I have a 99 SL2. Long story short, the car overheated and i realized the fan wasnt coming on. I changed out the temperature sensor and now the fan comes on when the AC is on but not when its off. As long as i run the AC the car is fine. If it turn it off the temp starts to climb and it will eventually start to boil over. What would allow the fan to work when the AC is on but keep it from coming on when the AC is off?

 

I'm not sure, because I'm not a mechanic, but maybe a bad relay?

 

There are two sets of criteria that turns on the fan.
One is that the A/C is on. If the A/C is on the engine control computor will turn the fan on directly.
The second criteria is a measurement of engine heat. the sensor at the end of the head on a 99, the drivers side end is used by the computor to determine engine tempeature to control the settings of the fuel injection,
To create information used to give a temerature gauge reading
and to determine when to turn on the fan because the engine is running hot.
If your temperature gauge is indicating engine temperture changing that is usally enough of a diagnostic to indicate the sensor is doing it, job correctly.
It also is usually an indication that the computor is getting the correct information and decoding it correctly.
If the fan comes on when the A/C is running that is usually and indication that the fan control is working correctly.
Between the two indications the assumption is the fan will turn on when needed to control engine heat.
Obviously there is a lie in there some where and as I am working from memory I have no clue so I will pull out my software Mitchel Saturn Diagnostics and see wht the wiring is. Of course if I was in the garage I would pull out a book. You do have one?
later.

 

Cooling fan relay located in Junction Block left side under hood.
Turned on by
Power Tran controll Module (PCM) behind left side of dash.
Inputs meede are engine coolant tempeature sensor located on left side of cylinder head, does guage work, if so then sensor should be good.
and
Vehical Speed Sensor VSS located on the transaxle.
If speedometer works VSS should be good.
If both check good and the Fan does come on when the A/C is running then it SHOULD come on when the A/C is not running and the engine gets hot.
So by definition there is no problem
Aint computers wunnerfull
Of course there is one possibility. There has been a long time problem with the reliability and the accuracy of the temperature sensor. When you hear the water boiling is the termpertue gauge reading HOT?
If not yur sensor is out of calibration and the computor thinks things are just fine. After market should have the correct sensor by now, but there is no guarentee that is true.
The best solution is to buy a correct sensor from your local saturn parts dealer. Supposedly any GM dealer has a friendly Saturn parts man.
I say that with tongue in cheek.
And supposedly you get a GM part not one that was purchased at the local auto parts store. And for those who don't think that won't happen? Dream on!
Good luck and report back.
Oh by the way, if you go that route see if there is a local GM dealer that actually advertises Saturn service. Close to where I am, there is one. And the parts guys are o.k. But I am building a Chevy engine and buying parts from another Chevy Dealer and although they are friendly and they have a reputation for supporting the custom car community I have had problems buying Chevy parts there and have politely refuse their offer to get me Saturn parts.

 

When it gets close to boiling over the gauge will read 1/2. When i turn the AC on the fan will kick on and the gauge will drop to 1/4.

I just bought this car today. I got it AS IS at an impound auction. When i got it, it had a very hard start and the temp gauge didnt work at all. From that i determined it needed a tempertature sensor. As soon as i changed the temp sensor it started starting fine and the temp gauge started working so i assume the sensor is working properly.

I did not change the sensor connector. Could this be my problem? The connector looked ok so i didnt worry about it.

 

Well, after extensive reading on the forum i am led to believe that the over heating problem is quite possibly a bad radiator cap. From what i have learned, the cooling fan will not come on until it reaches 3/4 on the guage. It starts to over flow from the reservoir at 1/2.

The reservior is part of the sealed pressurized cooling system and therefore should not be able to overflow. Is that correct? Also, if the cap is not sealing properly then that will lower the boiling point of the system preventing the coolant from reaching the temp it needs to reach to turn the cooling fan on. Correct?

Either way, i'm gonna go buy a new cap first thing and keep my fingers crossed.

 

A more likely culprit like Unclejohn says would be the ECTS/ connector. That is a very common problem on S series cars. You said you replaced the ECTS, but what did the connector look like?

 

When it gets close to boiling over the gauge will read 1/2.

Well you just described your problem.
At 1/2 gauge reading the computor, the little gremlin that run things, is quite happy there is nothing wrong.
If there is nothing wrong, there is no reason to turn on the fan.
The computor lives in it's own dream world and has no reality as to what is actually happening out in traffic.
When you turn on the A/C, whether the engine is hot or not makes no difference, turning on the A/C also turns on the fan.
The sensor is the little guy that tells the computor what to do.
Either your replacement sensor is not of the correct calibration.
A very strong probability depending on WHERE it was purchased, or the connector is bad, also a probability.
Connector damage is largly visible, the odds are if it looks good it probably is. If it looks crappy and ugly, it is NOT good.
That is the inside of the connector.
The outside is allowed to look crappy and ugly.
The general suggestion is to change both if you don't know.
Also the general suggestion is to buy from the dealer as that seems to be the only for sure location that you will get from re-designed stock when the problem was FINALLY addressed by GM.
By then though, the aftermarket has already flooded the network with the wrong sensor, 10 years into the product run apparently.
All of the autoparts stores did not run out and purge thier stock. They sell what is on the shelf and for something like this, it probably happens more than you think, and generally does not make much of a difference. So the guage reads slightly wrong. Big deal, when did gauges read exactly correct anyway?
But in this case having accurate heat readings to the computor makes a big difference the way the car runs.
Except, who cares, the company is out of business, who is going to buy these things anyway?
Sell'm.
Your choice on what to do.
My vote is the sensor is bad or incorrect.
The general consenses is the dealer is going to have the correct one or can get it along with the connector, a proven contributor to the problem.
The factory is closed, the dealer is gone and aftermarket no longer cares if they ever did.
Dealers will buy from local outlets to try to satisfy their customers.
Who actually reads 10 year old service bulletins anyway on cars that are no longer made. And that engine has not been produced in 10 years or so.
Where do you buy Studebaker, Packard, Hudson, DeSoto and Rambler parts now anyway.
I have 5 ramblers. Where was it you saw the last dealer?
The only thing you can be sure of is this.
If the gauge says it ain't hot, it ain't hot.
All that steam you see is a figment of your imagination.
From your description you need a better sensor, one that actually senses heat correctly. Where you get it at this point in time is now up to you.
Partially the problem is going to be where you find a dealer that has or can get one
and
whether the one they got was actually made correctly.
Good Luck.
I replaced the one in mine 11 years ago and it started working correctly then and still does today.
Damn, I like that car. The clear coat just crapped out for the second time. It will cost 20 grand plus or minus to buy another coupe I like as much as this and there aren't any.
It will cost 4o grand to buy one I like.
or a grand to repaint this again for the third time.
I promised myself I would replace the car when this happened the next time.
I think I lied.

 

All cheap parts sure ar hell doesn't hurt to replace all: Cap, ECTS, and the connector. All will be bagged or boxed in a GM labeled container.

 

Problem solved. It was radiator cap. The cap wasnt sealing properly so the system couldnt pressurize properly. I replaced the cap and then drove it around the block a few times to get it nice and warm. Came back and parked it and waited. The temp passed the 1/2 mark which is where it would start to boil over before and then it got to just below the 3/4 when the fan cut on. Temp dropped to 1/4 and then it cycled thru again. I watched it for a good 15-20 minutes and the fan cut on every time.

Now i just need to replace the valve seals and i should have a good little DD.