How to test the power transistors

Posted by luv2rallye, Jun 6, 2006
Articles: Electrical & Wiring -

  1. luv2rallye

    luv2rallye DSM Wiseman

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    How to test the power transistors (aka. ignition module or ignitor):

    This test will check the power transistors for go/no go operation. If it fails your transistor(s) are bad. Note that the power transistors may still be weak, intermittent, or fail with temperature which this test does not check for (although you can often make bad ones fail when you heat them with a hair dryer or heat gun which I strongly recommend).

    Background: This test is better than the one in the manuals (which use an ohmmeter) for the following reasons:

    1) The manual version requires an analog ohmmeter which most people don't have and can't get easily anymore (and why buy one for just this test?). A digital one, which all people have in their multimeter, will not read properly on any of its resistance scales (since the power transistor is non-linear and operates on current input instead of voltage). For that matter, an analog ohmmeter will not read consistently on its resistance scales either (since its output voltage on resistance scales varies with the scale and load, which then changes the resistance reading you get).

    2) The ohmmeter's probes polarity must be the correct way (analog ohmmeters actually have their polarity reversed which the manual version is counting on – digitals are not reversed).

    3) This test has the power transistor drive an actual load (a light bulb) for a more complete and realistic test, where the manual version doesn't drive any load.

    (Note: The pin numbers for the following are the ones on the power transistor side connector – not the ones on the harness side connector (for which I put the wire color in parenthesis to help identify the correct pin. Also know that any missing pins in the connector still count as a pin number so an 8 pin connector still has pins 1-8 even though one wire/pin may be missing (eg. pin 5). This is true for all connectors in the car.).

    Important: You must use a small incandescent 12V light bulb that draws some current, say between 3 – 25 watts (automotive tester light, taillight, turn signal bulb, brake light, dome light). DO NOT USE AN LED as this will glow brightly even when you only have 30mA of current flowing. They also are non-linear, have a low turn-on voltage, and only conduct in one polarity so it's misleading if it's backwards.

    Note: Often you can use a digital ohmmeter's continuity function (on the selector position with buzzer) in place of the 12v and light bulb in this test if you wish. Just put the positive lead on the pin that you connect the 1.5v battery negative to (eg. pin 3) and the negative lead on the pin that the light would have connected to (eg. pin 1 or 8). [If this doesn't work try swapping the leads as your ohmmeter may have polarity reversed for this function.] You should have continuity only when the 1.5v battery is connected as in the test (eg. positive to pin 2 or 7). This is not as good as using the 12v and light bulb method but usually will give the same go/no go result.

    On a 2g turbo (Note: 2g NT doesn't have power transistors):
    Unplug the power transistor 8 pin connector.

    To test the power transistor for cylinders 1 & 4:
    Connect the negative side of a 1.5V battery to pin 3 (black wire) of the connector. Connect the negative side of a 12V battery also to pin 3. Connect the 12V battery positive to one side of a 12V light bulb. Connect the other side of the 12V light bulb to pin 8 (black/white). The light should not be on. Now connect the 1.5V battery positive to pin 7 (black/blue). The light should go on.

    To test the power transistor for cylinders 2 & 3:
    Connect the negative side of a 1.5V battery to pin 3 (black wire) of the connector. Connect the negative side of a 12V battery also to pin 3. Connect the 12V battery positive to one side of a 12V light bulb. Connect the other side of the 12V light bulb to pin 1 (blue/black). The light should not be on. Now connect the 1.5V battery positive to pin 2 (brown/red). The light should go on.

    On a 91-94 turbo or NT:
    (Note: The pinout is the same as the 2g turbo one but the wire colors are different.)
    Unplug the power transistor 8 pin connector.

    To test the power transistor for cylinders 1 & 4:
    Connect the negative side of a 1.5V battery to pin 3 (black wire) of the connector. Connect the negative side of a 12V battery also to pin 3. Connect the 12V battery positive to one side of a 12V light bulb. Connect the other side of the 12V light bulb to pin 8 (yellow/green). The light should not be on. Now connect the 1.5V battery positive to pin 7 (yellow). The light should go on.

    To test the power transistor for cylinders 2 & 3:
    Connect the negative side of a 1.5V battery to pin 3 (black wire) of the connector. Connect the negative side of a 12V battery also to pin 3. Connect the 12V battery positive to one side of a 12V light bulb. Connect the other side of the 12V light bulb to pin 1 (yellow/black). The light should not be on. Now connect the 1.5V battery positive to pin 2 (yellow/red). The light should go on.

    On a 90 turbo or NT:
    Unplug the power transistor 6 pin connector.

    To test the power transistor for cylinders 1 & 4:
    Connect the negative side of a 1.5V battery to pin 3 (black wire) of the connector. Connect the negative side of a 12V battery also to pin 3. Connect the 12V battery positive to one side of a 12V light bulb. Connect the other side of the 12V light bulb to pin 1 (yellow/green). The light should not be on. Now connect the 1.5V battery positive to pin 2 (yellow). The light should go on.

    To test the power transistor for cylinders 2 & 3:
    Connect the negative side of a 1.5V battery to pin 3 (black wire) of the connector. Connect the negative side of a 12V battery also to pin 3. Connect the 12V battery positive to one side of a 12V light bulb. Connect the other side of the 12V light bulb to pin 6 (yellow/black). The light should not be on. Now connect the 1.5V battery positive to pin 5 (yellow/red). The light should go on.
    Last edited by a moderator: Jun 5, 2014
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