Question about Toyota Celica

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Messing around with 91 Celica, I caused one ignition switched circuit to go from 12v to 2.6v, apparently permanently. Just wondering what in general would cause a voltage drop like that. If it's a short, is it safe to leave the condition as is? What are the potential issues?

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  • coz Jun 03, 2010

    Excellent helper! Obviously very knowledgeable and explains clearly.

  • coz Jun 04, 2010

    I was afraid you might say something like that.

    I read some of your other postings. The one on laptop battery during sound spike was especially creative. You seem to know a lot in diverse areas.

    See ya around!



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Yes, it's a short. No, it's not safe to let it like that since something inside the ignition module is fried. A fried part doesn't do its job and draws a lot more power than it should which in turn overstresses all the parts connected (or depending) on it, causing them to fail.
Find out what this circuit was supposed to do and either isolate it or fix it.

Posted on Jun 02, 2010

  • 4 more comments 
  • coz Jun 02, 2010

    The circuit is the convertible top control. This is tied to the ignition & all other instruments on the same fuse are working. Over the weekend, I tried to pig-tail a + connection to this circuit to power an experimental hydrogen generator. This fuse (7.5A) never blew. The 30A relay fuse was blown & replaced. Are you suspecting the short to be in the ignition switch itself as opposed to the wiring from the ignition to the convertible top control switch?

  • Azrael SRL Jun 02, 2010

    The problem can't be in the ignition switch itself  - if it would have been then the car would be dead as a dodo.

    The problem is either in the wiring from the ignition to the convertible top control switch (but i would guess that you would have seen some melted wires by now) or, much more likely, in the control module itself. The only way to find that for sure is to remove it from the car and test them: start with the switch itself - melted plastic conducts electricity but not as much to appear as a pure short.

  • coz Jun 02, 2010

    Thanks for the clear suggestions. One thing before I accept the solution & close it out. The wire to the toggle switch (that's where I messed with & therefore most likely place the short may have occurred?) was the first thing I checked. No visible sign or a melt-down. So, could you clarify what you're referring to by "module" as the most likely suspect? Is there a module that plugs into the ignition switch? If so, do you suppose I can access it without disassembling the ignition mechanism?

  • Azrael SRL Jun 02, 2010

    By module i was referring to the electrical part of the mechanism: motors, connectors and all the wiring.

    How to test: start from the ignition key - you will get 12 volts there; plug in the wire to the switch still 12v unless its melted; plug in the switch and put it to close - if it's not shorted you should get 0; put it to open you should get 12v; plug in the next wire and continue the testing till you end up with a voltage that is different than 12 or 0 - there will be your fault.

  • coz Jun 03, 2010

    Sorry, my day job prevented me from logging on.

    Here's what I do know.

    1. The input voltage into the toggle switch now reads 2.6v. It read 12v at the beginning.
    2. Tested the circuit from the toggle down to the motor. Everything works.
    3. Toggle works as a switch as long as 12v is fed directly from the battery, i.e., activates the relay.
    4. So, my conclusion is the short is between the ignition and the switch.
    5. I wondered if it was safe to leave it like this. I now understand it isn't.
    6. If the module and/or the wiring melted or shorted, why didn't the fuse protect them? There are 2 fuses & 2 circuits, apparently. 30A fuse at the relay blew. 7.5A for the instruments & top control didn't blow. What's with that? I mean, shouldn't 7.5A have burned, not the 30A? Just want to understand.

    Thanks a bunch!

  • Azrael SRL Jun 03, 2010

    First of all if the fault is between the ignition and the switch then most likely it's a short in the wires - the most dangerous case. Fix it or you can set your car on fire.

    Why did the 30 amp fuse blew first ? Probably because it's a peak fuse (one that is designed to burn the instant the current reaches the rated peak) while the 7.5 fuse - like all standard car blade fuses - is a temp fuse (one that can handle very short current peaks - like ones generated when an electrical engine starts - without burning).
    As i said, the only way to really find out what happened is to find the actual fault. Sorry, but you will gonna have to check the entire wire loom, a long and tedious job.



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