Thread: External temperature reading is way low - A/C blows hot air

Hello,

I noticed that occasionally when I start my Grand Prix 1998 the external temperature reading shows low or negative numbers, e.g. -29F, even if the temperature is over 100F outside. This seems to cause A/C to blow warm/hot air since it thinks that temperature outside is cool enough and doesn't need additional cooling.

When I start driving the car and reach and maintain the speed of 65 mph or higher the temperature reading starts climbing and in a couple of minutes A/C starts blowing cold. I can achieve the same effect by cracking the driver and passenger windows open.

Online I found another complaint about a similar problem and the recommendation was to change temperature sensor. So, I changed the temperature sensor but that didn't fix the problem.

Any thoughts on this?

Thanks,

Alec

Yes, I replaced the temp sensor located in front of the radiator.

scottydoggs, are you referring to the wires leading to the temp sensor plug? Why would driving with 65 mph speed fix this?

I can almost promise you the issue is the low reference wire. I just spent the time involved to completely gut and install the automatic hvac you have and discovered that the sunload sensor, both door actuators, external, and internal sensors all share the same low reference (sensor gound wire) that goes to the hvac unit. If one of the wires in this system are shorting to chassis ground, then the temperature will as well.

Also, strangely enough, sometimes it is a calibration issue, or the hvac control unit issue. To re-calibrate, disconnect battery for a minute, reconnect, then turn key to accessory and DO NOT touch the hvac at all! I cannot stress this enough. Doing so will disrupt the learn cycle of the unit and you will have to start over again. Now start the car after it starts to blow air and leave it for another minute. Now check the readings and functions of your system.

Usually if it is a grounding issue, the temperature will ground to 55 degrees and you will have more issues with the blend doors, so I would wager on the hvac control unit itself. Unfortunately they are prone to failure.

The other frustrating thing about this system is that if any of the aforementioned parts actually short out for some reason to common, they take the WHOLE system out with it causing symptoms as you describe, other than replacing all the wiring and components one by one, its hard to pinpoint the issue.

The only sure-fire way of troubleshooting is to run a line test on both sensor reference wire and outside temp sensor wire to see if either are shorting, and then I would replace the hvac head unit.