2005 Sienna AC Evaporator Temperature Sensor

The sensor is of course on the evaporator coil, which is buried inside the front air conditioner assembly: Toyota Sienna Service Manual: Disassembly - Air conditioning unit - Air conditioning

To get that? Well...

thescot14 said:

Hey guys, first time question. I took my 2005 Toyota Sienna into the local trusted mechanic and got a high quote to fix my ac. They determined that the evaporator temperature sensor is faulty and said that the only way to get to it is to remove the dash. Anyone out there that can point me in the right direction to where this part is located? I am going to do the work myself, and want to remove as few parts as possible to do it.

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It's not just removing the dash. It's removing the windshield wipers, windshield cowl (my initial browsing shows this to already be a couple hours both ways, cause it has to come out for the spark plugs which I'm planning). Then you have to remove the dash. Including the passenger air bag assembly, intermediate steering shaft, transmission shift cable...well, everything. You are not just removing the dashboard. You are utterly and completely tearing the entire inside front of your vehicle apart. Oh, and not to mention, you need to remove the refrigerant first, since you're breaking AC lines, and so you'll need a new dryer for your AC system, and you'll either need a vacuum pump or have a shop evacuate and fill the AC system afterwards. Whatever you do, be absolutely sure that the AC system is leak checked and 100% leak free once the unit is back in place before you start piecing your dash back together... Toyota Sienna Service Manual: Removal - Air conditioning unit - Air conditioning

I hope I'm wrong, and there's a way to get this that bypasses all this. But looking at where it is...it seems doubtful. And if you need to do what the above service manual links are saying, well. I try to do a lot of my own maintenance. I just swapped my water pump. I don't think I'd do this. I'd live without AC or I'd get a small loan if needed to pay someone to do this. I mean maybe, if I wasn't too busy with other things and the weather allowed me riding my motorcycle for a month. If you choose to do this, I wish you luck. While you're at it, may swap the expansion valve as a PM cause you're pulling it anyway, and do your spark plugs since you've removed the windshield cowling. And make damn sure you've done all the testing you can possibly do to rule out the possibility that it's anything easier to access than that sensor is!

My final tip: I personally like having the factory service manual when I'm doing things. One particular 2nd gen model FSM is what the above links to tsienna.net are to. But it doesn't hurt to pay $20 to get a two day subscription to the real thing in case there's anything different for your particular model. Especially with something that will involve such huge amounts of time and effort as this does. https://techinfo.toyota.com/

sixto said:

Yes, the probe extends into the evaporator core but I’m 99.44% sure the sensor is accessible without splitting the air handler.

Here’s a picture from the passenger footwell of our ‘04 LE with basic AC after removing the inner kick panel. Hold the outer ring of the clip while turning the center counterclockwise with a Philips screwdriver. Once the head lifts the slightest bit, you can pull off the panel sending the clip flying.

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The yellow and green wires go into a connector off screen then emerge as black wires in the blue sheath. They go into the evaporator housing through that u-shaped thingy that conveniently fits in a 10mm spanner/open wrench. The scuff marks are from the wrench slipping but it seems a counterclockwise turn releases the sensor. I’d have to see a replacement sensor to guess how it’s held.

Sixto
‘04 LE FWD 198K miles

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thescot14 said:

Paul,

This is exactly what I was looking for! They said they were able to wiggle the wire and eliminate the issue, so having easy access to this makes complete sense. I have not tired to do the counterclockwise turn yet, today I was gathering more data. I can say at least that for whatever reason the issue is not presenting itself as obviously right now. I tried wiggling and pulling on the wires myself with the car on, but could not create a direct correlation. If I can replace the part as easily as turning this counterclockwise I think I will just order the part and do it, its $16.

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Hmm. Well if it can be reached from that plug, that's great news. According to the images at tsienna.net it looks like there's just a little plastic prong that shoves into the evaporator fins. Still seems unlikely you could reach to remove/replace it through that tiny hole. But I suppose another way, if that isn't possible, is to carefully cut a big hole in that plastic for more room, then just patch the hole up later.

thescot14 said:

We are driving around without it being loaded up, and this is happening. The belt door is also a thought I have had. I had to replace one already (the one that is accessible from the driver side). I believe there is another blend door that is accessible from the passenger side, but I have not replaced that one yet. Do you know what the passenger side blend door does? The driverside if memory serves changed the mode of the van's ac (blowing at feet, or face, or both) does the passenger side combine warm and cold air when the car is set to anything other than HI or LOW?

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I think by "loaded" he meant do you have a trim with digital temp controls and split zones vs the simpler single zone with simple analog dial temp control.

thescot14 said:

Paul,

With your question and some time during lunch today I hooked up 4 thermocouples to all the vents on the dash (driver left and right, passenger left and right) and plotted the data for about 30 min. No major failure was noticed while I was driving but I did notice smaller fluctuations. You can see in the plot below that during these smaller fluctuation whatever it was affected all the vents. To have something to compare it against I did unplug the sensor around 8-9 min into the test, then plug it back in. That is the major spike in the middle.

The pattern of failure times is very cyclical, every 30-40 seconds. It is as if the ac seems to fail but before it becomes more noticeable it recovers. I did try and wiggle the wires but cannot remember exactly when so its hard to attribute any small change to that. I will do a better job next time correlating the time when I am doing something different.
View attachment 57475

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Ohh, data! I love data. I'm a Test Engineer, and sometimes people will be talking about this or that could be the issue, and I just go and hook up a scope or something else and be like "here's what it's actually doing.

So, you've got the temp data. But we've got more data:

It's just a thermister. It says to remove it to test, but if you just run your vents with the AC off and you have the temp of the air blowing through the system, you could measure the sensor with it installed, since accessing the plug is easy.

Also, I'm pretty certain the purpose of this sensor is to shut off the AC if the evaporator starts icing. So you could do another test by sticking a resistor in the mating plug of the temp sensor (the side running to the control unit) to "fake out" the sensor and see if your AC runs without turning off. If so, it's definitely the sensor or wires to the sensor. If it continues turning on and off, and the resistance measurement of the sensor was good, then the issue is wiring downstream from the sensor.

I'd use something like a 2 kΩ resistor for this test, which will tell the car the evaporator temp is ~70 °F so the AC should not shut off (for this reason) with the resistor connected in place of the thermistor. And yes, I've totally used regular resistors to fake a temp to test things at work in place of termistors before to feed the sensor a temp I want it to see without needing to get the actual thermistor to a specific temp.

If you don't have access to a resistor easily, well, you said the new thermister is $16 so alternatively you could just order a new thermister, hook it up and leave it dangling in midair for the same sort of test.

thescot14 said:

I have always had the AC on high, and the fan blowing on high. Cabin air temperature is right at 74 degrees F, and I am pulling from inside.

The latest test with just a 2k resistor in place of the sensor did take the spikes away. Feels like I should just replace it and see if it improves, what do you think?

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Hmm. I would personally get the resistance value of the sensor with the AC off and just the fans running circulating air of a known temp within the chart of temp to resistance posted in the service manual snippet I posted previously. Then I'd use that value to determine the next steps.

• If you can't get a stable reading, then I'd replace.
• If you get a stable reading, but it's wrong, I'd replace.
• If you get a stable reading, and it's right, then then it may be doing exactly what it's supposed to do, shutting off the AC to avoid evaporator icing. But why it would be icing I'm not sure. I'm not an AC expert at all. I know dehumidifiers ice up when they get low on refrigerant. But those use fixed orifices for expansion. The Sienna uses a thermal expansion valve, so I don't know if it would behave the same. The thermal expansion valve could be messed up, as someone mentioned, and if so, that would definitely require tearing apart the dash to replace, cause if I remember from the linked service manual it's at the top of the assembly and removed from the side.

If your further tests point to replace, I'd still get a new sensor and plug it in quick and make sure it behaves similarly to how the 2k resistor did before I replaced it. In fact you could do further tests with a new sensor like make sure dangling your AC works normally, but when you press the sensor against an ice cube, or in a glass of very salty water and ice (to get the water below freezing) that the AC cuts out. That sort of test should indicate very effectively that the issue is just the sensor, I think.

BillG said:

Exactly! The loaded Sienna has digital temp controls. Every car I've had with this feature as far back as my '86 Celica would adjust blower speed and the blend between the cooled air and recirculated or outside air. The modern ones even have split systems where each zone of the car has its own temp control, blower and blend door. I only have the simplified system in my '06.

As for the source of the issue, I don't think I can be any help. Based on the data you captured, it seems like it's operating correctly... Your temps are fluctuating between 40 and 55, evenly across all vents. I would just write it off to normal behavior. You're saying it's 110 outside and the van cools to 74 with it on full blast? That's not totally unreasonable.

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I was a bit confused at first why you asked about a loaded Sienna, cause my Sienna is an LE, 1 step above the base, and has digital temp controls and split front temp zoning. Driver and passenger have separate temp controls, but the blower is shared with both sides. So two blend doors, but one blower. This is how my 2014 Pilot and 2020 Silverado were too. I haven't yet seen a car that has separate blowers for the driver/passenger up front. Rear has its own temp control and blower of course. But I have a 2014 2nd gen, and I realized the 1st gen probably didn't get digital controls and zoning until the very top trim levels.

wag3719 said:

You are a LOT confused. You do not have a 2nd gen Sienna, your 2014 is a 3rd gen. The OP has a 2nd gen. The 2nd gen LE does not have individual temp controls for the right and left sides.

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I wouldn't say a lot confused. I know I have a 3rd gen. I just had a momentary mix-up and mis-type.

bencahill said:

OP says "does the passenger side combine warm and cold air when the car is set to anything other than HI or LOW?" in post #20, making me think he has the automatic system/split controls.

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That's not what I got from that. He isn't asking about it splitting temp between driver and passenger. He's asking what blend door does what. He said he thought the one accessible from the driver side changed the vent mode and the one accessible from the passenger side did the hot/cold air mixing.

thescot14 said:

We are driving around without it being loaded up, and this is happening. The belt door is also a thought I have had. I had to replace one already (the one that is accessible from the driver side). I believe there is another blend door that is accessible from the passenger side, but I have not replaced that one yet. Do you know what the passenger side blend door does? The driverside if memory serves changed the mode of the van's ac (blowing at feet, or face, or both) does the passenger side combine warm and cold air when the car is set to anything other than HI or LOW?

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To specifically answer your question, yes, you are correct. The one you'd access from the driver side is the vent mode, the one accessible from the passenger side is hot/cold air mix. Well actually there's two on that side, with the third one being the fresh/recirculated air.


Speaking of which, if you already replaced the vent mode door (air outlet control servo motor), how bad was getting that out? Looking at that picture, it appears that if you can't get the evaporator temp sensor out through the hole on the bottom where the wire goes it (because it's attached to the evaporator) that pulling the vent mode door/motor out may leave a big enough hole to reach inside to remove/install the evaporator temp sensor?

Also, so we can settle the question, what trim level of Sienna do you have?