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Old 07-13-2021, 08:59 PM   #1
Clean05
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Default 05 Legacy Gt Rad Fans Constant On (Not Overheating)

Some background: Brand new block with heads reworked. I had the valves lapped and clearances to lobes are spec. New oil pump, water pump, timing belt, headgaskets, brand new engine harness. Im sure there's stuff im forgetting, but it really doesn't matter the engine runs great...

I'll get some other things out of the way to avoid unnecessary posts:

*Not the water pump
*Not an airbubble
*Not a bad thermostat
*Brand new coolant temperature sensor
*Torque app shows the coolant temp at 240° even when the engine was just started after sitting overnight

*Fans turn on and gauge reads hot even before engine is started. (The second line from the top)

The engine turns on goes through its normal closed loop operation and thermostat opens (I had to unplug the fans to get it hot enough to open). The engine is running fine and sounds great.

The fact that the ecu shows the obd2 app that the coolant temp is 240° (it does not fluctuate not even a half of a degree) makes me think that there could be a ground short after the sensor on the way to the ecu.

There was a recall on same year legacy (but in manaul transmission) that stated the fan relay could cause a surge in power and ruin the ecu. My car was not associated with that recall. I also did not have this problem with the old motor.

Can a bad relay cause the bad signal to the ecm? Or is the relay only engaged by the ecm after it reads its input from the sensor?

Any help is greatly appreciated.
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Old 07-15-2021, 09:50 AM   #2
heiche
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Is the new coolant temp sensor the only one that has shown this problem? If so, maybe swap it out with a different one.

If the sensor is good, it's probably time to get a dvm and check the wiring from the sensor.
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Old 07-15-2021, 10:32 AM   #3
subaru_gc8
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if you are getting aftermarket parts, sometimes they are just not good right out of the box.. you might want to try another just to make sure
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Old 07-15-2021, 11:24 AM   #4
Clean05
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Thanks for the responses.

The coolant temp sensor is new because this was happening with the old one. New sensor same problem.

Ive measured resistance between the sensor and the black plug going to the firewall and its continuity is good. The crankshaft position sensor and MAP sensor have continuity on ONE of the coolant temp sensor wires which i am assuming is the power circuit and not a short. Does that sound correct? I can explain better if that doesnt make sense. Thanks again.
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Old 07-15-2021, 12:39 PM   #5
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The coolant temp sensor should have 2 wires... one wire that is a signal ground (probably the one that you found continuity with other sensors, so that's probably good).

and the other wire is the signal that goes all the way to the ECU pin C14. Possibly there is a break or a short somewhere between the sensor and the ECU.
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Old 07-15-2021, 01:06 PM   #6
Clean05
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Hey thanks again for the help. I only use genuine subaru parts from the dealer to make sure.
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Old 07-15-2021, 01:11 PM   #7
Clean05
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Quote:
Originally Posted by heiche View Post
The coolant temp sensor should have 2 wires... one wire that is a signal ground (probably the one that you found continuity with other sensors, so that's probably good).

and the other wire is the signal that goes all the way to the ECU pin C14. Possibly there is a break or a short somewhere between the sensor and the ECU.
Well it looks like the continuity is good from the black plug to the sensor. I cant imagine how any damage could have happened to any wires after the plug going in to the firewall. I have never removed the ECM before. Is it possible to unplug the ecm, find the wire that goes to the Coolant Temp Sensor and measure continuity from there? If continuity is good between the ecm to the black plug, my last guess would be that somehow the computer is toast.

This is a brand new Genuine Subaru harness

Thanks again.
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Old 07-15-2021, 02:41 PM   #8
heiche
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with key off, and battery disconnected, disconnect the coolant temp sensor

-can you measure resistance between the 2 contacts in the coolant temp sensor connector (on the sensor itself)? how many ohms?

-can you measure resistance between the 2 contacts on the harness side plug? how many ohms?
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Old 07-15-2021, 05:45 PM   #9
Elbert Bass
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What does the temp gauge on the dash show?
Unplug the coolant temp sensor. Turn on the key, read coolant temp on ECM it should read -40. Anything else and you have a short to ground, short to power on the coolant temp sensor wiring, or you have swapped the plug with another sensor.
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Old 07-19-2021, 01:40 PM   #10
Clean05
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Quote:
Originally Posted by heiche View Post
with key off, and battery disconnected, disconnect the coolant temp sensor

-can you measure resistance between the 2 contacts in the coolant temp sensor connector (on the sensor itself)? how many ohms?

-can you measure resistance between the 2 contacts on the harness side plug? how many ohms?
Resistance between the contacts is 0.9 OHMS between the sensor and the black plug. Sensor is brand new and also has continuity.
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Old 07-19-2021, 01:44 PM   #11
Clean05
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Quote:
Originally Posted by Elbert Bass View Post
What does the temp gauge on the dash show?
Unplug the coolant temp sensor. Turn on the key, read coolant temp on ECM it should read -40. Anything else and you have a short to ground, short to power on the coolant temp sensor wiring, or you have swapped the plug with another sensor.

The temp on the gauge shows the second line from the top with the engine cold and the ignition turned to on. It shows hot on the ecm at 240 degrees flat with no fluctuation. it does this with the motor cold and off.

I cant see where I could have swapped a plug. The only sensors that plug could reach are different connectors.

Thanks again to everyone willing to take the time to help.
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Old 07-19-2021, 01:58 PM   #12
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Quote:
Originally Posted by Clean05 View Post
Resistance between the contacts is 0.9 OHMS between the sensor and the black plug. Sensor is brand new and also has continuity.

0.9 Ohms is basically a short. But I'm unclear what you were measuring... Was that across the 2 sensor contacts? Or across the 2 harness contacts?
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Old 07-19-2021, 02:10 PM   #13
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Just read this thread over on subaruoutback.org. This guys has an NA, but I am pretty sure the coolant sensor is probably the same as the GT/OBXTs. Interesting info on the kind of resistance you should be able to measure there. According to the info the OP reported, the lower the resistance, the higher the reported coolant temp.
I believe heiche is correct there where 0.9 Ohm is essentially a short which would result in maxed out reported coolant temp.

--> thread
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Old 07-21-2021, 08:46 PM   #14
Clean05
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Quote:
Originally Posted by enduroxt View Post
Just read this thread over on subaruoutback.org. This guys has an NA, but I am pretty sure the coolant sensor is probably the same as the GT/OBXTs. Interesting info on the kind of resistance you should be able to measure there. According to the info the OP reported, the lower the resistance, the higher the reported coolant temp.
I believe heiche is correct there where 0.9 Ohm is essentially a short which would result in maxed out reported coolant temp.

--> thread
Thanks a lot for that link. That article was very informative. I printed out the graphs and compared the resistence in the coolant temp to the ambient temp here in san diego and determined the sensor to be good. Which led to my conclusion....

Quote:
Originally Posted by Elbert Bass View Post
What does the temp gauge on the dash show?
Unplug the coolant temp sensor. Turn on the key, read coolant temp on ECM it should read -40. Anything else and you have a short to ground, short to power on the coolant temp sensor wiring, or you have swapped the plug with another sensor.
Elbert, thank you very much for the help. At first I was thinking there was no way I could have swapped a plug because they are all different connectors. Well I completely forgot about the purge solenoid. It's hidden so deep under the exhaust manifold, I haven't thought about it since I assembled the motor. It IS the same plug and of course fixed my ECT sensor problem.

Trouble now is I am getting a p0459 EVAP Purge Solenoid Voltage High. I think I may have damaged the solenoid or ECM by swapping the plugs. I still dont believe its the brand new harness because continuity is good from the black firewall plug to the sensor plug. There is a guy on ebay that sells a solder kit to fix the ECM because I guess it is common for the ECM to burn and cause that code (probably from people like me swapping the plugs on accident). I am going to start with testing the solenoid on a bench. Thanks again.
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Old 07-21-2021, 08:57 PM   #15
Clean05
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Quote:
Originally Posted by heiche View Post
0.9 Ohms is basically a short. But I'm unclear what you were measuring... Was that across the 2 sensor contacts? Or across the 2 harness contacts?
I think that reading was because of my $9 meter. Even when I hold the test probes together it jumps all over the place and finally settles to around zero but only if I hold it just right. Waiting on a new one coming in the mail.

Thanks again for input
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Old 07-22-2021, 09:14 AM   #16
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OK. You are getting closer to fixing your issue then.
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