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Help finding issue with this circuit.

Discussion in 'Troubleshooting and Repair' started by tlmills82, Jul 5, 2017.

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  1. tlmills82

    tlmills82

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    Jul 5, 2017
    I have a very basic understanding of electronics, and I had an issue with the "blower motor resistor" in my vehicle. I bought a new one to replace the unit and it fixed my issue. I'd like to repair the old one that's in there and keep it as a backup. I've checked everything I can think of but still haven't found out what's causing my issue. I've included a photo of the circuit and what results I got using my multimeter.

    When I tested the transistor Drain to source on my diode setting, it would flash a reading (around 1.5) then go to OL. I'm not sure if this is normal or not?

    Unfortunately I have no way of testing the cap, but it looks to be in good shape at least.
    Transistor.jpg
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,424
    2,621
    Nov 17, 2011
    A MOSFET has a built-in so called body diode from source to drain.
    The 2SK2313 is an N-channelMOSFET which means the diode has the cathode at the drain and the anode at the source.
    In the reverse direction there should be no conduction.
    You may have had an issue when handling the MOSFET out of the circuit. MOSFETS are rather sensitive to ESD. a small discharge can punch through the insulation between gate and channel and thus render the MOSFET useless.
    Here's a simple test to verify the basic operation of a MOSFET.

    Or you could simply buy a new transisitor and replace the old one.
     
  3. duke37

    duke37

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    Jan 9, 2011
    What was the trouble?
    Is this the type of car which likes to go on fire? If so, get it repaired free of charge on the recall.
    Make sure the MOSFET is properly fixed to the heatsink.
     
  4. dave9

    dave9

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    Mar 5, 2017
    On those if nothing else looks bad, burnt/cracked/etc, it's the transistor. Make sure it is heatsunk well, better than from the factory if you want it to last longer for the 2nd tour of duty.
     
    Arouse1973 likes this.
  5. tlmills82

    tlmills82

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    Jul 5, 2017
    The issue is that the blower motor stopped working, I did a bit of research online and found out this unit was likely the culprit. I replaced it and it works just fine now. this unit pulls the motor to ground based on what setting is selected on the panel and that adjusts the speed of the fan ( I'm guessing).
     
  6. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    The old system of control was three resistors switched in series with the motor to set the speed. Considerable heat was developed in the resistors.
    This transistor does not look as if it has been bolted to a heat sink. The transistor therefore has to be either fully on or fully off. To get different speeds the on/off period is varied. This may be done with a microprocessor which I do not see. The transistor may be good and the control faulty.
     
  7. tlmills82

    tlmills82

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    Jul 5, 2017
    The unit was attached to a rather large heat sink. I'll buy another transistor and solder it in and try that and see if that fixes it.
     
  8. dave9

    dave9

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    292
    Mar 5, 2017
    Often you would find both types in the same make and models of vehicle depending on whether it had manual (resistors) or automatic (transistor) climate control.

    Control of the transistor is handled by the console head unit. Head units can fail, but it is much less common. When that is the question then you can consult the wiring diagram to see if there's live 12V coming from the battery (through a relay, usually (another thing to check)), and drive voltage on the gate (transistor pin) from the console when it should be running the fan.
     
    Last edited: Jul 6, 2017
  9. tlmills82

    tlmills82

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    Jul 5, 2017
    The head unit is fine because I replaced the unit in question and it solved the issue.
     
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