Ever since I bought the car, my AC hasnt worked. Trying to fix, and now I am stuck. 1997 Vette, With ECC

The compressor clutch will not turn on.

The system was leaking, so a few components were replaced. I vacuumed it down to 1100 microns, and then added a 12 oz can of r134.

That gave about 130psi in the system (according to my gauges) which should be enough to engauge the compressor. Through Tech2, the pressure sensor was showing 137psi so the pressure sensor is working.

If you ground out the relay, the clutch will turn on, so the relay isnt being grounded by the PCM.

When I push the AC button on the electronic climate control, the tech two recognizes the request, and AC's status is ON. However, about 2 sec later, that status switches to OFF by itself. If you hold the AC button in, the status will stay ON, but the clutch style will not engauge.

Using tech2, if you command the compressor clutch ON, it will will stay commanded on, but will not actually engauge.

I am not sure if the HVAC control head is bad, or the PCM itself is bad. Is there a way to test? I think the data is sent on the bus line, so I dont think I can check voltages/grounds???

Also, if the PCM is bad, like I somewhat believe, how does HP Tuners deal with that? I have a single tuning license, but if I switch the PCM I wont be able to tune/upload anymore? Will they support transferring the VIN?

Thank for any help!

 

If I were going to try and fix it I would check to see if you have power going to the AC pump while commanding it to turn on. I would think you could use a multimeter to check the voltage at the electrical plug on the compressor. I am almost sure it isn't the pcm going bad. To me 12oz doesn't sound like enough but maybe it is. Have you looked to see how many cans that the system requires if the system is empty?

 

Some thoughts ....

1) 12 ozs is way too little ... the system wants 24 ozs.

2) Did you add any PAG oil when you recharged it ???

3) Display the DTC data .... sounds like the PCM is refusing to engage the clutch, and most of the time that will set a DTC that gives you the "reason" the PCM won't engage the compressor.

 

Yes I know 12oz isnt enough, but it is impossible to put anymore in without a clutch that engauges or without a R/R machine. That should be enough to at least turn on the compressor so that I can charge to proper level

The clutch isnt getting any power at the connector when commanded, however the wiring is good. I know this because if you ground out the relay, the clutch turns on. Therefore, the control side of the relay is faulty somewhere. The PCM controls the ground to the relay, but the HVAC head unit sends to command to the PCM.

No DTC codes, PCM or HVAC

 

What's the low side pressure? If the low side is too low, I don't think the compressor will engage. Also, there is a chart in the service manual for low/high side pressure.

 

Since I believe you should start with the easy stuff first, I'd try to put in at least a partial can of R-134. I think the vette is like most cars in that it reads the pressure (to make sure it isn't too high or too low) before it engages the clutch.

While it is true that you can't get a whole can of refrigerent in unless the clutch engages; I would still see if it will suck any more down without the clutch. I believe that the pressure in the second can, plus the pressure in the system should trigger the system to go on. Usually the clutch starts to cycle on and off as the presssure meets the minimum level.

It sounds like you know what you are doing, so this is a totally silly question but; your car is running and your A/C is set to max (60 degrees) while you add the r134, right?

If the most easy solutions aren't helping, What parts did you replace? Maybe one of those parts are bad, or not installed properly.

 

Not sure about C5, but on some other GM cars with ECC, it was nessary to disconect batt for a few minutes. Worth a try!

 

The pressure in the system is 130psi on both high and low sides. Without a compressor cycling you wont see a pressure difference, nor will any more force in from a new can.

Battery reset did not help.

Mechanically the system is fine, once again I can force the compressor on by jumping the relay to ground.

 

Sorry, I don't know much at all about the C5 a/c system, but does the C5 have a low pressure switch like most cars or is it completely computer controlled? If it has a low pressure switch, have you checked it? I know you said you jumped a relay and it started working so have you also replaced that relay? Just some thoughts and if nothing else, ttt for ya

 

The relay is good. I just grounded out the control side of the coil to simulate an AC request command. Something is preventing that command from reaching the relay.

The C5's use a pressure sensor, not a switch.

In other words, it feeds exact pressure information to the PCM. Mine is sending 137psi to the PCM right now.

Let me phrase this question, anyone know the minimum static (ac not on) pressure that must be seen to actually allow engaugement of the clutch? That will eliminate the charge as being an issue, or confirm it being a problem.

 

Hi, the pressure dual role pressure switch will not let the comp activate below 30psi, and above 410psi. The correct amount of refrigerant for the 97 was 24 ounces, later it was raised to 28ounces to help alleviate freezing evaporator complaints.
jump the relay to run the system untill you have enough ref in it for it to continue on it's own.
Also mini fuse 24 and mini relay 34 need to be good
good Luck
++++++++++++++++++++++++++++++++++++++++ ++++++++
Relays and Switches
A/C Refrigerant Pressure Sensor
An electronic A/C refrigerant pressure sensor is mounted in the evaporator tube between the evaporator and the condenser. The sensor output to the PCM is variable and is dependent upon pressure inside the line. A high line pressure results in a higher voltage output. The PCM constantly monitors the A/C refrigerant pressure in order to perform the following functions:

Disengage the A/C clutch if the pressures are above 2826 kPa (410 psi)
Disengage the A/C clutch if the pressures are below 207 kPa (30 psi)
Boost idle air control to compensate for A/C load at idle
Cooling fan control
The sensor is not serviceable. If the sensor fails, the sensor must be replaced.

More info
++++++++++++++++++++++++++++++++++++++++ ++++++++
The A/C system can be engaged by pressing the A/C switch. The A/C switch will illuminate when the A/C switch is pressed to the on position. Pressing the A/C switch the control module grounds A/C request signal circuit from the powertrain control module (PCM). The following conditions must be obtained before A/C compressor engagement is allowed:

The engine coolant temperature (ECT) is less than 121°C (250°F)
The engine RPM is more than 550 RPM
The A/C pressure is between 207 kPa (30 psi) and 2826 kPa (410 psi)
The A/C request signal circuit is grounded.
Once engaged, the compressor clutch will be disengaged for the following conditions:

The throttle position is 100 percent
The A/C pressure is more than 2826 kPa (410 psi)
The A/C pressure is less than 207 kPa (30 psi)
The engine coolant temperature (ECT) is more than 121°C (250°F)
The engine speed is more than 5,500 RPM
The transmission shift
The PCM detects excessive torque load
The PCM detects insufficient idle quality
The PCM detects a hard launch condition
When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

Automatic Operation
In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, the mode actuator and the recirculation actuator.

To place the HVAC system in Automatic mode, the following is required:

The Auto switch must be activated
The air temperature switch must be in any other position other than full hot or full cold position
Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

Monitor the following sensors:
Inside air temperature sensor
Ambient air temperature sensor
Sunload sensor
Regulate blower motor speed
Position the air temperature actuators
Position the mode actuator
Position the recirculation actuator
Request A/C operation

 

Thanks bestvette. One more question, all the conditions for the compressor to turn on are met, and none of the exclusionary conditions are met.

Even being low on r134a, it is still well above the min threshold for turnon. Relay/fuse are good as you can ground the control side of the relay to enable the clutch.

When I command AC with either the HVAC head, or Tech II, the command status is sent to the PCM, but then quickly turns back off. I have looked through service manuals, and one of the steps is this check.

http://alldatapro.com/alldata/PRO~V9...9508/131235946

How would you answer #23? I suppose it reads it, but it doesnt accept the command.

 

What components were replaced?

 

compressor, vacuum and suction lines, accumulator and orifce tube, belt and tensioner

 

Hi, below is a diagnostic outline that should help you test which component is bad. The choices are
1, Bad Control Head
2, Bad PCM
3, Faulty wiring or ground, or connector
4, Bad Pressure Switch
Also I would test the reference voltage being put out by the outside temp sensor,(on the radiator support), reading the dash readout should tell you, but the output should be somewhere between 0-5v, 0 being very cold out, and 5v being very hot out. A 0 reading will send a signal to the AC that it does not need to run.

One other thing you can try, turn the driver side temp dial to 65, and turn the system on to defrost. The defrost is programmed with different parameters. See if it runs the Comp. If it does, and the ac mode doesn't, that is a strong indication of a bad Control Head
I know the diagnostic is lengthy, but sometimes there is no shortcut to diagnose a problem. If I had to make a guess or decide where to look first from your description, I would look at the Pressure switch, as they seem to fail more than the PCM. The control heads are prone to fail but they usually show it, by getting dim or some other anomaly.
Good Luck
++++++++++++++++++++++++++++++++++
HVAC Compressor Clutch Does Not Engage
Test Description
The numbers below refer to the step numbers on the diagnostic table.

2, The A/C compressor relay output is disabled if engine coolant temperature is above 121°C (250°F).

3, Placing the mode switch in any position other than OFF, along with placing the air temperature control switch in a cold setting, activates A/C operation. For the purpose of this and future steps where A/C operation is necessary, bi-level mode is used for consistent testing.

6, The HVAC control module is inoperative when the module does not respond to any operator control requests to enable the A/C compressor. The blower motor may still operate independent of the A/C controls.

Step
Action
Yes
No

Schematic Reference: HVAC Schematics

Connector End View Reference: HVAC Connector End Views

DEFINITION: The A/C compressor clutch will not engage when an A/C request has been made and a Powertrain DTC has not been set.

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

2
Install a scan tool.
Start the engine.
With a scan tool, observe the ECT Sensor parameter in the Powertrain data list.
Does the scan tool indicate that the ECT Sensor parameter is below 121°C (250°F)?
Go to Step 3
Go to Engine Overheating in Engine Cooling

3

Important
For A/C compressor operation, ambient air temperature must be above 3°C (38°F).


Start the engine.
Place the blower motor switch in the maximum speed position.
Place the A/C request switch in the ON position.
Place the air temperature switch in the coldest position.
Does the A/C compressor operate?
Go to Testing for Intermittent and Poor Connections in Wiring Systems
Go to Step 4

4
Park the vehicle inside or in the shade.
Open the windows in order to ventilate the interior of the vehicle.
If the A/C system was operating, allow the A/C system to equalize, about 2 minutes.
Turn OFF the ignition.
Install the J 43600 ACR 2000.
Record the ambient air temperature at the vehicle.
Record readings of the low and high side STATIC pressures.
Compare the pressure values with the specifications for the ambient air temperature. 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 5
Go to Leak Testing in Heating, Ventilation and Air Conditioning

5
Turn ON the ignition, with the engine OFF.
With a scan tool, observe the A/C High Side Pressure parameter in the powertrain data 2 list.
Compare the A/C High Side Pressure parameter on the scan tool to the high side pressure value on the ACR2000.
Are the pressure values within 103 kPa (15 psi) of each other?
Go to Step 6
Go to Step 16

6
Is the HVAC control module inoperative?
Go to Step 15
Go to Step 7

7
Start the engine.
With a scan tool, observe the A/C Request Signal parameter in the powertrain data list.
Place the air temperature switch in the coldest position.
Place the A/C request switch in the ON position.
Does the scan tool indicate that the A/C Request Signal parameter is YES?
Go to Step 8
Go to Step 27

8
With a scan tool, observe the A/C Relay Command status in the powertrain data list.

Does the scan tool indicate that the A/C Relay Command status is ON?
Go to Step 9
Go to Step 28

9
With a scan tool, command the A/C compressor clutch relay ON and OFF.

Does the A/C compressor clutch relay turn ON and OFF with each command?
Go to Step 13
Go to Step 10

10
Turn OFF the ignition.
Disconnect the A/C compressor clutch relay.
Turn ON the ignition, with the engine OFF.
Probe the coil side voltage supply circuit of the A/C compressor clutch relay with a test lamp that is connected to a good ground.
Does the test lamp illuminate?
Go to Step 11
Go to Step 23

11
Connect a test lamp between the control circuit of the A/C compressor clutch relay and the coil side voltage supply circuit of the A/C compressor clutch relay.
With a scan tool, command the A/C compressor clutch relay ON and OFF.
Does the test lamp turn ON and OFF with each command?
Go to Step 25
Go to Step 12

12
Does the test lamp remain illuminated with each command?
Go to Step 19
Go to Step 18

13
Turn OFF the ignition.
Disconnect the A/C compressor clutch relay.
Probe the switch side voltage supply circuit of the A/C compressor clutch relay with a test lamp that is connected to a good ground.
Does the test lamp illuminate?
Go to Step 14
Go to Step 22

14
Connect a 10 amp fused jumper wire between the switch side voltage circuit of the A/C compressor clutch relay and the A/C compressor clutch supply voltage circuit of the A/C compressor clutch.
Turn ON the ignition, with the engine OFF.
Does the A/C compressor clutch engage?
Go to Step 25
Go to Step 20

15
Test the ignition 3 voltage circuit of the HVAC control module for a short to ground, a high resistance, or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 17

16
Test the low reference circuit of the A/C refrigerant pressure sensor for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 24

17
Test the ground circuit of the HVAC control module for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 27

18
Test the control circuit of the A/C compressor clutch relay for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 28

19
Test the control circuit of the A/C compressor clutch relay for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 28

20
Test the A/C compressor clutch supply voltage circuit at the A/C compressor clutch for a short to ground, a high resistance, or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 21

21
Test the ground circuit of the A/C compressor clutch for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 26

22
Repair the switch side voltage supply circuit of the A/C compressor clutch relay. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?
Go to Step 34
--

23
Repair the coil side voltage supply circuit of the A/C compressor clutch relay. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?
Go to Step 34
--

24
Inspect for poor connections at the harness connector of the A/C refrigerant pressure 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 34
Go to Step 29

25
Inspect for poor connections at the A/C compressor clutch relay. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 30

26
Inspect for poor connections at the harness connector of the A/C compressor clutch. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 31

27
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 34
Go to Step 32

28
Inspect for poor connections at the harness connector of the powertrain control module (PCM). Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
Go to Step 34
Go to Step 33

29
Replace the A/C refrigerant pressure sensor. Refer to Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement in Heating, Ventilation and Air Conditioning.

Did you complete the replacement?
Go to Step 34
--

30
Replace the A/C compressor clutch relay. Refer to Compressor Relay Replacement in Heating, Ventilation and Air Conditioning.

Did you complete the replacement?
Go to Step 34
--

31
Replace the A/C compressor clutch coil. Refer to Compressor Clutch Coil Removal and Compressor Clutch Coil Install in Heating, Ventilation and Air Conditioning.

Did you complete the replacement?
Go to Step 34
--

32

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 34
--

33

Important
Perform the programming procedure for the PCM.


Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement in Engine Controls.

Did you complete the replacement?
Go to Step 34
--

34
Operate the system in order to verify the repair.

Did you correct the condition?
System OK
Go to Step 4

 

After checking fuses, AUTO set at 60, I jumped out two wire sensor and no clutch still, I jumpered the clutch to make the AC pump work. Put 12 oz in, some cold but still not able to cycle w/o jumps....put in second can of 12 oz 134a and it got very cold and I could reconnect everything, remove jumps, and it was fine. I had to put in 134a about two years ago-a very slow leak I guess. I'll live with it.

PS-first sign of problem was the light blinking at HVAC setting.