I have been having problems with my HVAC system, and was wondering if anyone knows where the system gets its temperature information for the passenger compartment.

The car is a '99 C5 with the Automatic Dual climate control.

I don't know the exact temperatures, but it feels like it wants to keep the car at around 80-85 degrees when I have the thermostat set in the 60's. I use to keep the thermostat set at around 73-74, and was always comfortable, but now find myself with it turned down to the mid to low 60's and still too hot. The outside temperatures have been ranging between 25-50 degrees, but that doesn't seem to make a difference in the systems ability to control the temperature. I tried turning the system to manual, and turning the temperature as cold as it would go, to verify that the air conditioning can still provide cold air. It does with no problems, and the compressor comes on as expected.

With the passenger control set to the 12 O'clock position, the air blowing out of the passenger ducts is about the same temperture as the air coming out of the driver side. This leads me to believe that it isn't a passenger / driver air regulation problem.

I thought it might be a sensor, or a calibration problem with the control unit. It is acting like it thinks the car is colder than it really is. If anyone else has had a similar problem any information would be greatly appreciated.

 

The outside temp sensor is located under the passenger side by the front skid rail, the inside or return air sensor is inside just below the key behind the little grill looking plate that pops out. If the outside temp is reading correctly, that sensor is probably ok. If the return air sensor is bad, it could cause what you describe. It would keep trying to heat up to your set temp because it "thought" it wasn't there yet. It could also be a logic issue, but sorry, I don't know which module controls that. Post what you find.

 

You might want to pull the panel and check if the connector to the internal sensor in front of your right knee has become disconnected. The other possibility that the same thermcouple there has failed. Very easy to replace if needed. I haven't done this but with the panel out anbd the car running, HVAC on, try disconnecting that sensor and see if anything changes. If not, I am guessing you got a failed one.

 

I don't suppose it would be as easy as being low on freon???

 

I suggest you take it to your nearest or most trustworthy A/C service station and make sure there is no leak in a/c compressor.

The a/c compressor on that year has dual chambers, one being passenger and driver being the other if there is a leak on either side it will cause the freon to slowly leak out.

A quick die analysis on a car lift will narrow your problems down. Make sure they take a good look to see if it leaking.
Good Luck!

 

Low on Freon ???? I don't think so ... read the original post .... oh, and Freon hasn't been used in cars for years now ...

I second the suggestion of testing the interior thermocouple near the ignition key by disconnecting it. Last time I got one it was pretty cheap ... less than $10.00 IIRC.

HTH

 

DUEL CHAMBERS???????????????????????????:conf used: It has ONE evaporator! The evaporator has a plastic divider on top and under it that splits the air up into TWO zones (passenger & driver). Each Zone is temperature controllable via damper doors that mix heat with cool air.

You can purchase a set of R-134 freon gages and check to see if the system is operating at the correct pressures. Its dependent on outside temperature (load) & engine RPM. If you provide the two variables, Ill tell you what the pressures should be. There in the service manual if you have one.

BC

 

I did a little bit of follow up work, on the problem. Still don't have it solved, but found out some interesting stuff.

I tried unplugging the internal temperature sensor. I didn't notice much difference in the way the system worked. I'm not sure what exactly this tells me though, because at first I would have thought that it meant the sensor wasn't working. However, I did a little more investigating and am not sure this is the problem. That along with the fact that the temperature sensor looks to be a pretty simple thermocouple, and honestly probably not very likely to go bad.

I got curious on the temperature of the air actually coming out of the ducts, so I bought an infrared thermometer, and decided to take a few readings. Here is what I found.

Control was set to manual, and the air was directed only to the vents in the front of the dash.

Blower was set to high. (since I didn't have any way to measure the actual air temp, I measered the temperature on the plastic just under the tab for adjusting the vent flow direction). I had to give a little bit of time for the air to warm the plastic up to roughly the same temp.

Outside air temperature was 37 Degrees. (all temps in Farenheit)

Passenger temp control was set to the 12 o'clock position

Here is a table of the temps I recorded. The far left column is the commanded temperature setting

CS LD C LP RP
60 35 30 30 32
61 70 71 75 72
65 96 95 101 102
70 114 124 126 124
75 130 144 149 144
80 157 172 174 165
90 159 173 175 168


CS = control setting
LD = left driver duct
C = Center duct
LP = Left Passenger duct
RP = Right Passenger duct

What it looks like to me is that at full cold, the system can put out really cold air (although the outside air temp was pretty low)

Once the temperature is turned out of full cold (60), the system instantly has a 10 degree difference between the commanded temperature and the air coming out of the duct. The problem just keeps getting worse until you get to around 80 degrees. Between 80 degrees and the full hot temperature of 90, there is negligible difference between the duct temperatures.

I think the temperature differences between the ducts might be due to airflow differences. I think if I had given the system more time to stabilize the plastic vent temperature, it might have showed them all similar temperature. But, because the left driver vent seemed to have less airflow, it would make sense it would take longer for it to warm the plastic up.

So the automatic temperature control system might be commanding the correct temperature setting to attain the desired interior temperature, but the system might not be able to actually produce the commanded temperature. Without a thorough understanding of the system though, this is just speculation.

I had read some on the forum about HVAC problems, and found some other guys had problems with the vacuum hoses underneath the battery being eaten away from battery acid. I too had a battery leak on my car, and although I tried to clean the area out and neutralize the acid as best I could, now I am suspicious of the vacuum hoses. These guys complained of the driver being hot and the passenger being comfortable though. My system is equally controlling the two sides, so I'm not sure its the same problem.

I don't really know how the system controls the temperature of the air, so any insight into this would be greatly appreciated.

Does the control unit send some sort of signal to a switching mechanism that determines how far to open up a mixing door?

Where does the logic for the temperature control come from? Is it in the unit in the dash that the driver sets, or is it done in the BCM?

Thanks to everybody for the help and suggestions.

 

Take a look at this diagnostic. I wouldn't worry about the compressor now because your ambient air temp is so low the system should be able to control the temp without it. Anyways, the compressor will not come on if the ambient air temp is below 38. So skip the tests with the compressor and see if the other parts are working correctly. You should be able to buy an A/C system thermometer at an autoparts store. They look like a thin pencil with a big dial on the top. The sensor part sticks into the duct so you can make accurate temp measurements. You should also read the On Board Diagnostic system results to see if you have any HVAC codes showing.

Bill

Too Hot in Vehicle
Test Description
The numbers below refer to the step numbers on the diagnostic table.

This step clears the HVAC control module memory and checks for current air temperature actuator diagnostic trouble codes.

Ambient air temperature must be above 3°C (38°F) in order for this A/C Compressor test to be run.

The specified values are from the A/C System Performance Test

Step
Action
Values
Yes
No

Schematic Reference: HVAC Schematics

Connector End View Reference: HVAC Connector End Views

DEFINITION: The temperature cannot be adjusted, or the cooling is insufficient during A/C operation.

1
Did you perform the HVAC Diagnostic System Check?
--
Go to Step 2
Go to Diagnostic System Check - HVAC Systems - Automatic

2
Recalibrate actuators. Refer to Re-Calibrating Actuators .
Install a scan tool.
Turn ON the ignition, with the engine OFF.
Observe the Diagnostic Trouble Code (DTC) List in Heating and Air Conditioning.
Does the scan tool display any DTC B0361, B0363, B0365, B0367, B0441 or B0446?
--
Go to Diagnostic Trouble Code (DTC) List
Go to Step 3

3

Important
Ambient air temperature must be above 3°C (38°F).


Cover the sunload sensor.
Start the engine.
Fully open all panel outlets.
Adjust the temperature control to maximum cooling.
Select PANEL (upper) mode.
Adjust the blower speed to maximum.
Ensure that the VENT (outside air) mode indicator is not illuminated.
Does the A/C compressor operate?
--
Go to Step 4
Go to HVAC Compressor Clutch Does Not Engage

4
Place the blower motor switch in each speed position.

Does the blower motor operate correctly for each speed position?
--
Go to Step 5
Go to Blower Motor Inoperative

5
Does the blower motor provide sufficient air flow?
--
Go to Step 6
Go to Blower Motor Malfunction

6
Place the blower motor switch in the maximum speed position.
Place the mode switch in the bi-level position.
Place the outside air switch in the ON position.
Place the recirculation switch in the ON position.
Does the recirculation door operate properly?
--
Go to Step 7
Go to Air Recirculation Malfunction

7
Perform the A/C system performance test. Refer to Air Conditioning (A/C) System Performance Test in Heating, Ventilation and Air Conditioning.

Does the system pass the A/C System Performance Test?
--
Go to Step 8
Go to Air Conditioning (A/C) System Performance Test in Heating, Ventilation and Air Conditioning

8
Install a thermometer near the inside air temperature sensor.
With a scan tool, observe the Inside Temp Sensor data parameters in the Heating and Air Conditioning data list.
Does the scan tool indicate that the sensor temperatures are within 3°C (5°F) of the thermometer temperatures?
--
Go to Step 10
Go to Step 9

9
Inspect for a blocked or malfunctioning inside air temperature sensor. Refer to Inside Air Temperature Sensor Replacement .

Did you find and correct the condition?
--
Go to Step 20
Go to Step 13

10
Turn the engine OFF.
Install a scan tool.
Cover the sunload sensor.
Start the engine.
Adjust driver side temperature to 22°C (72°F).
With a scan tool, observe the Sun Load Sensor data parameter in the Heating and Air Conditioning data list.
Does the scan tool indicate that the sunload sensor parameter is greater than the specified value?
4.3 V
Go to Step 11
Go to Step 15

11
Uncover the sunload sensor.
Direct a light source at the sunload sensor.
Does the voltage change?
--
Go to Step 12
Go to Step 15

12
Does the customer need to change the set temperature frequently to maintain comfort?
--
Go to Symptoms - HVAC Systems - Automatic
System OK

13
Inspect for poor connections at the harness connector of the inside air temperature sensor. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 20
Go to Step 14

14
Turn OFF the ignition.
Test the inside air temperature sensor resistance. Refer to Sensor Resistance Table .
Does the resistance measure near the specified value?
--
Go to Step 16
Go to Step 17

15
Inspect for poor connections at the harness connector of the sunload sensor. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 20
Go to Step 18

16
Inspect for poor connections at the harness connector of the HVAC control module. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 20
Go to Step 19

17
Replace the inside air temperature sensor. Refer to Inside Air Temperature Sensor Replacement .

Did you complete the replacement?
--
Go to Step 20
--

18
Replace the sunload sensor. Refer to Sun Load Sensor Replacement .

Did you complete the replacement?
--
Go to Step 20
--

19

Important
Perform the recalibration procedure for the HVAC control module.


Replace the HVAC control module. Refer to HVAC Control Module Replacement .

Did you complete the replacement?
--
Go to Step 20
--

20
Operate the system in order to verify the repair.

Did you correct the condition?
--
System OK
Go to Step 3

 

I think you need a trip to the dealer so that they can read the BCM while its running, with a Tech 2 and determine where the commands for heat are coming from in the given temperature situation. Outside amibient temps of 37 will most likely keep the AC Compressor turned off. At the low outside temp, I'm sure the algorithm that compares the outside temp with the projected heat loss and then the dialed settings from the driver and the passenger override, will compute some serious heat to keep up with the heat loss. It does sound like its too much though which I why I suggested looking at the thermocouple. You are right that they don't fail very often, but they can fail. A TC works on a generated voltage, microvolts, from a difference in disimilar metals. If that connection between the two metals breaks down, then the resulting microvolt value of temperature can change a lot and be unreliable. As someone here said, they are cheap to replace if needed.
One extra item that isn't given its credit is on top of the dash. That small black dome shaped item is a "Sun Sensor", a true piece of spacecraft technology. Its sole purpose is to determine which side of the car the sun is shining on, then the bcm will adust the temps on both sides of the car for the most comfort. Make sure you don't have it covered up with something. I've never heard of one failing so I doubt that it did, but just know that it is important to the HVAC. I'm thinking you've got vacuum valve issues in there or the bcm needs looking at.

 

Stupid question, but have you checked for codes?

 

I think you're confused. He has dual climate control, which has 2 doors that are electronically actuated to control the airflow to the driver and passenger. There is no 'special' dual chamber compressor.

 

Check the vacuum line from the back of the intake manifold to under the battery box. My 1998 had a leaky battery years ago and just recently caused a hole in the plastic vacuum tube/hose under the battery box. You will need to remove the battery and then the box below. Next carefully cut off the black tape around the wire bundle. Trace the vacuum tube and splice it with a hose. It may have more than one hole. Be careful not to kink it. The connection on the back of the intake manifold is almost impossible to reach. It is the small one.

 

I still think it sounds like the return air sensor on the dash. If it doesn't sense the temp, it will continue to try to heat until it reaches that setting. Please post what you find either way.

 

I had the same problem. The lines were broken and the corrosion ate the lines up. The lines are vacums that control the head until. The head unit in turn goes belly up becuase the vacum lines are broken. The dealer will have to replace the head until and the lines and treat the corrosion. This means taking out the complete dash which means big buck. Good luck. Sounds like there is a lot of this going on with those lines, my vette friends all at one time or another got a trip to the dealer to help them lighten their wallets as well. I have the factory manual and normally work on my car but this is one time where the money is well spent to take it to the dealer.

 

I did check the codes on the DIC, and it didn't show anything.

I'll probably change out the internal temp sensor, just because its a cheap, and easy check. Although, I'm not convinced this is it, when I checked the vent temperatures I had the system in the manual mode, and it still couldn't seem to produce the set temperature.

I probably won't get a chance to check the hoses until sometime late next week, but if everything looks fine there, I'll take it into the dealership.

I'll post the solution as soon as I get it figured out. With temps in the 40s-50s, it still isn't a real big problem for me.

So if the vacuum lines got ate up, then the control unit will also need to be replaced????

 

Speedkills

I have reviewed this post and have a couple of things to clear up!

The vacuum system is ONLY used to control the vent doors. If you can switch from upper to lower to defrost and select air from the outside or recirc etc,,,you have ample vacuum and the vacuum lines are intact and working. If the vacuum system fails, the default vent position is defrost.

If your vacuum lines are dissolved by acid, I seriously doubt that it will effect the HVAC control module. The acid would go towards the intake not the control module. I have lost vacuum cause the hose popped off and the module was fine.

Vacuum reservoir and lines:



The BCM has little or nothing to do with the HVAC system, the PCM however interfaces with it but has little to do with this issue.

As cold as it is out,,,don't worry about the compressor running.

Lets get to the bottom of your problem:

Check fuses:

- Instrument panel fuse center, fuse 27 (HVACCON), 10 amp is power to the control head.


- Instrument panel fuse center, fuse 18 (HVAC) 10 amp. Fuse 18 is the fuse that supplies the Temperature Valve Electric Actuators and the solenoids that control the vacuum actuators that control the vent vacuum motors.

If your having temp problems, the TWO Temperature Valve Electric Actuators are not working properly.

Passengers side Actuator:;



Drivers side Actuator:




Actuator close up:



If you turn on the ignition and adjust the temperature from cold to hot while you watch the center operating shaft on the actuator, you should be able to see it rotate.

The sun load sensor & the interior air temp sensor & the external temp sensor all go into the computation for the interior temperature.

Do you read the correct temp for the outside and the inside as displayed on the HVAC Control center?? That info will tell you if the sensors are working!

The main ground for the HVAC electrical system is Chassis Ground G-202. The grounds go through Splice Pack SP-202. G-202 in under the passengers side door sill cover:



Check all that stuff out and let me know what you find! I have a ton more photos, information and suggestions.

Your web site is phenomenal!!! I love all that you have done!

Bill Curlee

 

I just thought of something. Change the HVAC system air intake so that it takes a suction from the EXTERNAL AIR and see if that makes a difference in your temp control problem. I doubt if it will but, I need to figure out something.

BC

 

Okay, so I got a chance to look at some things.

The vacuum lines didn't seem to have any problems.

I checked the fuses, and all of those were fine too.

I also am reading correct temp for outside the car.

I looked under the passenger side sill cover, and the ground there looked fine. (no surprise, because I just had a problem with that about a year ago, and had it fixed then).

So before I got any further, I took the car out of town this past weekend, and sometime in there, the HVAC started working fine. I switched the controls over to manual, to see if I would get the same results as when I did the test before. The system worked great, providing correct temperatures for the desired settings I put in.

So as of right now, I tend to think it is an electrical problem, but don't know what to do about it. If it works fine, its hard to diagnose, so I'm just waiting on it right now to do something permanent.


A couple of other things that may or may not be related, but are electrical nuances that have been happening.

The driver side headlight sometimes doesn't come up when I release the parking brake and its dark outside. The passenger side always seems to work, but not the driver side. This problem is easily solved by going into the options, and selecting twilight sentinel off then on again. As of yet, this has always worked.

The other thing that sometimes happens, is that the backlighting in the buttons on the side of the driver display (the ones that are for trip, gauges, fuel, options,....etc), doesn't always light up. It seems to only be a couple of them at a time though, I've never had them all go out at once. This problem comes and goes, seemingly unpredictable to me.


Bill,

Thanks for the great pictures, and suggestions.

 

Hi Bill,

You seem to know what you are talking about. I was wondering if you had any input on a strange A/C problem I have with my 97 Vette with dual climate control. I recently purchased this vehicle, so do not know much history on it.

Outside temp is around 80 degrees F.

I press the A/C button, the light comes on and the A/C blows cold.

Within minutes, the A/C light goes off and the air is no longer blowing cold.

Press the A/C button again, and the light will blink a few times and then turn off.

If I let the car sit for a few hours, I can get the A/C to blow cold and the light to stay on for a few minutes again, but then fails the same way. I have had the system checked and the freon level is where it should be and not leaking. Something is failing and turning off the A/C but no one local can figure this out.
Any ideas??
Oh, I also beleive the heat has a similar problem where it will blow hot for a few minutes, then seems to stop blowing hot even if you have the temp set as high as possible.

Thanks in advance