Tahoe A/C System Good but Compressor Won't Engauge
March 10th, 2013, 6:16 PM
#11
CF Junior Member
Thread Starter
March 10th, 2013, 7:26 PM
#14
Super Moderator
Professional Mechanic
Professional Mechanic
Ok let me rephrase that take to an Automotive Technician. Just because someone puts parts on for a living it does not mean they can diagnose electrical problem. If he was any good he could have found the real problem in less than a hour.
March 10th, 2013, 7:41 PM
#15
Super Moderator
Professional Mechanic
Professional Mechanic
Here is how the the clutch circuit works...... Hope it helps
The compressor for the air conditioning is clutch regulated and driven by the engine by means of a belt drive. The clutch allows the compressor to be disengaged when compressor operation is not required.
The battery supplies voltage to the following components.
The HVAC Control Module (when the ignition switch is in the RUN position.)
The A/C Compressor Clutch Relay Coil (when the ignition switch is in the RUN position.)
The normally open A/C Compressor Clutch Relay contact.
At the HVAC control module, B+ voltage is supplied by CKT 141 (BRN) to C3 pin 5 and to C2 pin G at all times from the 25 amp Fuse 12, HTR-AC, at the IP Fuse Block. Battery voltage is also supplied at all times to the A/C Compressor Clutch Relay from the 10 Amp A/C Fuse located in the Underhood Fuse Block through CKT 1240 (ORN). Ignition voltage is supplied to the A/C Compressor Clutch Relay from the 10 Amp fuse, IGN E from the Underhood Fuse Block through CKT 639 (PNK). The HVAC Control Module grounds through CKT 150 (BLK) to G105 and the A/C Compressor Clutch Relay grounds through CKT 150, G200.
To drive the A/C Compressor Clutch, the Powertrain Control Module (PCM) on diesels or the Vehicle Control Module (VCM) for gasoline engines, provides a ground on CKT 459 (DK GRN/WHT) which in turn activates the coil in the A/C Compressor Clutch Relay which is fed voltage through CKT 639 (PNK). This activates the A/C Compressor Clutch Relay which is fed voltage to CKT 59 (DK GRN) to the A/C Compressor Clutch, which grounds through CKT 150 (BLK), G105. Diode D104 in the harness is reverse-biased and prevents current flow bypassing the clutch coil to ground as well as serves as a means of de-energizing the magnetic field when the relay coil is de-energized.
For Gasoline Engines, the VCM will provide a ground path when a B+ voltage signal is received on CKT 762 (DK GRN/WHT). This voltage signal is provided by the HVAC Control Module when the A/C Push Button Switch is depressed or if defrost is selected through the Mode Dial Defrost Detent, and feeds B+ voltage through CKT 66 (LT GRN) to the A/C Compressor High Pressure Cutout Switch. Provided that the switch is closed, the signal is then supplied to CKT 762 (DK GRN/WHT). The A/C Compressor Clutch can also be turned ON/OFF by the VCM if a voltage drop is detected on CKT 604 (DK BLU) which feeds the A/C Compressor Refrigerant Pressure Switch and grounds through CKT 451 (BLK/WHT) to G104 or if a voltage drop is detected on CKT 603 (DK GRN) which feeds the A/C Compressor Cycling Switch and grounds on CKT 451 (BLK/WHT). These switches are normally closed.
For Diesel Engines, the signal is fed inn the same way as Gasoline Engines with exception of an A/C Compressor High Pressure Cutoff Switch and an A/C Compressor Cycling Switch is included in the circuit. CKT 66 (LT GRN) is fed B+ voltage from the HVAC Control Module, the A/C Compressor Cycling switch must be closed to continue the voltage path through CKT 603 (DK GRN) to the A/C Compressor High Pressure Cutoff Switch, which feeds the PCM through CKT 762 (DK GRN/WHT).
When the Front Blower Switch is in the OFF position, the HVAC Control Module feeds B+ voltage to CKT 71 (DK GRN) from C2, pin G to C3, pin 3. This voltage signal provides the HVAC Control Module with an input to the Compressor Logic Controller internal to the HVAC Control Module to prevent B+ voltage being fed to CKT 66 (LT GRN), preventing A/C Operation when the system is turned off.
When either the Re-circulate switch or MAX A/C is selected, CKT 1641 (DK GRN) provides a ground for the Recirculation Door Motor through either the VCM for a gasoline engine or through the A/C Compressor Low Pressure Cutout Switch to CKT 150 (BLK) to G105 for a diesel engine. The Recirculation Door Motor's normal position is for outside air. When the Recirculate Switch is depressed, this provides a ground on CKT 1641 (DK GRN) at the HVAC Control Module, that has an internal 1 K resistor, to activate the Recirculation Door Motor. When the VCM on a gasoline engine or the A/C Compressor Refrigerant Pressure Switch on a diesel engine closes or provides a no-resistance ground on CKT 1641 (DK GRN), the Recirculation Door Motor moves to recirculation position.
The compressor for the air conditioning is clutch regulated and driven by the engine by means of a belt drive. The clutch allows the compressor to be disengaged when compressor operation is not required.
The battery supplies voltage to the following components.
The HVAC Control Module (when the ignition switch is in the RUN position.)
The A/C Compressor Clutch Relay Coil (when the ignition switch is in the RUN position.)
The normally open A/C Compressor Clutch Relay contact.
At the HVAC control module, B+ voltage is supplied by CKT 141 (BRN) to C3 pin 5 and to C2 pin G at all times from the 25 amp Fuse 12, HTR-AC, at the IP Fuse Block. Battery voltage is also supplied at all times to the A/C Compressor Clutch Relay from the 10 Amp A/C Fuse located in the Underhood Fuse Block through CKT 1240 (ORN). Ignition voltage is supplied to the A/C Compressor Clutch Relay from the 10 Amp fuse, IGN E from the Underhood Fuse Block through CKT 639 (PNK). The HVAC Control Module grounds through CKT 150 (BLK) to G105 and the A/C Compressor Clutch Relay grounds through CKT 150, G200.
To drive the A/C Compressor Clutch, the Powertrain Control Module (PCM) on diesels or the Vehicle Control Module (VCM) for gasoline engines, provides a ground on CKT 459 (DK GRN/WHT) which in turn activates the coil in the A/C Compressor Clutch Relay which is fed voltage through CKT 639 (PNK). This activates the A/C Compressor Clutch Relay which is fed voltage to CKT 59 (DK GRN) to the A/C Compressor Clutch, which grounds through CKT 150 (BLK), G105. Diode D104 in the harness is reverse-biased and prevents current flow bypassing the clutch coil to ground as well as serves as a means of de-energizing the magnetic field when the relay coil is de-energized.
For Gasoline Engines, the VCM will provide a ground path when a B+ voltage signal is received on CKT 762 (DK GRN/WHT). This voltage signal is provided by the HVAC Control Module when the A/C Push Button Switch is depressed or if defrost is selected through the Mode Dial Defrost Detent, and feeds B+ voltage through CKT 66 (LT GRN) to the A/C Compressor High Pressure Cutout Switch. Provided that the switch is closed, the signal is then supplied to CKT 762 (DK GRN/WHT). The A/C Compressor Clutch can also be turned ON/OFF by the VCM if a voltage drop is detected on CKT 604 (DK BLU) which feeds the A/C Compressor Refrigerant Pressure Switch and grounds through CKT 451 (BLK/WHT) to G104 or if a voltage drop is detected on CKT 603 (DK GRN) which feeds the A/C Compressor Cycling Switch and grounds on CKT 451 (BLK/WHT). These switches are normally closed.
For Diesel Engines, the signal is fed inn the same way as Gasoline Engines with exception of an A/C Compressor High Pressure Cutoff Switch and an A/C Compressor Cycling Switch is included in the circuit. CKT 66 (LT GRN) is fed B+ voltage from the HVAC Control Module, the A/C Compressor Cycling switch must be closed to continue the voltage path through CKT 603 (DK GRN) to the A/C Compressor High Pressure Cutoff Switch, which feeds the PCM through CKT 762 (DK GRN/WHT).
When the Front Blower Switch is in the OFF position, the HVAC Control Module feeds B+ voltage to CKT 71 (DK GRN) from C2, pin G to C3, pin 3. This voltage signal provides the HVAC Control Module with an input to the Compressor Logic Controller internal to the HVAC Control Module to prevent B+ voltage being fed to CKT 66 (LT GRN), preventing A/C Operation when the system is turned off.
When either the Re-circulate switch or MAX A/C is selected, CKT 1641 (DK GRN) provides a ground for the Recirculation Door Motor through either the VCM for a gasoline engine or through the A/C Compressor Low Pressure Cutout Switch to CKT 150 (BLK) to G105 for a diesel engine. The Recirculation Door Motor's normal position is for outside air. When the Recirculate Switch is depressed, this provides a ground on CKT 1641 (DK GRN) at the HVAC Control Module, that has an internal 1 K resistor, to activate the Recirculation Door Motor. When the VCM on a gasoline engine or the A/C Compressor Refrigerant Pressure Switch on a diesel engine closes or provides a no-resistance ground on CKT 1641 (DK GRN), the Recirculation Door Motor moves to recirculation position.
Last edited by MDTAHOE; March 10th, 2013 at 9:35 PM.
March 10th, 2013, 9:02 PM
#16
CF Junior Member
Thread Starter
Here is how the the clutch circuit works...... Hope it helps
The compressor for the air conditioning is clutch regulated and driven by the engine by means of a belt drive. The clutch allows the compressor to be disengaged when compressor operation is not required.
The battery supplies voltage to the following components.
The HVAC Control Module (when the ignition switch is in the RUN position.)
The A/C Compressor Clutch Relay Coil (when the ignition switch is in the RUN position.)
The normally open A/C Compressor Clutch Relay contact.
At the HVAC control module, B+ voltage is supplied by CKT 141 (BRN) to C3 pin 5 and to C2 pin G at all times from the 25 amp Fuse 12, HTR-AC, at the IP Fuse Block. Battery voltage is also supplied at all times to the A/C Compressor Clutch Relay from the 10 Amp A/C Fuse located in the Underhood Fuse Block through CKT 1240 (ORN). Ignition voltage is supplied to the A/C Compressor Clutch Relay from the 10 Amp fuse, IGN E from the Underhood Fuse Block through CKT 639 (PNK). The HVAC Control Module grounds through CKT 150 (BLK) to G105 and the A/C Compressor Clutch Relay grounds through CKT 150, G200.
To drive the A/C Compressor Clutch, the Powertrain Control Module (PCM) on diesels or the Vehicle Control Module (VCM) for gasoline engines, provides a ground on CKT 459 (DK GRN/WHT) which in turn activates the coil in the A/C Compressor Clutch Relay which is fed voltage through CKT 639 (PNK). This activates the A/C Compressor Clutch Relay which is fed voltage to CKT 59 (DK GRN) to the A/C Compressor Clutch, which grounds through CKT 150 (BLK), G105. Diode D104 in the harness is reverse-biased and prevents current flow bypassing the clutch coil to ground as well as serves as a means of de-energizing the magnetic field when the relay coil is de-energized.
For Gasoline Engines, the VCM will provide a ground path when a B+ voltage signal is received on CKT 762 (DK GRN/WHT). This voltage signal is provided by the HVAC Control Module when the A/C Push Button Switch is depressed or if defrost is selected through the Mode Dial Defrost Detent, and feeds B+ voltage through CKT 66 (LT GRN) to the A/C Compressor High Pressure Cutout Switch. Provided that the switch is closed, the signal is then supplied to CKT 762 (DK GRN/WHT). The A/C Compressor Clutch can also be turned ON/OFF by the VCM if a voltage drop is detected on CKT 604 (DK BLU) which feeds the A/C Compressor Refrigerant Pressure Switch and grounds through CKT 451 (BLK/WHT) to G104 or if a voltage drop is detected on CKT 603 (DK GRN) which feeds the A/C Compressor Cycling Switch and grounds on CKT 451 (BLK/WHT). These switches are normally closed.
For Diesel Engines, the signal is fed inn the same way as Gasoline Engines with exception of an A/C Compressor High Pressure Cutoff Switch and an A/C Compressor Cycling Switch is included in the circuit. CKT 66 (LT GRN) is fed B+ voltage from the HVAC Control Module, the A/C Compressor Cycling switch must be closed to continue the voltage path through CKT 603 (DK GRN) to the A/C Compressor High Pressure Cutoff Switch, which feeds the PCM through CKT 762 (DK GRN/WHT).
When the Front Blower Switch is in the OFF position, the HVAC Control Module feeds B+ voltage to CKT 71 (DK GRN) from C2, pin G to C3, pin 3. This voltage signal provides the HVAC Control Module with an input to the Compressor Logic Controller internal to the HVAC Control Module to prevent B+ voltage being fed to CKT 66 (LT GRN), preventing A/C Operation when the system is turned off.
When either the Re-circulate switch or MAX A/C is selected, CKT 1641 (DK GRN) provides a ground for the Recirculation Door Motor through either the VCM for a gasoline engine or through the A/C Compressor Low Pressure Cutout Switch to CKT 150 (BLK) to G105 for a diesel engine. The Recirculation Door Motor's normal position is for outside air. When the Recirculate Switch is depressed, this provides a ground on CKT 1641 (DK GRN) at the HVAC Control Module, that has an internal 1 K resistor, to activate the Recirculation Door Motor. When the VCM on a gasoline engine or the A/C Compressor Refrigerant Pressure Switch on a diesel engine closes or provides a no-resistance ground on CKT 1641 (DK GRN), the Recirculation Door Motor moves to recirculation position.
The compressor for the air conditioning is clutch regulated and driven by the engine by means of a belt drive. The clutch allows the compressor to be disengaged when compressor operation is not required.
The battery supplies voltage to the following components.
The HVAC Control Module (when the ignition switch is in the RUN position.)
The A/C Compressor Clutch Relay Coil (when the ignition switch is in the RUN position.)
The normally open A/C Compressor Clutch Relay contact.
At the HVAC control module, B+ voltage is supplied by CKT 141 (BRN) to C3 pin 5 and to C2 pin G at all times from the 25 amp Fuse 12, HTR-AC, at the IP Fuse Block. Battery voltage is also supplied at all times to the A/C Compressor Clutch Relay from the 10 Amp A/C Fuse located in the Underhood Fuse Block through CKT 1240 (ORN). Ignition voltage is supplied to the A/C Compressor Clutch Relay from the 10 Amp fuse, IGN E from the Underhood Fuse Block through CKT 639 (PNK). The HVAC Control Module grounds through CKT 150 (BLK) to G105 and the A/C Compressor Clutch Relay grounds through CKT 150, G200.
To drive the A/C Compressor Clutch, the Powertrain Control Module (PCM) on diesels or the Vehicle Control Module (VCM) for gasoline engines, provides a ground on CKT 459 (DK GRN/WHT) which in turn activates the coil in the A/C Compressor Clutch Relay which is fed voltage through CKT 639 (PNK). This activates the A/C Compressor Clutch Relay which is fed voltage to CKT 59 (DK GRN) to the A/C Compressor Clutch, which grounds through CKT 150 (BLK), G105. Diode D104 in the harness is reverse-biased and prevents current flow bypassing the clutch coil to ground as well as serves as a means of de-energizing the magnetic field when the relay coil is de-energized.
For Gasoline Engines, the VCM will provide a ground path when a B+ voltage signal is received on CKT 762 (DK GRN/WHT). This voltage signal is provided by the HVAC Control Module when the A/C Push Button Switch is depressed or if defrost is selected through the Mode Dial Defrost Detent, and feeds B+ voltage through CKT 66 (LT GRN) to the A/C Compressor High Pressure Cutout Switch. Provided that the switch is closed, the signal is then supplied to CKT 762 (DK GRN/WHT). The A/C Compressor Clutch can also be turned ON/OFF by the VCM if a voltage drop is detected on CKT 604 (DK BLU) which feeds the A/C Compressor Refrigerant Pressure Switch and grounds through CKT 451 (BLK/WHT) to G104 or if a voltage drop is detected on CKT 603 (DK GRN) which feeds the A/C Compressor Cycling Switch and grounds on CKT 451 (BLK/WHT). These switches are normally closed.
For Diesel Engines, the signal is fed inn the same way as Gasoline Engines with exception of an A/C Compressor High Pressure Cutoff Switch and an A/C Compressor Cycling Switch is included in the circuit. CKT 66 (LT GRN) is fed B+ voltage from the HVAC Control Module, the A/C Compressor Cycling switch must be closed to continue the voltage path through CKT 603 (DK GRN) to the A/C Compressor High Pressure Cutoff Switch, which feeds the PCM through CKT 762 (DK GRN/WHT).
When the Front Blower Switch is in the OFF position, the HVAC Control Module feeds B+ voltage to CKT 71 (DK GRN) from C2, pin G to C3, pin 3. This voltage signal provides the HVAC Control Module with an input to the Compressor Logic Controller internal to the HVAC Control Module to prevent B+ voltage being fed to CKT 66 (LT GRN), preventing A/C Operation when the system is turned off.
When either the Re-circulate switch or MAX A/C is selected, CKT 1641 (DK GRN) provides a ground for the Recirculation Door Motor through either the VCM for a gasoline engine or through the A/C Compressor Low Pressure Cutout Switch to CKT 150 (BLK) to G105 for a diesel engine. The Recirculation Door Motor's normal position is for outside air. When the Recirculate Switch is depressed, this provides a ground on CKT 1641 (DK GRN) at the HVAC Control Module, that has an internal 1 K resistor, to activate the Recirculation Door Motor. When the VCM on a gasoline engine or the A/C Compressor Refrigerant Pressure Switch on a diesel engine closes or provides a no-resistance ground on CKT 1641 (DK GRN), the Recirculation Door Motor moves to recirculation position.
Last edited by MDTAHOE; March 10th, 2013 at 9:35 PM.
March 11th, 2013, 7:16 PM
#17
CF Junior Member
Thread Starter
wow. you know ur stuff. i'll be taking the truck to a ac specialist in the morn. im actually going to print and take this info along. Thanks for all the help. also, just an added two cents. my brake lights are f***ed up as well. there's no way there's some kind of grounding issue that is messing up the truck's entire elec system you think?
August 14th, 2023, 11:17 AM
#18
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AC blowing hot
Hi. Really discouraged. I just bought a 2000 tahoe limited 5.7. The button on the dash will NOT enguage the compressor even though it lights up. My mechanic jumped the compressor clutch and it spun. A/C blew cold air. It is NOT the really, NOT the low-pres cutoff switch and NOT any fuses. Both myself and my mechanic are at a huge loss as what is could be. The climate cnt pannel seems to be ok. Temp control will go to hot and heater works, fan speed works, defrost works, and recirc button is operational. Anything else that would disable the compressor from engauging? Thanks in advance the help is greatly appreciated.
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