Coolant sensor/switch questions

[m3f_circuit_ic_e9caa35a59]

I checked the schematic on the coolant temperature sensor/switch that sits in the radiator.



It says that the black/gray wire (high temp wire) should be +12V. It also says that the black/green wire (low temp wire) should also be +12V. The brown wire is a ground.



My question is - how exactly does this setup work?



When I measure either of the black/green or the black/gray wires when the car is on, they both show +12V. Shouldn't it be one or the other?



Also, I've noticed that if you ground out the high temp wire, it will turn on the OEM electric fan. This seems odd, as grounding a hot wire should result in an arc.



If I had to take a guess at how this SHOULD work, I would assume that the ground is constant, and something inside of that switch would decide which (if either) of the other wires (black/gray vs black/green) should get power based on the temperature of the sensor.



But since they both always have power, either I'm completely wrong, or my stock wiring is bad, or the sensor/switch is bad.

[m3f_vmbray_980ce7cf9e]

I don't think you have 12v on those wires. They work exactly by grounding when the temp hits the threshold so all that is normal.


The fan switch connects one or both of them to the third ground wire.


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[m3f_circuit_ic_e9caa35a59]

So since I never see a ground on either of those two wires (black/gray or black/green), then perhaps my switch is broken?

[m3f_RightYouAr_088582bd0b]

So since I never see a ground on either of those two wires (black/gray or black/green), then perhaps my switch is broken?


I'm not an electrical guy, so please forgive me. I don't really understand your question about not seeing a ground on the 12v wires. I'm not sure what that means. But I checked my fan wiring recently to ensure my fan relay was working. I didn't check the temp switch in the radiator, because my symptoms were a fan that stayed on even when the temp switch was unplugged and the A/C was off, and the ignition was off!. I think my relay was sticky and pulling it and reinstalling it fixed my issue for now.


Here's the test I did, in my layman's terms:


1) Unplug the temp switch connector from the temp switch in the radiator

2) Short the brown wire in the connector to the black/gray wire. The fan should come on high speed.

3) Short the brown wire in the connector to the black/green wire. The fan should come on low speed.


How you test the switch itself, I'm not sure without heating it up to test operation. As a previous poster said, the switch appears to ground the 12v from either the high or low temp wires, completing the circuit and tripping the relay to trigger the fan.

[m3f_circuit_ic_e9caa35a59]

By fan you mean your OEM electric fan?


I guess what's confusing me here is if you ground either the black/green or the black/gray wires, shouldn't it result in arcing rather than turning a fan on? That's normally what happens when you ground a hot wire.

[m3f_RightYouAr_088582bd0b]

Yes, I'm referring to the OEM electric fan. It also comes on low when the AC is on. As to your other question, I'll have to defer to those who understand electrical systems better than I.

[m3f_RightYouAr_088582bd0b]

Found some interesting stuff searching Google for "automotive ground switched"


Here's an interesting thread that may be relevant.


http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2462446

[m3f_circuit_ic_e9caa35a59]

Yup, I'm familiar with autos using a switched ground. For example, fuel injectors always recieve power, and the ECU switches the ground off and on to complete the circuit and fire the injector.


What's throwing me for a loop is the fact that I'm seeing +12V from those two wires instead of nothing.

[m3f_RightYouAr_088582bd0b]

Yup, I'm familiar with autos using a switched ground. For example, fuel injectors always recieve power, and the ECU switches the ground off and on to complete the circuit and fire the injector.



What's throwing me for a loop is the fact that I'm seeing +12V from those two wires instead of nothing.



Why would it be nothing? I'm assuming you're using a multimeter, with one probe on the terminal in the connector and one probe on a ground? If so, you're also completing a circuit, and should see 12v. Or are you measuring voltage some other way I'm not familiar with?

[m3f_circuit_ic_e9caa35a59]

Why would it be nothing? I'm assuming you're using a multimeter, with one probe on the terminal in the connector and one probe on a ground? If so, you're also completing a circuit, and should see 12v. Or are you measuring voltage some other way I'm not familiar with?



Right, that's how I'm measuring it.



I guess what I don't understand is why a wire intended to be a switched ground would carry any voltage at all.



Basically, I would expect that wire to be completely dead until it was switched to become a ground.



In reality, I'm getting +12V and then maybe at some point it becomes a ground (which I haven't seen yet).



Feel like doing a quick test on yours to see if you get the same thing?

[m3f_RightYouAr_088582bd0b]

Right, that's how I'm measuring it.



I guess what I don't understand is why a wire intended to be a switched ground would carry any voltage at all.



Basically, I would expect that wire to be completely dead until it was switched to become a ground.



In reality, I'm getting +12V and then maybe at some point it becomes a ground (which I haven't seen yet).



Feel like doing a quick test on yours to see if you get the same thing?



Sure, I can check in a few hours when I get home. I assume measure the voltage on the two black/xxx wires?

[m3f_RightYouAr_088582bd0b]

Circuit_icon, I just confirmed that I have 12v on both black/green and black/gray wires on the temp switch connector, and ground on the brown wire on my car. Hope that helps.

[m3f_circuit_ic_e9caa35a59]

That does, thank you. Alright, so maybe I don't have a bad sensor.


I'll try wiring it to the low speed side and see if it ever triggers it.

[m3f_englishtom_70cc2c7eca]

the wires are hot. the switch works by connecting them to ground depending on the temperature.


when the switch is triggered it connects the low speed wire to ground (turning on the low speed fan setting). When the temperature reaches the next temperature it moves to connect the high speed wire to ground causing the high speed fan to kick on.


you gave yourself the correct analogy then seemed to confused yourself again. You have it correct with the fuel injectors so lets run with that. The fuel injector is ALWAYS getting 12v to the plug. What it doesn't have is a ground until the computer closes the switch to complete the circuit. This is how every switch works in one way or another. All the temp sensor is doing is acting as the "computer" to determine which wire gets to go to ground and turning on the fan.


Also your straight to ground Arc theory only occurs when you go straight to ground without some form of resistance.

[m3f_circuit_ic_e9caa35a59]

you gave yourself the correct analogy then seemed to confused yourself again. You have it correct with the fuel injectors so lets run with that. The fuel injector is ALWAYS getting 12v to the plug.



Right, I get that...the one wire is always hot, but doesn't the injector itself prevent the other wire from seeing any current?



Also your straight to ground Arc theory only occurs when you go straight to ground without some form of resistance.



I think this is my hang up. I get that the case of the injector it acts as a resistor...but what's the resistor in this case? The sensor? The fan?



Anyway, good news - I swapped it over to the low temp fan and the car heats up enough to trigger it.



:thumbsup2:



It also kicks the fan on for about 20 seconds during initial startup. Not sure why.