Cooling Fan won't start on 97 SC2 -- advice on what to check next?

Discussion in 'General Motoring' started by Ivan Cooper, Jun 6, 2005.

  1. Ivan Cooper

    Ivan Cooper Guest

    So, on my mother's 1997 SC2, the radiator cooling fan won't turn on and
    the car therefore runs too hot if it sits in traffic. It will get to
    over 3/4 and the cooling fan would apparently never turn on.

    First, I replaced the Temp Sensor (turned out the old one in there was
    already the brass type and looked fine). No change. The temp readout
    on the dashboard also seems to be working normally.

    I checked the fuse and the relay -- both good, no change when I swapped
    them. Tried turning on the AC, still the fan did not start.

    So, I thought it was the fan motor. But when I tested the connector to
    the motor, realized no voltage was being sent to the fan. At this
    point, the dashboard temp sensor was reading almost 3/4 and you can
    feel the extra heat from the engine.

    Then I tried removing the fan relay and put a jumper wire in its place
    -- suddenly the fan started right up! I confirmed that the relay is
    just not being switched on (checked voltage on the relay connector at
    the fuse/relay box).

    Then I put everything back together and put the relay back in place.
    Bam! Suddenly the fan started up with the engine temp gauge reading
    almost 3/4! I am thinking somehow it fixed itself or the connectors
    were corroded in the relay connector, or something.

    Stopped and started the car, back to nothing. The car's still hot and
    the fan won't turn on again. Argh! But when I tried bridging the
    relay again, it started right up.

    So... I'm pretty sure it's NOT the fuse / relay / fan motor / wiring
    from fusebox to fan / temp-sensor. It might be loose wiring or
    corrosion in/under the fuse box but I'm not sure about taking it apart
    - how hard is that? Or it might be the computer. Maybe it's sometimes
    supposed to get up over 3/4 before switching the fan on -- but I don't
    remember it getting that hot when I used to drive it! Or maybe
    something else.

    Oh, one other thing I almost forgot! There is a little one-wire sensor
    (?) in the front of the engine, near the cooling fan. It goes out a
    few inches from the engine and connects with a white plastic
    barrel-style connector a bit smaller than my finger, to a wire which
    goes (somewhere) routed next to the fan power wires. That white
    connector thing used to be attached to a clip on the cooling fan cowl.
    That clip melted or broke in the past, and the plastic connector sat
    right next to the manifold on the engine and got partially melted. I
    disconnected and reconnected it and it still seems to make a
    connection. Perhaps unlikely this is the cause, but as the only thing
    obviously damaged/out of place under the hood, I thought it was worth

    I'm tempted to just put a manual fan switch in Mom's dashboard and be
    done with it. But I'd much rather fix it right if I can figure out
    how, and I wanted to get a second opinion. Thoughts?

    Ivan Cooper, Jun 6, 2005
  2. Ivan Cooper

    MR Guest

    If the relay is not relaying then it seems as though that would be the
    problem. Sounds like you checked everything out very well.
    Good luck,
    MR, Jun 7, 2005
  3. Ivan Cooper

    Ivan Cooper Guest

    Then I tried removing the fan relay and put a jumper wire in its place
    Well, actually the problem is, it's apparently not even *trying* to
    switch the relay on. I even put in a different relay and it still
    didn't come on (except that one time when it suddenly did but then
    wouldn't again). So it's not the relay, it's something further
    "upstream" like the fuse box wiring, computer (?) or something else.

    Unfortunately that's where I may need a little help or advice to
    troubleshoot properly! Thanks...

    Ivan Cooper, Jun 7, 2005
  4. Ivan Cooper

    Chris Guest

    Not sure about your SC, and perhaps this is the sensor you referred to, but
    on an older Audi, the relay was switched by a sensor in the rad header tank
    (i.e. NOT the engine temp sensor). Jumpering that switched everything on
    immediately. I replaced that switch and made certain that coolant level was
    correct and problem was resolved (I also installed an illuminated toggle
    switch in parallel with the rad switch, which allowed me to both override
    the temp switch if I felt I needed to, and see when the circuit was
    energized - sort of the "belt and suspenders" approach)

    Best of luck
    Chris, Jun 8, 2005
  5. Ivan Cooper

    MR Guest

    Perhaps one of the code readers would give you some hint as to where
    the problem is hiding. I think one of the parts stores will plug you
    in for free; just can't remember which one. Maybe someone on the
    group could help with that info.
    Good luck,
    MR, Jun 8, 2005
  6. Ivan Cooper

    Kirk Kohnen Guest

    Replace the fan motor.

    If I recall correctly, the fan relay switches the ground contact of the
    motor, not the power contact (but I could be wrong).

    It's almost always the motors - the brushes plain wear out.

    And, they fail intermittently.

    Do yourself a favor - get and install a new motor.
    Kirk Kohnen, Jun 27, 2005
  7. Ivan Cooper

    Ivan Cooper Guest

    Thanks Kirk, at first I thought the same thing.

    But I was able to confirm that it was not the motor by shorting across
    contacts 87 and 30 of the fan relay. The fan would start up every
    time. To make a long story short, since I had satisfactorily tested
    the fan, relay, fuses, temp sensor and almost everything else I could
    think of, that only left the computer, or the wiring between the sensor
    / computer / fusebox. (I bet it's probably the computer.)

    But since I didn't want to replace the expensive computer or tear apart
    the wiring harness, I installed a secondary 30 amp relay with contacts
    87 and 30 in parallel with the original one. Then, I just put a
    lighted switch in the dashboard which powers this relay and turns on
    the fan immediately (regardless of whether the factory relay is being
    turned on).

    The mod is all installed with removable connectors. So if I manage to
    fix the problem which stops the computer from turning on the fan, I can
    pull the whole secondary circuit out and return it to stock condition
    in 5 minutes, except for the small hole where the switch is installed.
    And it's a cool looking switch. :)

    The other thing I was thinking I might do is wire up a voltage detector
    circuit to the engine temp sensor in parallel with the computer. I
    might be able to just create my own simple automatic fan control
    circuit so that Mom doesn't have to remember to flip the switch in slow
    moving traffic. Hmm...

    Ivan Cooper, Jun 28, 2005
  8. Ivan Cooper

    blah blah Guest

    You may never have let it get hot enough.
    Cooling Fan Motor Control

    The PCM controls the cooling fan motor by controlling the cooling fan
    relay. The PCM controls the cooling fan relay by controlling an internal
    driver that pulls the relay coil circuit to ground. When the relay coil
    circuit is pulled to ground, the switch side of the relay closes
    allowing current to flow to the cooling fan motor.

    The PCM turns the cooling fan relay ON based on the reading from the
    engine coolant temperature (ECT) sensor. The ECT sensor is a thermistor
    that varies resistance according to changes in engine coolant
    temperature. The PCM supplies a 5 volt reference through a pull-up
    resistor to the sensor, which is connected to ground. When the sensor is
    cold, it has high internal resistance, high signal voltage at the PCM.
    As the sensor temperature increases, its resistance decreases, a low
    signal voltage at the PCM. The PCM uses the signal voltage to determine
    the engine coolant temperature.

    Under normal operating conditions, the cooling fan motor operates
    whenever the ECT is greater than 105.5°C (222°F) or the A/C relay is
    commanded ON below 113 km/h (70 mph). The cooling fan motor will turn
    OFF when the temperature drops below 100°C (212°F). If the engine is
    turned OFF and the ECT is above 107°C (225°F), the cooling fan motor can
    run up to 4 minutes or run until the ECT drops below 107°C (225°F) with
    the ignition OFF. If a low/high ECT circuit fault resulting in DTC P0117
    or P0118 is set, the PCM will command the cooling fan relay ON to
    protect the engine and transaxle until the condition is corrected.
    If you know what the resistance of the ECT sensor is just below 210 and
    above 222 you can use resistors to simulate those values. If the fan
    kicks on with the simulated 222deg resistor and the gage on the dash is
    just above 2/3rd's then you know you're fine.

    I couldnt find these values so I cant tell you but I know they're
    blah blah, Jun 28, 2005
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