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

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
mentioning.

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

 

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

 

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

 

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

 

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

 

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.

 

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

 

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
somewhere.