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Old 11-30-2007, 07:36 AM   #4
MrHorspwer is on a distinguished road
Join Date: Dec 2006
Posts: 125
Default Re: "Sticky" temperature display

The ambient air temperate is received and processed by the HVAC control assembly, not the BCM. Your software update for the transmission didn't do anything to the ambient air temp display.

From your description, it sounds like the system is operating perfectly normal. As noted in the post above, the system will only update the ambient air temp after the vehicle has been turned off for 3 hours. It will do an instant update if the temperature has decreased in that time, but like you said, the temp increased from the 30s to the 60s. You would have to leave the vehicle sit for more than 3 hours to get an instant update when you turned on the vehicle again.

The temp display won't even attempt a filtered update until you've traveled at least 20 MPH for greater than 80 seconds. If you stay less than 45 MPH, the temp updates at a slow filtered rate. At 45 MPH, it should do an instant update.

All vehicles use some type of filtering and update process. The values and times may change, but filtering is the only way to prevent wild swings in the ambient temperature display. The ambient air temp sensor needs to be in an area which receives outside air. This is typically in the front fascia somewhere. The problem is, when you stop, all the latent heat from the radiator and condensor surge forward into the front fascia cavity, warming the ambient air temp sensor. This is why there is a need for movement before an update, to get outside air flowing over the sensor instead of the hot air coming off the radiator. When the vehicle is shut off, it needs to see a predefinied cooldown period. In your case, it's 3 hours.

Here's a trick. It may or may not work, but give it a try. If you want to do an instant air temp update, press and hold the A/C and RECIRC on the HVAC controls for a few seconds. This should force the display to show actual air temp seen at the sensor, not the filtered value.


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