Need A/c Troubleshooting Help

MikeInCtown · April 25, 2009

2004 Sierra 1500 LTS package with the digital display and auto climate control.

When I press the snowflake/AC button the snowflake flashes twice and goes back off. The A/C will not come on. I put my guages on and I get about 110 psi on high and low sides. Never changes when I try and turn the air on. I tried jumping out the pressure switch on the accumulator to no avail. Fure for compressor (10amp) is good. I did see a connector on an in line switch on one of the A/C pipes near the compressor but was unsure what it was for. Upon unplugging it I though I saw three or four pins and didn't want to take a chance trying to jump out the wrong wires.

I have ordered a service manual set on DVD from a member here but was wondering if someone could help me until it comes in.

Mike

bill190 · April 26, 2009

Because I am not familiar with how auto climate control "should" work, I searched google for this and found something which said you would turn the A/C on by pressing "Auto" and the snowflake is used only to turn the A/C off!

I don't know if this is the way it should work, but does it say that in the owners manual?

MikeInCtown · April 26, 2009

Thanks for posting that. I went out this morning and pressed theh "auto" button. The A/C came on and worked great. I made my first stop and the A/C never would turn on after that.

I pulled out my owners manual and it says that the auto function will turn it on but you can also do it manually. According to the manual if the temperature sensor inside the cab is reading that it's too cold outside then the snowflake will flash three times letting you know that. My panel does just that and theh A/C will not turn on. What's odd though is that except for the A/C everything works correctly. The fan speeds up and slows down and the temp inside is regulated automatically. All winter it has worked flawlessly, so I'm not sure it's the sensor. (outside temp sensor always reads correct on my mirror display, but I'm not sure if that's the same)

bill190 · April 27, 2009

Generally these types of things (temperature sensors) are called "thermistors". They change electrical resistance depending on temperature.

You can do two things at this point. Search google.com for the following words and maybe get lucky...

2004 gm A/C outside temp sensor

or

2004 gm sierra A/C outside temp sensor

or

2004 gm A/C "outside temp sensor"

Something like the above is hard to search for. It brings up the wrong year, or wrong vehicle or wrong temperature sensor. Then maybe the right year and vehicle, but it just says replace the sensor! (Nothing about how it SHOULD work and testing it.)

Or you could get a factory service manual set. Sometimes the information to test something like this will be in the regular service manual. Sometimes in the wiring diagram book. Or both.

I would be looking for WHERE this specific temperature sensor is located. Then I would want to know what resistance (ohms) it should read at a particular temperature. Then I would find the sensor and first look to see if it was touching something nearby like a piece of metal (which could be cold) and giving it a wrong reading. Or if airflow was blocked to it and keeping it from getting a proper reading.

Then if that was not the problem, I would disconnect the sensor and use a multimeter set to "ohms" and along with a thermometer, see what resistance it was reading and if that was correct for the temperature. If the reading was off, I would buy a new sensor.

If the sensor was working as it should, but its location was in a poor spot or I did not like the way it was designed. (Say I wanted the A/C on when it was cooler outside. Like maybe it is 60 outside, but HOT inside the vehicle and the spot where the sensor is located is 50 degrees due to being around cold metal parts in the shade...) Then I would modify the sensor to turn the A/C on when *I* wanted it on!

I would do this by looking at the range of resistance in relation to temperature data which I would get from the factory service manual(s). Then install a resistor of a certain value to "modify" the temperature the A/C unit thinks it is (warmer than it actually is at the sensor). I would do this by placing a resistor in series or parallel with the thermistor as needed to modify its reading.

So if the resistance went higher as the temperature went higher, and it needed to read say 100 ohms to turn on the A/C, but it was reading 95 ohms when I wanted the A/C on, then I could add a 5 ohm resistor in "series" with the thermistor and this would alter its reading. This is just hypothetical. I have no idea what your sensor values would be or even if it is a thermistor. But usually things like this can be redesigned/modified.

Actually I've been fixing or modifying a LOT of products I buy lately because they don't work like they should!

Thermistors...

http://en.wikipedia.org/wiki/Thermistor

MikeInCtown · April 28, 2009

Well thanks for the tips. I think i may have to take the truck to a dealer who has the scanning equipment outlined in the service manuals. every repair or test procedure involves plugging in the scan tool to check for temperature change when you do this or that. Found there are craploads of temp sensors in the auto temp controlled trucks. Lucky me. In reading what the display does when I press the A/C button (flash three times then go off) and reading that the temp sensor for outside air is what could cause this I am inclined to try and replace the temp sensor. Found a schematic and it shows that it is mounted below the parking lights on the front passenger side mounted to some bumper cover just above the bumper and just below the light assembly. I'm hoping I don't need to remove too much to get to it. I'm also hoping that the temperature sensor that reads for the mirror display isn't the same one as that reads 100% accurate.