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Discussion Starter · #1 ·
I was told to replace the ECM, the Engine Control Module, to fix an idling issue. Got it from eBay and replaced and fixed it. But to my surprise, AC is acting. Compressor and blower run, but apparently thermostat is not functioning. It blows hot air even I set the Temp at 66! the "Auto" button is not working.
I am afraid to take it to an AC mechanic, as it seems to electronic and thermostat issue. Any advice is highly appreciated.
 

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If it blows cold air at the very bottom setting (60 on most Nissans, I don't know about yours specifically), then chances are you just have a bad temperature sensor. Without pulling out the wiring diagram, most Nissan Auto AC systems have three, an ambient sensor in the grille, an in-car sensor under the dash, and an intake sensor on the AC evap. If any one of them fails, they usually read -20 to -44 degrees. The HVAC thinks you're at the North Pole and stops blowing cold. Pushing the temp to the bottom is sort of an "override" that tells the HVAC to "just blow cold", so that's the tipoff to a failed sensor.
 

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Discussion Starter · #3 ·
If it blows cold air at the very bottom setting (60 on most Nissans, I don't know about yours specifically), then chances are you just have a bad temperature sensor. Without pulling out the wiring diagram, most Nissan Auto AC systems have three, an ambient sensor in the grille, an in-car sensor under the dash, and an intake sensor on the AC evap. If any one of them fails, they usually read -20 to -44 degrees. The HVAC thinks you're at the North Pole and stops blowing cold. Pushing the temp to the bottom is sort of an "override" that tells the HVAC to "just blow cold", so that's the tipoff to a failed sensor.
Thanks very much. Something like that I guess, except the lowest in my 2001 Pathfinder is 65. I used to "override" manually setting at 65 to cool and manually adjust fan speed. But now it blows hot air even at 65!. Do I have to replace all sensors? any idea on costs involved in parts and labor? BTW, I have already ordered the AC control module on the dash, from eBay. Will arrive next week. Sharmanp.
 

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Sounds like you had a bad sensor and now something else in the system has gone south, so you need to fix the "no cold" problem first. Is the compressor turning on? If not, you may just be low on refrigerant or have a leak somewhere. Once you get the system to blow cold at 65, then you can troubleshoot the sensors. They're all "balance resistor" setups that produce an analog voltage that changes with temperature, so you're looking for one that's stuck at a high voltage with no movement. The ambient and in-car sensors are generally easy to access and replace. If it's the intake sensor you're probably into a much bigger job, most of them are inside the HVAC box attached to the evap. Fortunately, those rarely fail.
 

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Discussion Starter · #5 ·
Sounds like you had a bad sensor and now something else in the system has gone south, so you need to fix the "no cold" problem first. Is the compressor turning on? If not, you may just be low on refrigerant or have a leak somewhere. Once you get the system to blow cold at 65, then you can troubleshoot the sensors. They're all "balance resistor" setups that produce an analog voltage that changes with temperature, so you're looking for one that's stuck at a high voltage with no movement. The ambient and in-car sensors are generally easy to access and replace. If it's the intake sensor you're probably into a much bigger job, most of them are inside the HVAC box attached to the evap. Fortunately, those rarely fail.
Looks like you nailed the problem. I can dabble by trial and error, I cant do more damage than what it is now. I will try.
Could you possibly point to any youtube videos on this? or keywords I dig up some?
I need some visual before I can pretend to be a doing anything under the hood.
Thanks very much!
Sharman.
 

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The ambient sensor is right up in the grille on the radiator support:

7460


The stuff in the passenger compartment is all accessible by removing covers and the glove box. It turns out the AC Auto Amp is integrated into the display unit, and even the intake sensor is reachable and replaceable without any major surgery. So you should be able to get a voltmeter on all the sensors without too much difficulty. For all 3 sensors, one wire will be ground and the other is signal. You'll want to see voltage on each signal wire change with temperature. Holding a Bic lighter on them for a few seconds should suffice for testing. When you find the one that doesn't move for Mr Bic, that's the culprit. Pretty simple.
7461
 

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PS - To back-probe the voltage with the sensors plugged in, get a safety pin assortment from Walmart or Dollar General. Push the pin underneath the rubber seal on the back of the connector to make contact with the metal pin inside. No expensive tools needed.
 

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Discussion Starter · #8 ·
PS - To back-probe the voltage with the sensors plugged in, get a safety pin assortment from Walmart or Dollar General. Push the pin underneath the rubber seal on the back of the connector to make contact with the metal pin inside. No expensive tools needed.
Man, you make sound so simple....the graphics from the service manuals are great. Let me tinker a bit and see if this all makes sense to me. Are these from the Nissan 2001 pathfinder service manual?
Thank you! Sharman
 

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Discussion Starter · #10 ·
Thanks.
I recharged gas using A/C Pro ($39) and now the AC is cooling, at 65 (min). Not super cool, however. I could fill more, up to 50psi. Looks like my one or more senses are bad. Blower, all ventilators/door motors are working.
 

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The most likely one to fail is the ambient, the sensor and harness can get whacked by road debris up in the grille. Check that first.
 
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