Cooling fans coming on max after cold engine start, stays on till engine shutdown

[ValleyTahoe]

08 Tahoe.

Weird it started the other day while driving on the first really hot day out. I noticed the snowflake icon on my a/c flashing and my ac got warm. I got out the truck at my next destination with it running and the fans were on wide open. I checked coolant, good. A/C belt, good. Got out my scan tool. No codes. Coolant temp showing 190. So I shut it down. Next morning did a cold start. Same thing fans wide open when cold and A/c wont work. I replaced the coolant temp sensor like some suggest but its still present. I thought maybe a bad relay but they stick on even when truck is off. I see thread but none solved. My thoughts are in the direction of a high pressure switch or maybe and issue causing a high pressure backup in the system.

Any direction is appreciated.

 

[Fless]

Moved you here from General Discussion section to Engine & Drivetrain.

 

[j91z28d1]

you can read the high side ac pressure with most scanners, even torque app. the low side takes a bit better scanner it's under the hvac live data. atleast on mine.


bag high side pressure transducer isn't a bad guess. would probably have some hvac trouble codes if so, but that takes a good scanner to read too.

 

[Doubeleive]

I think you can unplug the ac and see if the fans still go screwy

 

[ValleyTahoe]

Yep youre both correct. You can unplug to test. Unfortunately same result. So far unplugged high side and also replaced coolent temp sensor. Going to throw a new thermostat in just for the hell of it approaching summer. Some have fixed with a thermostat but I dont understand that theory when the gauges read 190 degrees and all ok.

No codes other than a recent P1744 fuel imbalance code.

 

[j91z28d1]

there might be some other codes besides the P codes in the ecm. not all scanners can read them. like my truck has a few codes for seats motors that will only show up on a scanner that can read all 30 sometimes modules.


besides that, I got nothing useful to even guess at

 

[Geotrash]

Yep youre both correct. You can unplug to test. Unfortunately same result. So far unplugged high side and also replaced coolent temp sensor. Going to throw a new thermostat in just for the hell of it approaching summer. Some have fixed with a thermostat but I dont understand that theory when the gauges read 190 degrees and all ok.

No codes other than a recent P1744 fuel imbalance code.

I'll try to find a flow chart for the factors impacting cooling fan speed on these. Pretty sure Alldata has one. Stay tuned.

 

[Geotrash]

Here's a wiring diagram from a 2012 Yukon XL Denali that should be substantially similar:

 

[Geotrash]

And here's a description of operation:

Cooling Fan Description and Operation (Gas)

Cooling Fan Control w/o HP2

System Overview

The engine cooling fan system consists of 2 electric cooling fans and 3 fan relays. The cooling fan relays are arranged in a series/parallel (S/P) configuration. This allows the engine control module (ECM) to operate both fans together at low or high speeds using 2 fan control circuits. The cooling fans receive positive voltage from the cooling fan relays, which receive battery positive voltage from the underhood fuse block. The cooling fans are supplied a ground at G100 and G101.

Low Speed Operation

The ECM applies ground to the coil side of the cooling fan low speed relay. This energizes the coil and applies voltage directly to the left cooling fan through the switch side of the low speed relay. The left cooling fan is connected in series to the right cooling fan through the de-energized series/parallel (S/P) cooling fan speed control relay. The result is a series circuit that operates both fans at low speed.

High Speed Operation

The ECM applies a ground to the coil side of the cooling fan low speed relay, the S/P cooling fan speed control relay, and the cooling fan high speed relay. When energized, the high speed fan relay applies voltage directly to the right cooling fan through the switch side of the relay. Simultaneously, the low speed fan relay and the S/P speed control relay provide ignition voltage and a direct path to ground for the left cooling fan. During high speed fan operation, both engine cooling fans have their own ground path. The result is a parallel circuit with both fans operating at high speed.

Cooling Fan Control w/ HP2

System Overview

The hybrid engine cooling fan system consists of two electrical cooling fans, two resistors, three engine control module (ECM) fan output control drivers, and five fan relays. The relays are arranged in a series/parallel configuration that allows the ECM to operate the fans at low, medium, or high speed depending on engine cooling requirements. The ECM controls the five relays by grounding the relay control circuits.

Low Speed Operation

The ECM applies a ground to the fan 1 control circuit for the low fan relay. This energizes the low fan relay coil, closes the relay contacts, and supplies battery positive voltage from the fan 1 fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the de-energized fan control relay, through the right cooling fan in-line resistor, and right cooling fan. The result is a series circuit with both fans running at a reduced speed.

Medium Speed Operation

The ECM applies a ground to the fan 2 control circuit, which energizes a bank of three relays consisting of the FAN Mid 1 relay, the fan control relay, and the FAN Mid 2 relay. When these three relays are energized the Mid 1 relay supplies battery voltage to the left cooling fan through the left cooling fan resistor. A direct ground path is provided for the left fan through the energized fan control relay. At the same time the Mid 2 relay is energized which applies battery voltage to the right cooling fan through the right cooling fan resistor. The result is a parallel circuit where each fan has voltage applied through a separate resistor, causing each fan to operate at a medium speed.

High Speed Operation

The ECM applies a ground to the fan 1, fan 2, and fan 3 relay control circuits simultaneously, which activates all five relays. This allows battery voltage to be applied directly to the right cooling fan from the Fan High relay, and battery voltage to be applied directly to the left fan from the Fan Low relay. The left fan is supplied a direct path to ground through the energized fan control relay. This arrangement provides each fan with direct battery voltage and its own ground path. The result is a parallel circuit with both fans running at full speed. When the ECM is commanding a fan relay ON, the voltage of the control circuit should be low, near 0 volts. When the ECM is commanding a fan relay OFF, the voltage of the control circuit should be high, near battery voltage.

 

[Geotrash]

And here are the diagnostic procedures:

Cooling Fan Always On (without HP2)

Diagnostic Instructions

• Perform the Diagnostic System Check - Vehicle See: Testing and Inspection\Initial Inspection and Diagnostic Overview\Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis See: Testing and Inspection\Initial Inspection and Diagnostic Overview\Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions See: Testing and Inspection\Initial Inspection and Diagnostic Overview\Diagnostic Procedure Instructionsprovides an overview of each diagnostic category.

Circuit/System Description

The engine cooling fan system consists of 2 electric cooling fans and 3 fan relays. The cooling fan relays are arranged in a series/parallel (S/P) configuration. This allows the engine control module (ECM) to operate both fans together at low or high speeds using 2 fan control circuits. The cooling fans receive positive voltage from the cooling fan relays, which receive battery positive voltage from the underhood fuse block. The cooling fans are supplied a ground at G100 and G101.

Diagnostic Aids

The ECM has the capability of providing command to the fan relays even while a scan tool output control is being used. Always refer to the fan control command parameters on the scan tool to know which fans are being commanded ON by the ECM.

Reference Information
Schematic Reference

Engine Cooling Schematics See: Diagrams\Electrical
Connector End View Reference

Component Connector End Views See: Diagrams\Connector Views
Description and Operation

Cooling Fan Description and Operation (Gas) See: Description and Operation\Cooling Fan (Gas)
Electrical Information Reference

• Circuit Testing See: Testing and Inspection\Component Tests and General Diagnostics\General Electrical Diagnostic Procedures\Circuit Testing\Circuit Testing
• Connector Repairs See: Testing and Inspection\Component Tests and General Diagnostics\General Electrical Diagnostic Procedures\Connector Repairs\Connector Repairs
• Testing for Intermittent Conditions and Poor Connections See: Testing and Inspection\Component Tests and General Diagnostics\General Electrical Diagnostic Procedures\Circuit Testing\Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs See: Testing and Inspection\Component Tests and General Diagnostics\General Electrical Diagnostic Procedures\Wiring Repairs\Wiring Repairs

Circuit/System Verification

1. Verify that the following DTCs are not set: P0480 or P0481.

¤	If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle See: A L L Diagnostic Trouble Codes ( DTC )\Testing and Inspection\Diagnostic Trouble Code Descriptions\Diagnostic Trouble Code (DTC) List - Vehicle.

1. Ignition ON, verify with a scan tool that the control module is not commanding fan activation.
2. Ignition ON, observe that the fans are not activated.

Circuit/System Testing

Note: The following tests must be performed on each cooling fan relay.

1. Ignition OFF, disconnect each cooling fan relay one at a time.
2. Ignition ON, observe that the fans are not activated.

¤	If the fans are activated, test for a short to voltage in the relay controlled output circuit.
¤	If the fans are not activated, test or replace the appropriate cooling fan relay.

Component Testing

1. Ignition OFF, disconnect the cooling fan relay.
2. Test for 70-110 ohm between terminals 85 and 86.

¤	If not within the specified range, replace the cooling fan relay.

1. Measure the resistance between the terminals listed below:

• 30 and 86
• 30 and 87
• 30 and 85
• 85 and 87 The DMM should display OL.

¤	If less than the specified range, replace the cooling fan relay.

1. Test the Fan Control Relay for less than 2 ohm between terminals 30 and 87A.

¤	If greater than the specified range, replace the cooling fan relay.

1. Connect a 20 A fused jumper wire between relay terminal 85 and B+. Connect a jumper wire between relay terminal 86 and ground. Test for less than 2 ohm between terminals 30 and 87.

¤	If greater than the specified range, replace the cooling fan relay.

Repair Instructions

Perform the Diagnostic Repair Verification See: A L L Diagnostic Trouble Codes ( DTC )\Testing and Inspection\Verification Tests after completing the diagnostic procedure.

• Relay Replacement (Attached to Wire Harness) See: Service and Repair\Removal and Replacement\Relay Replacement (Attached to Wire Harness)Relay Replacement (Within an Electrical Center) See: Power and Ground Distribution\Power Distribution Relay\Service and Repair\Relay Replacement (Within an Electrical Center)
• Underhood Electrical Center or Junction Block Replacement See: Maintenance\Fuses and Circuit Breakers\Fuse Block\Service and Repair\Removal and Replacement\Underhood Electrical Center or Junction Block Replacement