94 SL2 Overheating

Hi,

I have a 94 saturn SL2 that's overheating. So far I've done the following:

1. The coolant doesn't leak
2. The radiator is new
3. Connected the radiator fan to the battery and the fan spins
4. Had a mechanic look at all the fuses and relays, all are fine.
5. Had three mechanics told me that it's the temperature sensor (one of
them)

So my question is as follows:

My Hanes manual says that I should replace the thermostat. Is the
thermostat the same as the temperature sensor? If not are there
instructions on replacing it?

Where would I get the part numbers for what I need?


Thanks,
Andrew

 

To clarify a bit,

the car overheats when it is sitting idling. If I'm driving over 30 or
40 mph the temperature comes back down.

Thanks,
Andrew

 

Sounds like the radiator fan isn't turning on. There is enough
airflow over the radiator when driving and the fan isn't needed. Stop
moving and it eventually has to come on to prevent overheating.

I always thought there should be some indicator that the fan is on.
It's a handy thing to know.

Does your car have A/C? Turn that on and see if the fan starts. Then
you know the wiring to the fan is OK.

 

How did you determine that? Just checking for drips underneath the car isn't
enough -- for example, a leaking head gasket could leak coolant into the
combustion chambers where it will be evaporated and blown out the tailpipe,
and you'll never see any drips. Have you verified that the cooling system
actually is, and remains, full?

I doubt it.

It's possible that a failed temperature sensor is causing overheating by
preventing the electrically operated fan from coming on -- but as long as the
vehicle is moving at any appreciable speed, there's enough air being forced
through the radiator that the fan isn't really needed for cooling, so this is
unlikely IMO unless you're doing a lot of driving in very heavy stop-and-go
slow speed traffic. The more likely symptom of a failed sensor is a gauge
that's *falsely* reporting the engine to be too hot, when the temperature is
actually normal.

IS the engine actually overheating? (Noticeably hotter than normal when you
lift the hood, smell of hot coolant, wisps of steam drifting out from under
the hood, clouds of steam pouring out, etc.)

If so, I'm betting the problem is a failed thermostat. *Especially* if the
thermostat wasn't replaced at the same time the radiator was. (It should have
been. They're cheap, and they fail fairly regularly in older vehicles. As long
as the cooling system is drained anyway, it's just silly not to replace it.)

I agree.

It is not. The temperature sensor does just that: senses the temperature, and
sends a signal to the temperature gauge. The thermostat controls how hot the
engine gets; specifically, the thermostat is a valve that sits between the
engine and the radiator. It stays closed until the coolant reaches the correct
temperature (190F, I believe), and then opens. This prevents water from
circulating through the radiator until the engine is warmed up.

If the thermostat is stuck open, the engine takes a very long time to warm up
(which is bad in a variety of ways) -- and if it's stuck *closed* (or doesn't
open fully), the engine overheats. Of course, that's also bad.

If you have the same Haynes manual that I do, it's on pages 3-2 and 3-3.
Or you could look in the index under "Thermostat, check and replacement".

The instructions for checking and replacing the temperature sensor are a few
pages later, listed under "Coolant temperature sending unit - check and
replacement".

You should be able to find a thermostat for your car at just about any auto
parts store, or maybe even in the automotive department at Wal-Mart.

 

From what you state, it sounds like either a failed coolant temperature
sensor or the fan relay.

Bob

 

Hi

Yes it remains full over several months.

Because my other radiator cracked and I had to replace it.

This is exactly what is happening. The fan doesn't come on even when the
temperature indicator on the dashboard is way over half. As I said, the
fan is fine and the relays and fuses have all been tested (by a
mechanic) and are fine. If the relay is shorted the fan turns on.

-- but as long as the

This is exactly when the overheating happens: during stop and go
traffic. If I'm driving at an appreciable speed the engine stays at the
1/4 mark.

The more likely symptom of a failed sensor is a gauge

Yes I've had it steam up before and it is *very* hot otherwise.

So given the above do you still agree?

So is this what I'm looking for? Does the coolant temperature sending
unit cause the engine fan to come on?

Does this apply to the coolant temperature sending unit?

Thanks,
Andrew

 

Hi

I had the relay tested and it is fine (according to the mechanic). Does
the coolant temperature sensor control when the fan comes on?

thanks,
Andrew

 

Hi,

I tried turning on the A/C and the fan does not start.

Thanks,
Andrew

 

Andrew,

It's time to get a schematic diagram for the car. The official Saturn
manual has one, don't know about other sources.

Figure out which relay pins are the coil.
Pop the relay and check for voltage to the coil. Have someone turn on
the A/C or something like that.

Then take it from there.

 

Hi,

Already done. That's what the mechanic did: he opened the relay and
shorted the coil and the fan turned on.

Andrew

 

Why did it crack? Overheating, or some other cause?

Depending on which terminals are shorted, that may or may not be a valid test.

Maybe not -- but I'd sure check it anyway. It's an easy test to perform.

Not directly, no. The coolant temperature sensor, and the fan relay, are both
connected to the PCM (Powertrain Control Module). That's what causes the
fan to come on. It's possible that a failure in the PCM, or in the wiring
between the PCM and the fan relay, could be preventing the fan from coming on.

By the way -- the Haynes manual gives a detailed procedure for testing the
coolant temperature sensor, using an ohmmeter. It's in Chapter 6, Engine &
Emissions Control Systems, under "Engine coolant temperature sensor".
Don't bother trying to find part numbers; there's no need -- just call up your
friendly neighborhood auto parts retailer and say "I need _____ for a 94
Saturn SL2. You got one?"

You won't find that at Wal-Mart, I don't think... but I'd be really surprised
if the major auto parts chains wouldn't have it.

 

Or the wiring between them. Also there is two temp sensors on that
year engine. One for gage and one for ECM/PCM (engine control module
or powertrain control module which ever you prefer to call it) The
second one is used to control fan.

 

Hi,

Yes this is exactly what two mechanics told me. But I don't see anything
in the hanes manual about two sensors. Do you know where they are located?


Thanks,
Andrew

 

From your description, remember I am not there to do the work, so this
is all guessing, and isn't a How-to:

Something with coolant flow must be happening. If you have a fan
comeing on, and coolant level is sufficent, I would suspect coolant
flow is the issue.

I would check the water pump for operation, the dumb way I would do
it, is check if I get heat (strong heat) from the heater. Second, I
would then figure what could be interupting flow, and the internal
coolant thrmistat (a valve that opens and closes allowing flow to the
radator). I would be suspect of that, since I've been told they fail
over time.

I was once told how to check one, a valve thermistat, but figured that
once I open the housing, it saves time to just install a new one.

If a person can safely work yourself, and follow directions from the
Haynes book, I like them better than chiltons, since more pictures.
A person could replace the item themself.

later,

tom @ www.NoCostAds.com

 

Hi,

So now I have replaced the temp sensor. It had a crack in it and was
misreporting the temperature. So now when the car's temp gauge gets to
3/4 the fan comes on and the engine cools down.

Here's the rub. I've had this car for 3 years and it's never gone beyond
the 1/4 mark on the temp guage. So now I'm thinking that I might have
misdiagnosed the problem. The temp guage was definitely cracked and
broken, but I didn't realize that the car had to be at 220 degrees (3/4
of the guage) before the fan would come on.

Do you think this is a thermostat problem as you originally suggested?
I read the thermostat "Check" portion in the hanes manual and it says if
the upper radiator hose is hot then it's not a thermostat problem.

So I could just ignore the fact the engine gets to 3/4 of the temp guage
because the fan will come on and cool it before it overheats.

Any suggestions?

Thanks,
Andrew

 

... probably because the gauge wasn't getting the correct input from the
faulty sensor.

You mean the sensor, right?

Considering that (according to the Haynes manual) the thermostat isn't even
fully open until 212, that's really not too surprising.

Not anymore, no.

Well, is it?

Have you actually checked the upper radiator hose yet? If it's hot, then, like
the book says, the 'stat is OK -- and I was wrong. If it's not hot, then
replace the 'stat too.

 

Yes the upper hose is very hot.


Andrew

 

3/4 is known to be normal.

 

Ok, so that said, why would have the car never gone to that? The sensor
that was cracked was the PCM sensor, not the guage... and the guage
never showed the car overheating before...


Andrew

 

Then your thermostat's probably good, and the problem was indeed the sensor.
Everything should be OK now.