Thread: 2005 Grand Prix - temp gauge/cooling fan issues.

Man, are you lucky.

I just happen to be working with another guy on a near identical issue. First things first though. Have you ever replaced the thermostat? Ive never seen a normal thermostat stick in the open position. They have specialized ones that will, but not the typical ones that most use.

I just want to make sure, when you say cooling fan, you mean the radiator fans, correct? Im working from that assumption so if Im wrong, let me know, but you said it still blew hot air so I want to make sure we are talking about the same thing.

Working from the assumption that its the radiator fans you are talking of, if they stay running, then they are getting a faulty signal from the coolant temp sensor. The fact your temp gauge never rises tells me that there is something going on with the coolant sensor as well.

The Coolant temp sensor sends out two signals. A temp signal to your dash and to the PCM. If you thermostat is good, that would tell me that the sensor is not sending a signal to the dash gauge. The fact your fans are on all the time, since they are controlled by the coolant temp sensor (well activated based on information from the sensor) tells me that the sensor has or is failing and the PCM has gone into some default mode and keeping the fans on as possibly as a precaution.

Yeah, I believe the two are related. The symptoms you describe lateralus20 are nearly identical to spenc with regards to the starting, but opposite with regards to fan operation.

I dont know how to really explain it, it makes sense in my head and when I look at the electrical diagrams but in your case lateralus20 I believe your coolant temperature sensor to be faulty and now that you have posted and with all the work spenc has done Im kinda leaning that way with spen's sensor too. Hes tested everything so far including the wiring harnes and I believe I had mentioned to him that if the harness was good then it had to be an issue with the sensor, its all thats left. But we were reluctant to go there because he has already replaced the sensor.

But in your case lateralus20, Im thinking the sensor is likely bad. There are three wires from the Engine Coolant Temperature sensor or CTS for short (Coolant Temperature Sensor). One wire gives the temperature signal to your dash temp gauge, one gives a temperature signal to the PCM, and the other provides a low reference signal to the PCM and I believe provides the signal to turn on the fans.

The fact you, lateralus20, have no temp reading on your dash tells me that the CTS is possibly faulty because as far as I know that is the only place the dash temp gauge gets its signal from. I have a diagram posted below, well a couple actually.

If you are getting no signal to your dash, then that means, by default that there is probably no signal making it to the PCM since the wire that goes to the dash is just the other side of the sensor that goes to the PCM.

My belief is that if the PCM sees no coolant temp sensor input, then it goes into some kind of default mode that runs the fans all the time as some kind of protection measure.

You can see in the last diagram that the fan relays go directly to the PCM to get the ground (or low reference) signal in order to engage and that comes directly from the CTS.

Now what I dont know is the resistance of the CTS and whether that resistance increases or decreases with temperature. I believe it decreases. But if anyone knows for sure, please post up.

Well if you prep the coolant sensor ahead of time, that is, put the teflon tape on it before you pull out the old one, you wont loose that much. Ive done it like that before on 3800. Just as the old sensor comes out, plug the hole with your finger, then get the replacement sensor and get it in there and start screwing it in. Shouldnt loose that much coolant at all.

In your case, Im kinda out of ideas if it isnt that sensor, we've proved out everything else including the wiring, so all that is left is that blasted sensor.

Right now, with all the work we've done testing this thing, particularly with testing the harness today and it showing good so far, Im thinking odds are looking good.

I do believe that is it. I dont think GM has changed that coolant sensor in a number of years so you should be good to go. Just the same though, I would see if you can look up the part number for the 97-03 sensor and the 04+ sensor and make sure they are the same.

First, spenc, if the sensors are not the same, and it appears they are not and would mean GM has finally changed their sensors, then I would just contact Ed and see if he has a sensor and harness for the 97-03 model. I was not aware of the differences as its hard to tell in that one pic on ebay.

lateralus20, Im an advocate for not spending any money that isnt necessary so here is how I am seeing this.

1. You could change the thermostat because its likely been a while since it was or probably has never been changed and may be a good maintenance thing to do. Furthermore, you're going to loose 'some' coolant changing either the thermostat or CTS, so it may be worth doing both actually.

2. You could just change out the sensor and see if that takes care of it, which I believe it will. But since you will likely loose some coolant in replacing that sensor, you might as well just replace the thermostat as well and make sure thats out of the way.

Now, in your case lateralus20, thanks to all the testing we've done on spenc's car, Im pretty certain its the sensor causing your issues. So if you get a repalcement, be sure to have some teflon tape to wrap the threads of the new sensor with. Im not sure if the new sensor already comes with sealant on the threads or not.

Disconnect the electrical connector to the sensor in the car. Unscrew it and do so slowly. What you want to do is just as the sensor comes out, put your finger over the hole so that you loose as little coolant as possible. Then get the new sensor ready, and get it in the hole as quickly as possible and start screwing it in by hand to stop the coolant from leaking out. Tighten the new sensor down and reconnect the electrical connector

Now since yours is an 05 GP, your sensor is going to be different, which I now know thanks to spenc. I dont have any electrical diagrams for an 05 GP since my AllData information covers only 97 - 03. But I still believe your issue is with the sensor since the CTS still feeds information to the PCM to control the fans. Your dash gauge signal may not come directly from the sensor, but Im willing to bet it comes from the PCM as a result of information from the CTS.

Personally, if you are going to change only one of the items, the CTS or the thermostat, you might as well change the other, it just makes no sense not to with the symptoms you have.

Well I guess that begs the question if either person has a block heater.