engine overheating?

Take a look at the wiring diagram. The thing that I am concerned with is the "nothing" on the black wire. This is the sensor return to the PCM and tells the PCM what the temp is based on what the voltage is. So the PCM supplies the 5 volt reference on the ECT Signal wire and then based on the temp the resistance changes. This means as the temp goes up, the resistance goes down and causing the return voltage to go up. So the fact that you don't have voltage here is where I would start.

Do you have any codes? If not, check this wire for a short to ground. Let me know what you find. Thanks

so what I am reading is that the black wire going back to the PCM has a bad spot in it?

Yes. With no voltage on it, the PCM has no idea what the temp sensor is seeing. So we need to figure that out first.

I will check tomorrow. thanks

All wires hook up and ck voltage on black I get 0.05 with car running.. With the key to run the black has 0.01 then start it goes to 0.05. So pulled all the back to the PCM and I didn't find any bad spot. So I did a ohm on the black to the PCM was good and the yellow was good too. If I check volts on the black wire one at a time it has 0.01, yellow has 5.08 and the green 11.74 volts. When you unhook the green the temperature gauge goes down to 100. Car running green has 3.61 and yellow has 2.03 and the black has 0.05 volts. I also pulled the green wire out all the way to the plug that goes the the fire wall. Couldn't find anything wrong with that wire too.

On the sensor, what is the resistance between the 5 volt reference and the signal return? So unplug the sensor and take this resistance reading on the sensor pins itself. Take it when the engine is cold. Then start the engine and tell me what the number does. It should move smoothly as the engine warms up. You only have to run it for a few minutes, just to see what the number does.

At hook up it was at 1,500 then at start up. I let run for maybe a minute. It drop down to 650.

It moved smoothly, it didn't jump any numbers, it just counted it way up a little fast.

Okay. Does the temperature indicator come on when the gauge is reading high like it is? Not sure why I didn't think of this sooner. The PCM controls the indicator on the cluster but not the gauge. So if the indicator is not coming on then the PCM is not seeing the ECT as too high.

The light should come on at start up and then go off. If it stays on then the PCM is seeing the same as the gauge but the sensor appears to be working.

We are clearly missing something but let me know if the light stays on when the gauge is showing hot and we can go from there. Thanks.

The light comes on at start up. Then it goes off. When the gauge goes up the light stay off. I can unhook the green wire (the coolant temperature signal) the gauge would go back down to 100. That wire has 12.0 volts on it. But if I unhook the cluster it don't have the voltage.

Okay. So that means the PCM doesn't see the "high" temperature, only the gauge.

So the way that side of the sensor works is the cluster supplies 12 volts to the sensor. The sensor has a resistor on that circuit as well and as the temperature rises, the resistance drops and the voltage increases which raises the gauge. The fact that you always have 12 volts is why the gauge is high.

So unhook the connector again on the sensor and see what the resistance is of pin c and ground. Do this the same way. See what it is when the engine is cold and then see what it is when it starts to warm up. You can let it run a little longer because I am pretty confident that the engine is not actually overheating. Attached the diagrams if needed.

The car was still warm. When I hooked it up, my reading was 840 with car off. Then I started it started going down. I let it run tell it got to 375. It didn't skip any numbers. This was all done with the pin C and on a good ground.

Based on all this my current thought is the cluster is shorted or this circuit is shorted. Here is why:

The way this works is the cluster is supplied 12 volts through the PCM fuse. However, the ground for the coolant temperature gauge is past the sensor. This means between the 12 volt source and ground is a resistor. Based on the temperature of the coolant the resistor varies its resistance which will increase or decrease the voltage that the gauge is reading.

Taking a step back, remember that a load in a circuit will use all available current except for just enough to get back to ground. This means the gauge will use 12 volts when resistance is low and will have the needle high in the scale. When resistance is high the gauge will use less voltage because it needs more to get past the resistance and return to ground.

Applying that to this circuit, when the coolant is cold, the resistance is high as you found, this means the gauge is not able to use all 12 volts and the needle should be low on the gauge. As the resistance decreases because the coolant is heating up, the gauge is able to use that amount of voltage due to the lower resistance and the needle will rise.

However, your gauge is always reading 12 volts. So where is that 12 volts coming from or where is it finding another path to ground so it does not go through the resistor? Remember current is going to find the easiest path to ground.

So if it were me, I would go to the back of the cluster and cut the wire coming directly out of the cluster. Then run a new wire directly to the temp sensor and retest. If the wire is okay then the short has to be inside the cluster on the ground side of the gauge. If it were before the gauge then the gauge would read low all the time.

Let me know if this makes sense. I know you said you inspected the wire but running a new one is much more definitive.

So I ran the new wire and it still did the same thing. From back of the cluster to the sensor. When I started it up. The gauge went all the way over. The light came on at start up then went out. I change the cluster out with a other one I have. It did the same thing.

I installed a remote temperature gauge at the motor. It did the same thing the other two are.

Do you think that the parts store gave me the wrong sensor for it? Or do I need to find a better one? This one on it came from O'Reilly it's a master pro p/n 2-9385.

I took the sensor out and hook the plug up and tested. With my yellow 5v wire unhook from the sensor. When I hook it up to the sensor. It goes to 2.47v. That's with all hook to the car and the jumper wire going to the cluster. Then I unhook it from the car. I used a heat gun to heat it up using my meter. With the ground hook to the black off the meter. Yellow would move numbers smoothly and the green didn't do anything. Showed nothing on the meter. Didn't know know if it would show anything. I am still going to get a new sensor for it and run the test before installing it. I am up to try anything now. Need to get this thing back on the road.

Okay. I need to work through all that information.

Just want to clarify something though. When you say you ran a new wire, did you cut the wire from the cluster and the sensor and solder in a new one? or did you splice it in or just back probe the connectors and run another one without isolating the original wire?

Take a look at the diagram. We need to completely remove the wire that is there and run a completely separate one. You may have done this but a lot of people interrupt an overlay as just running a redundant wire. This will not solve the issue as it will still find the short to ground.

Just want to make sure we are on the same page because what you are seeing doesn't make much sense at this point.

Given you ran a separate wire, I think you are on the right path with a sensor issue but they appear to be working. So maybe it is one for a different application and the resistors are different causing the gauge to go high? Let me dig into it more but if you can confirm the separate wire we can go from there. Thanks

Yes, I cut the at the back of the cluster and the at the sensor. Ran a new wire. I just tied the ends together. Just got a new sensor going out to try it out now.

Sounds great. Keep me updated with new sensor. I am hopeful because there has to be something going on with those sensors.

I installed the new sensor and it works. Who would say someone that can buy two new sensors that are bad. Right now working the air out of it to make sure it all good. Thanks for all your help.

Unbelievable. The only reason we found this was your testing was spot on. You really did an excellent job in working through this.

Good work. Thanks for the update and come back to 2CarPros next time you need some help. Thanks