how do I tell if this sensor is bad?

vmb · 09-02-2017, 07:03 AM

my wife has a 2003 4Runner, and the A/C only blows cold air if I set the temperature to [Lo]. Any other setting/temp causes only hot air to blow out the vents. A search on the web suggests that the cabin air temp sensor is bad. Most stories are that this got disconnected when people had stereo work done under the dash. I have not. So, I found my sensor, which was covered in 14 years of dust, and tried to source a new one but before I do that how do I tell if this one is bad or not? I hooked up a multitestor and got no response on resistance/ohms (the omega sign, right?). Should it vary the reading by temp? I'm hoping this is dead and I don't have to replace the $1k head unit. Thanks!

Bob Kontak · Bob Kontak's Garage

Just for hip pocket information.

Recently worked on an 05 Lexus RX330. Roughly the same issue. Web searches referenced the same sensor. If you turned your head and coughed the AC would work on high for a bit then nothing but warm air.

Coolant reservoir indicated plenty of fluid. However, radiator had a small leak and big air was in the cooling system. Reservoir did not give a correct indication of coolant level. Confuses wet sensors, I am guessing. Radiator and thermo.

I'll look to see if you can test your sensor. Set the resistance range fairly high. It may be in the thousands of ohms if this is the correct test.

Bob Kontak · Bob Kontak's Garage

I found nothing on resistance. We probably looked at the same sites.

One thread did mention if the sensor is dirty, clean it. Looks like yours falls into that category. Suggest mass air flow sensor cleaner in aerosol as I don't know how delicate the sensor is.

quicksix · quicksix's Garage

I just picked up an 88 4x pickup and went over to Yotatech.com
Friendly place with tons of knowledge, tech section is pretty good too.
Somebody over there will have an answer.
Good luck

aschen · 09-02-2017, 12:04 PM

No responce meaning 0 ohms(short) or open circuit? Im not sure if it is a thermistor or a thermocouple but either way it should have a low resistace value. If it is open circuit there is a better than 99% chance it is toast i think

Unless it is some sort of micro thermostat swich which i am un aware of.

asphaltgambler · 09-02-2017, 03:23 PM

Agree with above^^^^^^

MBAtarga · 09-02-2017, 06:04 PM

Quote:

Originally Posted by aschen

No responce meaning 0 ohms(short) or open circuit? Im not sure if it is a thermistor or a thermocouple but either way it should have a low resistace value. If it is open circuit there is a better than 99% chance it is toast i think

Unless it is some sort of micro thermostat swich which i am un aware of.

No response = infinity = open
"0" ohms = short

vmb · 09-02-2017, 06:49 PM

Thanks everyone! Looks like I'll start by ordering a new sensor.

pen15 · pen15's Garage

It is a thermistor. Very simple. Resistance will vary with temp. Either increase or decrease with temp rise. Most will read somewhere between 0-1000 at room temp. If it reads open Or OL it's no good.

john70t · 09-02-2017, 10:45 PM

thermistor sensor
vs
Magnetic pickup generating voltage
vs
Hall Effects interrupting blade
vs
Some other type I'm too lazy to research at this moment.

vmb · 09-03-2017, 07:04 AM

I'm just curious, what makes these things go bad? Corrosion? overheating? cracks in the epoxy "bulb?" I found it interesting how they designed this. Has a hose hookup that looks like it actively draws cabin air past the bulb to get a more accurate reading than if it was just measuring stale air (surrounded by other electronics under the dash)