# 94 Sentra Coolant Temp Problems



## FlavorOfSound (Jul 12, 2012)

Hi, I am new to the forums and am a mechanic looking for some help on a 94 Sentra with a 1.6L and an automatic.
The customer brought the vehicle in with complaints of the A/C working until it reaches operating temperature. I connected my scan tool to check for codes and view the PIDs. The coolant temp. PID was sitting at 242 degrees, yet my temp. gun said it was 205 degrees. I unplugged the sensor to check for voltage and ground, and noticed that the A/C clutch engaged. The PID also said at this point that the coolant temp. was 176 degrees. I know that the computer will have a set value to run if there is no signal, so this was expected. The sensor passed the resistance test, and there is contenuity between the sensor and the ECM for both wires. At this point, we decided the ECM was malfunctioning, and replaced with a used unit (nissan no longer offers new ECMs for this model), yet the malfunction is still present. We also installed a new sensor, but it still kicks the A/C clutch out when it hits "242 degrees."
I am lost at this point, could both the ECMs be bad, or am i missing some detail here? Sorry for any poor spelling or grammar, neither are my strong point.


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## LvR. (May 26, 2010)

Battery voltage when engine running around 2000rpm? (13.5+)
Voltage drop over the length of the positive and negative cables on the battery? (0.1V with all electrical loads on)
Voltage difference between the body and engine and battery negative terminals? (close to 0)

Do the cooling fans engage when you disconnect the temp sender?


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## FlavorOfSound (Jul 12, 2012)

-Battery voltage when running at around 2000rpm is 13.86v
-Voltage drop on the positive is 0.43 and negative is 0.02 with all loads on. (I am an engine guy and have very little experience with electrical diag, so this is what I ended up with. Please feel free to explain how you would check voltage drop so I can verify my results)
-No voltage difference exists
-Unplugging the temp sender does engage fans


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## i r teh noobz (Apr 26, 2007)

176 sounds a bit off. Every ECT sensor I've messed with sets a -40 on the scanner when unplugged. Usually full cold is max resistance and full hot is min resistance. Try jumping the harness connectors and watching the scanner. It should go full hot. 

See what kind of voltage is available at the sensor. They're usually around 5 volts.

Voltage drop is measured by connecting a volt meter to both ends of an active circuit and seeing what is displayed. .4 volts means that .4 volts has been lost/used from point a to point b. General spec is around .2v lost per connection.


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## jdg (Aug 27, 2009)

i r teh noobz said:


> 176 sounds a bit off.


The ECU sets a default value of 176 (or 184 on my '98 GA16DE) if it detects a fault in the ECT circuit. Kind of a backup plan sorta thing. I remember reading about it somewhere in the FSM's.


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## dreamteam (Jul 29, 2005)

My experience with Nissan has been that the A/C will "kick out" when it is low on freon. You didn't mention whether it was full or not.
Also, why was the PID at 242? Check the feed wire for resistance/bad ground.


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## i r teh noobz (Apr 26, 2007)

jdg said:


> The ECU sets a default value of 176 (or 184 on my '98 GA16DE) if it detects a fault in the ECT circuit. Kind of a backup plan sorta thing. I remember reading about it somewhere in the FSM's.


I suppose that makes sense. At least the car would run halfway decent at that value. How often does it get to -40 outside?


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## jdg (Aug 27, 2009)

i r teh noobz said:


> I suppose that makes sense. At least the car would run halfway decent at that value. How often does it get to -40 outside?


Up where I'm at? Maybe a couple times a year


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