94 impreza overheating - no fan

[Remco]

Our beater 94 Impreza is overheating.
The fan does not appear to ever come on so that would be a obvious
cause.

This is what I've found:

Shorting the main fan relay contacts (the one in the under dash
fusebox) turns the main fan on - the fan and associated wiring is
good.

Tested the relay on the bench and it is good.

The relay coil has 12V on one side so one would assume that the ECM
switches the other side to ground when cooling is deemed necessary. I
put a meter across those two points and never see this relay be driven
on.

When the AC is turned on, neither fan turns on. The AC is broken so I
am not sure if it is supposed to behave this way.

When it overheats, the temperature gauge stays in the normal range. It
seems to behave normally in that when the car is cold, it is indeed
indicating cold and it slowly creeps up.

I am wondering what the ECM uses to determine when to turn the fan on.
On the schematic, the thermometer used for the dash has a wife going to
the ECM as well.

From experience I do know that the thermo sensor (a sensor mounted

right next to the thermometer on the manifold) is also wired to the
ECM.
It would be hard to believe that the thermo sensor is bad, because that
would mean the thermometer is bad as well.

Currently, I am leaning towards replacing the thermometer because
clearly the indication seems to be bad. I might just see what it would
do with a potentiometer to determine whether the ECM truly uses that
signal.

Does anyone have any other suggestions?

Remco

 

[Carl 1 Lucky Texan]

Our beater 94 Impreza is overheating.
The fan does not appear to ever come on so that would be a obvious
cause.

This is what I've found:

Shorting the main fan relay contacts (the one in the under dash
fusebox) turns the main fan on - the fan and associated wiring is
good.

Tested the relay on the bench and it is good.

The relay coil has 12V on one side so one would assume that the ECM
switches the other side to ground when cooling is deemed necessary. I
put a meter across those two points and never see this relay be driven
on.

When the AC is turned on, neither fan turns on. The AC is broken so I
am not sure if it is supposed to behave this way.

When it overheats, the temperature gauge stays in the normal range. It
seems to behave normally in that when the car is cold, it is indeed
indicating cold and it slowly creeps up.

I am wondering what the ECM uses to determine when to turn the fan on.
On the schematic, the thermometer used for the dash has a wife going to
the ECM as well.

right next to the thermometer on the manifold) is also wired to the
ECM.
It would be hard to believe that the thermo sensor is bad, because that
would mean the thermometer is bad as well.

Currently, I am leaning towards replacing the thermometer because
clearly the indication seems to be bad. I might just see what it would
do with a potentiometer to determine whether the ECM truly uses that
signal.

Does anyone have any other suggestions?

Remco

You could pull those parts and test them with hot water with a ohmmeter
on them. I'd suspect the engine temp sensor - not the gauge
sensor(pretty sure have both - anyone???).Seems odd the A/C doesnt't
kick a fan on though. Also, confirm there is no air entrained in the
coolant system, thermostat and rad cap are OK.
Soobs are sometimes know to trap air bubbles and can have 'localized'
hot spots. Put front tires up on ramps or curb with rad cap off and
heater on max. Idle until thermostat opens and fill with coolant. let
cool and reapeat. Fill o'flow bottle, drive and monitor for correct fill
in radiator and o'flow. Even if the system is now full, you will need
those instructions if you open it up.

Carl


Carl

 

[Remco]

You could pull those parts and test them with hot water with a ohmmeter
on them. I'd suspect the engine temp sensor - not the gauge
sensor(pretty sure have both - anyone???).Seems odd the A/C doesnt't
kick a fan on though. Also, confirm there is no air entrained in the
coolant system, thermostat and rad cap are OK.
Soobs are sometimes know to trap air bubbles and can have 'localized'
hot spots. Put front tires up on ramps or curb with rad cap off and
heater on max. Idle until thermostat opens and fill with coolant. let
cool and reapeat. Fill o'flow bottle, drive and monitor for correct fill
in radiator and o'flow. Even if the system is now full, you will need
those instructions if you open it up.

Carl

Thanks, Carl.
I'll try that first. That's a really good thought, because most sensors
don't work well when they aren't immersed - I forgot about that until
now.

Not sure if the AC switch is the one that signals the ECM to turn the
fans on or if the AC truly has to be working for it to turn on. Not
knowing what signals what, I can't tell whether this fact is
significant or not.

I do find it odd that the temperature gauge doesn't seem to indicate an
overheating condition. Perhaps those things are for reference only?
I've replaced the thermosensor a while back (had a problem with hard
starting, both fans on, rich mixture) but maybe it went again.
What is a hot and cold condition on those things, does anyone know?
(high resistance = cold?, low = hot?)

Remco

 

[Carl 1 Lucky Texan]

Thanks, Carl.
I'll try that first. That's a really good thought, because most sensors
don't work well when they aren't immersed - I forgot about that until
now.

Not sure if the AC switch is the one that signals the ECM to turn the
fans on or if the AC truly has to be working for it to turn on. Not
knowing what signals what, I can't tell whether this fact is
significant or not.

I do find it odd that the temperature gauge doesn't seem to indicate an
overheating condition. Perhaps those things are for reference only?
I've replaced the thermosensor a while back (had a problem with hard
starting, both fans on, rich mixture) but maybe it went again.
What is a hot and cold condition on those things, does anyone know?
(high resistance = cold?, low = hot?)

Remco

Hmmm... I'd hate to suggest it could be another bad sensor - though I
have read that a certain one (with a specific color of plastic - can't
remember which is which though - white vs. gray vs. blue???) perhaps the
replacement you got was from the same 'bad run'.
Don't overlook the possibility of a bad/corroded ground wire somewhere.
many folks have repaired odd problems by reworking
engine/battery/chassis ground points.
I'd like to have you visit www.ultimatesubaru.org and search the term
'engine temp sensor' and similar wording. If no help, post your issue
there. Those guys have a lot of experience keeping older soobs running well.
I think Rick might be able to help if he is monitoring this.

Carl