Bullet points: Car overheated a month ago in traffic; this after installing a new cooling fan and burping the system 9or so I believed). After getting the car home I watched it idle and saw that only one fan was working. I pulled the fuses, nothing, disconnected the battery and PRESTO all fans working, which led to my false sense of security. All was well until I was sitting in the car using WiFi on my laptop near a library one night, the temperature about 86 degrees and night. All was fine until about twenty minutes and the temperature gauge started rising to the top third. I turned off the AC and switched the temp to 90 to siphon off the heat and it went back to normal. This presented a quandary: If there was air in the system then how was hot air coming from the vents? If it is a bad thermostat then is that a symptom? My guess is that either the thermostat is going or that there is air trapped in the system that prevented it from getting to temperature and opening.

I have ordered a Lisle funnel to burp the system but just spitballing as to if I need a new thermostat? The car is a 2005 and I am almost sure that the original is still there as the records I received said nothing about changing it. Someone on another forum said that these cooling systems are notorious for overheating in traffic but I can't believe that such a car would be prone to that problem and it is just "oh well".

 

You can test the thermostat really easily. Take it out and get a pot of water boiling on the stove. Drop the thermostat in it and if it opens you know it's working. If it doesn't then it needs to be replaced. Given the fact our cars are notoriously hard to bleed air is the most likely culprit. It's also possible you have a fan motor that's beginning to die, if you're over 100K miles it's not a bad idea to just replace them. You can get a whole new assembly with the fans/motors and shroud from Rockauto for less than $100 and it's a ridiculously easy DIY repair.

 

Replaced the fans about a month ago. At what point should both fans come on is my dilemma. Right now its low 70s-65 here and I let the car run and one fan came on, the AC fan. I even took it on the highway and ran it pretty hard, brought it back and watched...still nothing. Heat comes from the vents when I turn on the heat so that checks. After using the lisle funnel I saw no air bubbles in the coolant and I revved it high for good measure just so that the coolant could boil up and force whatever air was trapped out. The dial went up and then back down into the safe zone but still only one fan. All of those check working thermostat I assume. I am just curious if the computer goes by ambient engine temperature/engine bay temperature or coolant temperature before activating both fans.

 

The ambient sensors are only for the air conditioner, the only thing that matters for engine fan speed is the temperature sensor on the motor. When you turn the AC on you should have the fans kick onto HIGH SPEED regardless of engine temperature. There are 3 relays in the middle of the IPDM that are for fanspeed, it's possible you have a relay failure, swap the relays around to see if you lose low speed but gain high speed. Normally the engine turns on low speed cooling when it passes 98 degrees, on a cool day at idle you will likely never even have the high relay turn on. This is why most folks just press the AC button to test high function since it forces it on immediately.

 

You can also unplug those harnesses and check for voltage, if you only have ONE FAN SPINNING you either fried the relay or the fan motor itself stopped working. They both turn on low and high at the same time, there are a different number of BLADES on the two fans though so the airflow is different on each side.

 

Just to be clear, I know that the AC side fan should come on when you start the AC but you are saying that both should? What if the coolant temp hasn't reached minimum operating temperate and the temperature outside is, say 65 degrees? Are both fans still supposed to be on with the AC on or just the AC side fan on and operating at low speed? Secondarily, using the same 65 degrees outside and cool scenario, are both fans operating once the minimum coolant level is established? If so, then I might have a problem with the coolant temperature sending unit or is that off base?

Is there any scenario where I would switch on the AC and just one fan, the AC side runs, other than a bad relay? I guess that is the simplified version of the question.

 

No, both fans should be turning on either low or high speed but they both run the same speed relatively, it's hard to tell when you look at them because one has more blades so they might LOOK like one is moving slightly faster but there will be a noticeable difference between high and low speeds on both fans.

If you have a fan not functioning at all there's a really good chance the fan motor is dead or the relay is physically BROKEN inside and the two latching mechanisms are not fused together anymore. You can bench test relays as well to verify function, I'm pretty sure they are the 2M relays but you can verify them against the picture when you remove to test. Get a bunch of female spades, crimp on some insulated wire so you have proper test leads, I like to take the other end of the leads and strip the ends and staple them down to a piece of wood and write on the wood what the lead numbers are. Then you are able to apply test voltage with aligator clips and not worry about anything moving around and shorting out. Make sure to always use an inline fuse on your positive battery side aligator clip lead or you might accidently make a heating element out of your wire. Using the aligator clips and having everything on a bench frees up both your hands so you can use a digital multimeter to read continuity across the 2 sets of contacts in the relay.

Apply the voltage to pins 1 and 2, polarity doesn't matter.

Use the digital multimeter to read the first contact pins 3 and 5, the circuit should be CLOSED with full continuity, remove voltage and verify the circuit opens.

Next read the second contact pins 6 and 7, should be exactly the same readings as the first contact.

 

You can also unplug the fan harness and read for power on it as well but it's fairly tight working quarters and everything is hot so wear gloves/sleeves and use the same procedure of making test leads stapled to a piece of wood to relocate your test area to under the vehicle where it's easier to work.

Make damned sure you don't overheat your car, I like to have a second person in the driver seat watching temps in case I get distracted.

 

It was a bad relay, #2 to be exact. Thanks for the insight. For those of you who haven't yet had to deal with fan relays they are from the top, 3-2-1; 3 being the passenger side, 2 being driver and 1 being the fan speed/off-on switch controlled by the ECU. The retainers are not logically positioned but you just have to wiggle a flathead screwdriver back and forth on the retainer as you pull them out.

 

Awesome, glad everything is working again.