View Full Version : Climate Control Wiring Diagram - Auto Climate Control Problem.

12-24-2010, 03:48 AM
Hello all.

I have been doing a lot of forum searching and have found a couple of great threads to help but need a couple questions answered.

First off, I have the automatic climate control in my Celica. Its a 91 GT-S.

First off, the A/C works and the heat works so I do not believe there is any problem with the heater core, heather valve or coolant flow.

The issue is however, that the amount of heat emitted cannot be controlled by the knob itself. It is either full heat or full cold. Indicated 66 degrees is full heat as is 67-85 degrees. Only 65 is full cold.

I have had to disconnect the cable running to the heat control valve in the engine compartment. Reason being, that if I adjust the temp from 65 (coldest) to 66, the cable reacts by extending fully giving me full heat.

I have disconnected the cable from the valve and adjusted the heater valve in the engine compartment by hand to about 1/2 heat so that now my options are full cold or 1/2 heat.

If I move the temperature knob in the car the cable does move, just too far.

My understanding is that on the auto climate models, the blue heater cable that runs through the dash and into the engine compartment is controlled electronically and does not physically connect into the back of the auto climate control HVAC unit.

On the manual climate control models, the blue cable actually runs into the rear of the HVAC controls. I have purchased a replacement auto climate control model (from a '91) and do not see a spot for the blue line to hook into. Therefore, I have drawn the above conclusion. Is this correct?

So, seeing that my car thinks that 66 degrees means full hot, I have drawn two conclusions:

The first is that there is a problem inside my auto climate control unit (bad solder, fried board). The thinking here is that a replacement HVAC control unit would fix this issue. Problem here is that the previous owner says he replaced the unit with no fix.

The second conclusion is that the problem may lay elsewhere in the auto climate system. Maybe the temp sensors under the dash.

This is the reason I need a diagram so I can attempt to troubleshoot.

If anyone else has any other ideas or has experienced this problem, I would appreciate any input. Full heat or full cold is kind of annoying.

I am sorry if this is long winded but I am trying to supply as much info as possible.


12-24-2010, 09:09 PM
On your climate control system, all valves and air dampers are controlled by servo motors (so no cables connected to the climate module itself, just wiring harnesses).

I think (but am not sure) that the temperature knob on your climate control is a rheostat. It sounds to me like your rheostat is worn out / broken, or a wire in one of your wiring harnesses to the control unit is broken. If the previous owner replaced the control unit with a salvage one, it is possible the salvage one had the exact same problem. If your replacement has not been tried yet, I would install it and see if it does anything different. If not, I would be thinking you have a broken or shorted wiring in the harness somewhere.

Just my 2 cents worth.

02-17-2011, 01:33 AM
***UPDATE*** Problem solved

Thought I would update this thread in case anyone comes along later with a similar issue.

The problem in my Celica came down to the cabin air temperature sensor. The vent for this sensor is just below the ignition as seen in the pics below.

It turns out that my temperature sensor was disconnected... Seems that when the previous owner had the stereo installed, the installer took off the lower dash trim, unplugged the sensor, and then reinstalled the dash trim without plugging the sensor back in.

I cleaned out the sensor with a Q-tip (gets quite dusty), plugged it back in and now I have a working temperature gauge.

You never realise how much you miss being able to adjust the temperature when all you have is full hot and full cold.

Pics of the location of the sensor, a not so good pic of the built up dust inside the sensor, and the sensor itself.




02-17-2011, 02:55 AM
Thanks for posting up the fix for this. I'm sure it'll be useful for someone in the future.

02-17-2011, 02:57 AM
Nice to see someone come back to provide the solution to the issue, makes for a valuable thread instead of another stub.

02-20-2011, 04:52 AM
MAF cleaner works really good to clean those out.

02-20-2011, 09:42 PM
Great info. I didn't realize there was a sensor there.