I have a 1998 Saturn SL2 DOHC with ~140,000 miles. The car runs great, but I am having trouble with the coolant temperature. When first starting the car up in the morning, the coolant temperature gauge does not come off of the Cold mark, preventing the car from shifting into overdrive. Eventually (5-10 minutes), the gauge will “pop” up to the normal running range and the car will shift into overdrive. Sometimes it goes about halfway between cold and normal running range and then goes back down to cold again, and then goes to normal running range. However, almost always, once it’s in the normal operating range it’s fine and doesn’t go back down to cold or anything like that.

Does anyone have any ideas on what could be causing this? Here’s what I’ve done so far:
The service engine light is NOT on and there are no engine codes. Nevertheless, I have hooked it up to the computer scanner and observed the coolant temperature readings while driving. The temperature reading on the scanner seems to coincide with what the gauge is showing, that is, when it’s on Cold the temperature reading is also cold (80-100F). When the gauge goes halfway between normal and cold and then goes back to cold, the scanner is also showing a similar change in temperature. So I believe the gauge is showing correctly.

I have replaced:
Coolant Temperature Sensor – Several of these over the years, never has resolved the problem.
Thermostat
Coolant Recovery Tank Cap

This problem has also existed for several years, but used to only happen once in a great while. It has now gotten worse to the point that it happens virtually every time I start the car in the morning (but only after letting the car sit overnight or at work). The above part replacements have caused no change in the behavior or occurrence frequency of this problem. To my knowledge, I’ve replaced every part on the car relating to the coolant and its operating temperature and I am at an absolute loss as to what could still be causing this! Is there an onboard computer of some kind that could be malfunctioning and causing this? Any help would be greatly appreciated! Thanks!

 

when was the coolant system last flushed/

 

The gauge gets it's signal from the ECTS, so either the ECTS or the connection is the culprit. Since you have already changed the ECTS more than once, maybe a new connector will fix the problem.

 

I have to agree with HosePPi on this one for the following reason --

The ECTS is a resistance device that changes with temperature. When the temperature is cold, the resistance is high (little current flows through the guage), and as the temperature heats up, the resistance decreases (allowing more current to flow).

A loose or defective connector to the ECTS would introduce more resistance into the circuit, making the system think everything is still cold - preventing the overdrive from engaging

 

Everything you have written points at the sensor being defective or as Hoseppi said, the connector if not the sensor.
3 for 3

 

Well, yes ....... but it could be a bad wiring harness too .........

 

ok well i got a 93 saturn sl 1.9 single overhead cam with a 5 speed. i had just put a tranny in this car and soon after and its happen and gone back to normal and now it is back to having no acceration. ive been told to replace the sensor so i did and it still has no power im new to the forum thng so im not sure wat im saposed to ask really but if the connecters are bad or shorting would that cause it to act up. i actually am having the same proublem with the gauge i go to school at 5 am so its cold, the car seams or atleast says it doesnt warm up at all even today on my way home in 95 degree weather it didnt even get warm any way u can send me looking for the cause thanks for ur time...