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Coolant Temp Sensor wiring question

thomcasey

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Sep 24, 2014
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907
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Indianapolis, IN
I am chasing a ECT sensor issue with a constant code 21. Here is what I have found:

1. Straight short between the two pins for the sensor (so I believe this is bad)
2. Wiring has been traced and verified good connectivity
3. There are TWO Yellow/green line wires going to 1 pin , the other is green/black line (sensor ground)

I plan on replacing the sensor as it seems to not measure out correctly. BUT, are there supposed to be two Yellow/green line wires to the same pin? I can only find 1 on the harness connection at the ECU, so I am wondering if something is amiss there?
 

iceman69510

Turn Right Racing
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I do not remember that being the case when I have repaired that wiring before. Can you find where it goes?
 

thomcasey

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Sep 24, 2014
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Indianapolis, IN
They both go into the same loom. I will have to see if they both trace to Pin 20 on the ECU. Now, it should also be pointed out that I do not have the VR4 thermostat housing installed, but rather a DSM model. SO, it does not have the sensor on the back side that the VR4 model does.
 

strokin4dr

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Aug 30, 2005
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Savannah, GA
They should not be going to the same pin. Here is the break down in that area.

Blue/Black, single wire sensor, A/C Condenser Fan, Normally open

Green/Red and Green/Black, 2 wire sensor, A/C Clutch lockout, Normally closed

Green/Black and Yellow/Green, 2 wire sensor, ECU Temperature sensor, variable ohm reading

Yellow/Green, single wire sensor, Cluster temp gauge, variable ohm reading.
img.php
 

thomcasey

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Messages
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Indianapolis, IN
Thanks alot!!

Ok, so according to this, I need to remove both Yellow/green line wires from the connector, and only connect the one going to pin 20 on the ECU to the ECT. The other one should go to the gauge single pin sensor. I can do that. Currently the gauge works, but I assume it is reading from the ECT. There is a single Blue/black line wire (that you mention) connected currently. It was a single connector, so the DSM guy in me said it went there. I do not have AC, so I will ditch the wire.

So, I will correct the ECT wiring and the gauge wiring. I assume I will still see the ECT CEL , but hope I will not need to replace it.
 

iceman69510

Turn Right Racing
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You may not see a CEL. Remember the system in these cars is fairly primitive and really only senses open circuits. It may not sense a misreading sensor, but hopefully will a dead one.
 

thomcasey

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Sep 24, 2014
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Indianapolis, IN
Well, it is telling me the ECT is bad with the CEL, and it is ohming a direct short. So, I may replace it anyway. I have a good factory one off a low mileage JDM motor pulled from a galant last year, so at least it is factory.
 

thomcasey

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I fixed the wiring, verified it, swapped the sensor, still tripping the CEL with a 21 code. Took ECU out and looked it over. Looks good, no questionable runs or components. This a 94 DSM ecu that is socketed and has a keydriver chip for the 850's, get solenoid delete, 2g-Max, etc...
 

iceman69510

Turn Right Racing
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When you say verified it, you are certain that you have the right wires (of the two same colored ones) connected to the right source?
 

GSTwithPSI

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My guess would be that you may not have fixed the correct wire.

You need to test for continuity between the CTS sensor wire under to hood and the ECU pin 20. Only one of the Yellow w/ green tracer goes to the ECU, and the test above will identify which one. For a complete breakdown, see the following...Pretty much all the info you could possibly want to know in one thread: click
 

thomcasey

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Indianapolis, IN
I will check out that thread, but I did fix the correct wire. I verified it was correct using a volt meter from pin 20 to that wire by itself. It take 1 minute of idle, then sets the code
 

thomcasey

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Indianapolis, IN
Reading those posts, do I understand correctly that the Yellow/green line wire from Pin 20 (ECU) goes to the stem of the T and the green/black line (Sensor ground) goes to the cross bar on the top of the T?

I believe that is the way I have it, but will verify after work.
 

GSTwithPSI

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That's correct, you just need to ensure you have the correct Green w/ black tracer and Yellow w/ green tracer since there are a pair of each in the harness at that location.
 

GSTwithPSI

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^ Correct. The positions noted in my post just describe how the connector is configured from the factory.

As long as the ECU sees a variation in the 5V signal it sends to the CTS, you're good to go.
 

thomcasey

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Excellent. Unfortunately that is how I have it configured and I still get the CEL and code 21 after 1 minute from startup. Guess it is time to check voltages
 

thomcasey

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Sep 24, 2014
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907
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Indianapolis, IN
I guess my next step will be to swap out my injectors for some 550's from my son (I have a 2g MAF) so I can install a factory ECU and see if it clears the CEL. As all wiring and sensors check as good, only thing left is bad voltage from ECU.
 

thomcasey

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Sep 24, 2014
Messages
907
Location
Indianapolis, IN
Is it known which sensor ground the green/black wire needs to go to (pin 17 or 24)?? I have verified the sensor is working with different resistance due to temp, and ran a new + wire from the sensor to pin 20 on the ECU plug, and verified continuity from the ground wire connection to pin 24 on the ECU(sensor ground), but DSMLink still shows -74°. I also shorted the two wires together and it still showed -74° (I understand a shorted wire should register a +300°).
 
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