's when the starter just keeps right on cranking after the key has been returned to the run position. People run out and buy themselves a new starter, or beat on the starter with a hammer or something, when, if you think about it, the starter itself is the one thing that you know for sure is NOT the problem. It's down there cranking like crazy, working beautifully. The reason that it's doing that is because it is still getting power when it's not supposed to, so something along the line of power flow must be the problem. And that is the starter solenoid. It is continuing to supply power after it was supposed to stop doing that. The most common cause of the starter solenoid hanging up is a weak battery. Insufficient current combined with having system engaged far longer than it should be makes stuff start heating up. When the solenoid gets too hot, the contacts of the switch inside it try to weld themselves together.
Another thing that can cause what appears to be a starter problem is a bad ignition switch. You have returned the key to the run position, but, internally, the switch has refused to comply with your wishes and is still over there in the start position. But, it is far more common to have it be the starter solenoid.
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6 comments:

  1. dellSeptember 2, 2009 at 9:02 AM

    I have an issue I could use some help with. I have a 66 with 200 6cyl. If the car doesn't start immediately and I have to crank it for 8 or 10 seconds, the battery goes dead. I replaced the battery, battery cables and the cable from the starter solenoid to the starter. I removed the starter and took it to Auto Zone and they tested it and said it was working properly. Any ideas? Thanks!

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  2. VeronicaSeptember 2, 2009 at 9:44 PM

    That sounds more like a charging system problem to me. A fully charged battery won't die that quickly, but, if it also has to run all of the car's electrical systems while the motor is running, that will cause it to be sufficiently weakened to die after a bit of cranking. All that the battery is supposed to do is start the car. After that everything is supposed to be running off of the alternator, which is also rechargeing the battery. I think that I would take a look at my charging system.

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  3. Braydn TannerNovember 13, 2012 at 9:15 PM

    Hi Veronica, I really appreciate your contribution to the mustang community, both with your blog and help on allfordmsutangs.com. i have a question for you. which wires receive power in the accesory, run, and start positions respectively? any help would be very much appreciated. Thanks!

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  4. VeronicaNovember 14, 2012 at 5:50 AM

    When the key is in the 'start' position power is on the red/blue wire. When the key is in the 'run' position you have power on the red/green wire and all of the wires attached to the post on the back of the ignition switch, and when the key is in the accessory position, that has power on the wires attached to the post on the back of the ignition switch, but not on the red/green wire that goes to the coil.

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  5. Phil BMay 23, 2014 at 10:19 AM

    Veronica, great page! Thanks for all the info.
    Do you have any idea what the "acceptable" electrical draw in on a 1964 1/2 Mustang, with a generator?
    I am chasing a parasite and got it down to about 1.5a .

    Thanks,
    Phil

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  6. VeronicaMay 23, 2014 at 4:55 PM

    Hi, Phil. Thanks. The answer to that question is easy. The only acceptable electrical draw is 0. If you have a rally pack, which has a clock in it, there will always be a draw, but, if you unplug the clock, the draw should be zero. If you have a draw with the key in the 'off' position, there aren't very many things that can cause that on your car. If you look in the November of '09 section here, there is a post that might help on finding the draw.

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