Coolant Temp Sensor Replaced

[travelair]

After reading every related thread on this forum I decided to replace the CTS on my 96 3.1. I've been told that when one fails, the computer assumes a default of 32 degrees. This causes a rich mixture and reduces fuel economy slightly. It also causes the two fans to behave erratically.

Well, the strangest thing has happened. Upon turning the ignition on, the temp gauge pegs past the redline (stone cold engine) and is accompanied by the red "hot" light.

So I reset the ECM, let it go through its relearning process, and drove it around for a few days. Guy's, it's very scary to drive around with a temp gauge buried past the red. The condition has not improved.

--The radiator hoses get hot (thermostat/pump are okay)
--Thermostat is two years old
--The AC still activates the left fan (normal)
--Neither fan activates on its own any longer (not good)
--Unplugging the CTS causes both fans to activate and run continuously regardless of temperature
--The gauge remains pegged past the red (I have no way to know the coolant temperature)
--When I chicken out and turn on the AC to cool it back down, both fans come on for a while and then the right one shuts off

I understand from reading previous posts that the fans are operating normally. Personally, I think allowing an iron block/aluminum head combo to get over 200 is asking for the sort of gasket problems so common among these engines. I speak from experience here (lower intake victim two years ago). G.M. is entitled to their own opinion however.

Anyway, the fans no longer work normally and my temp gauge is all messed up. Both CTS’s gave identical resistance levels when tested with boiling and ice water in my kitchen. The old sensor can still be used in the car and every symptom goes away except the slightly lower fuel economy and less power. The engine seems to have a little more power with the new CTS (this could be in my head I suppose, if it is more, it’s minimal). I think the new sensor is bad but why does it bench test the same as the old one? Also, the positive lead to the sensor is only providing 4.2 volts when it’s supposed to be receiving 5.0 from the ECM. Is this close enough? If you think that’s the problem, what could be done to correct it and why would the symptoms disappear with the old CTS installed? If you have any other ideas I would sure be appreciative. Replacing the CTS did wonders for my Saturn (highway mpg went from 28 to 37).

I would also like to thank whoever came up with the idea of tilting the engine forward to replace the back three spark plugs. Pure genius.

[sysopt]

I believe there are two temp sensors, the one you speak of is one of many parameters for the ecu to control fuel and it also controls the fans. The other temp sensor controls the temp gauge on the instrument cluster. Check the other temp sensor. If youre only receiving 4.2 volts, youve gotta find the high resistance part of the wire and repair it. That high resistance is throwing everything off; the ECU "sees" a considerably lower voltage, and therefore a completely distant temperature.

[travelair]

Thank you for reading all that and replying. I´m stuck in Mexico at the moment and won´t be home for a few days. I will look for the other sensor you mentioned. I´m all for replacing sensors. They´re cheap and if they haven´t failed yet they probably will soon.

I still wonder why the symptoms disappeared when I reinstalled the old CTS. The CTS harness is a mystery in and of itself. I can´t trace it far before it disappears under the fuel rail. It maintained the 4.2 volts even when I wiggled it around so I don´t think it shorted.

[JAL]

Thanks for sharing. I don't know the answer, but I had the same behavior upon replacing the coolant temperature sensor. I had to leave the new sensor unplugged to get the fans going whence they would run non-stop under all conditions. I did not put the old sensor back in for two reasons. I damaged the plastic end, and I can't get the threads to "catch." Does anyone know a simple trick for catching threads? Elbow grease usually helps but damaged the plastic end in my case!
- Anyway, maybe the aftermarket sensor is mis-designed?
- I did not have any problems with the temperature gauge.
- I have never had my A/C serviced. I don't need it.

[travelair]

Problem solved. I returned the the sensor and got a new one. Everything works fine now so it must have been a defective part.

JAL, be careful with these threads because they are tapped into aluminum which is a very soft alloy. The CTS is brass and it would be easy to damage the cylinder head. I used a woble socket to get the threads started; it works like a U-joint. The book says torque the CTS to 10 ft/lbs which isn't very tight at all. Don't overtighten.

[JAL]

Thanks so much for the advice about the threads.

[Dr Sparks]

Travelaire, did you get that first faulty part aftermarket or OEM? I ask this because your dilemma brings to mind my hard won wisdom that taught me to source engine control electrical items as well as secondary ignition components from the OEM parts channel. I especially stress this on EEC sensor items. I think most of my grey hair is due to aftermarket O2 and temp input sensors that either don't work at all or introduce a whole different set of problems. Make that some of my grey hair. The kids are responsible for most of it.

[travelair]

I used an aftermarket CTS. I forgot the brand name of both the defective sensor and the new one. I did have the presence of mind to ask for a different brand when I returned the faulty one. I like your opinion on using OEM and if I had it to do over again I would. But it's just an old beat up, paid for Lumina that I just want to keep running well. When I get more proficient with Bondo it just might look as well as it runs.

P.S.

Krylon Appliance White enamel is very close the white GM used that year (in anything less than direct sunlight that is).