2013 Altima 2.5 L. Turn car on and virtually immediately the radiator fans run at maximum speed and stay running all the time (idle and at speed). I have done the following to diagnose: ran auto active test and the test executes perfectly; fan starts off in the off state, then runs for five seconds at slow speed than runs five seconds at high speed then turns off. This would seem to indicate that the entire control path from the control computer through the power control module (which houses the fan relay) through to the fan is intact and is communicating. This seems like strong evidence that the fan relay(s) are operating properly.

Also, I checked the coolant temperature sensor unplugged by reading the resistance value. I got about 339 ohms when the coolant was warmed up. I also tried to place a fixed resistor in place of the coolant sensor (value of resistor: 2.5Kohms) corresponding to about 20 degrees F (a temperature that should not command the fan to be a high speed. Based on the indicator light on the dash control and the fact that the A/C runs properly, it would not seem to be that the system thinks that the A/C is on (I understand that if the A/C is on, the fan runs at high all the time). I have tried disconnecting the battery and keeping it disconnected for several minutes and then reconnecting...no difference. I have seen mention of a "radiator fan control module" for some nissan models that is mounted very close to the fans but per the service manual, I don't think my model has one????

Is my car haunted? If I can't figure this out, I am thinking either: live with it and let the fan wear out. Or remove the fans from the on board control system and install a generic fan controller that operates based on a sensor on the radiator.

Any lights on the dash or "U" codes in the CAN system? The IPDM will turn the fans on full as a failsafe measure if it loses communication with the ECM or BCM.

Vstar,
Thanks for quick response. No lights on dash (I assume meaning indicator lights such as check engine light or other back lit symbol lights)...no indication of any problem. I don't know about "U" codes...I am guessing I need a more sophisticated OBDII scanner (I have a $40 "pocket" scanner that reads P codes...no P codes)? The possible cause you raise (failsafe measure turning fans on full) is appealing and seems likely though I would have thought that the auto active test passing (able to control wipers, lights, fan speed, A/C compressor clutch, etc..) would be indication that the IPDM was communicating with the ECM/BCM...or is that a bad assumption...is it that the ECM/BCM is not involved in the auto active test?

Just checked and my scanner is nominally capable of reading "U" codes. I just checked and there are no codes of any kind including "U" codes. I failed to mention I topped off the coolant just for completeness and it did not help. I will probably pull the fan control relay if the oft chance it is failing though I would think the auto active test would not have passed. I also may rig up a variable resistor in place of the coolant temperature sensor and play with simulating cooling and heating of the radiator fluid and see if I can some kind of clue.

Auto Active does confirm that there's BCM-IPDM communication, but not ECM-IPDM. However, if your cheapie OBD is reading the engine then it's unlikely the IPDM is out of contact with it. One other thing which can cause constant fan operation is an A/C pressure sensor reading off scale high, but that should also make your A/C inoperative (and it would be a weird failure anyway, those sensors virtually always flatline low and not high). FYI, there's no fan control module on a gen5 Altima. There are three fan relays which control the speed. IIRC two of them are underneath the front of the Fuse & Relay Box and one is inside the IPDM. But I don't think that can be the problem either, since it worked correctly in Auto Active. You may need a scan with a much better scanner that can read the IPDM, to find out who's requesting the fan and why.

VStar,

Thanks again. The information is great and well explained. Probably my last question if you don't mind: are there non-OEM scanners that may be up to the job to read more detail or it is something only a dealer would have? Any scanners that I might consider?

I know some of the high-end Foxwells and Autels will do it, and the AutoEnginuity with the Nissan/Infiniti package will do it. Nothing cheap that I'm aware of.

Vstar,

Just checked and you are correct in your recollection. There are two removable fan relays in the "relay/fuse" box...one labelled "MID/HIGH" and one labelled "HIGH". I listened more closely when turning on the car and the sequence is: ignition, no fan on for a couple of seconds, sounds like mid fan for a couple/three seconds, then high fan indefinitely. So per your thought, there seems to be some some reason commands going out to sequence the fan as opposed to a stuck relay or control line. The prices of the high end scanners are eye watering! But I agree, the obvious next step if I want to figure this out is to figure out what is commanding this fan sequence and why. Thanks again, over and out.

VStar,

Happy to report that I found and fixed the problem. I decided based on your analysis that it much have something to do with the A/C system. I pulled the front bumper/plastic cowling off to get a look at the A/C pressure sensor. As you know there are three wires on the connector. I barely touched the red one (I think it may be the signal wire as opposed to the DC supply wire and the ground wire) and it separated from the connector. On closer examination, I could see green copper corrosion on both ends of the wire....somehow the insulation much have been compromised and the annual winter salt bath here in New England must have infiltrated and ate through the wire. Some wiring microsurgery and I was able to solder the two ends together and save the connector. A good smathering of liquid tape and good as new...well good enough. One take away is that apparently there can be a break in that wire and the A/C still works and no error codes are thrown (as these that OBDII code readers find) and no indicator lights.