Tahoe 2004 / 2005 differences?

For the most part yes the Navigation system is the only add on now

"Ken Williams" wrote in news:R4rte.1667$gt5.1006@fed1read02:

2005 Tahoe gets 1 MPG better then the 2004 Tahoe and has a bigger alternator (by 20 amps I believe).

Adam

Wonder what they changed to get the extra mpg out of it.

Not sure if this accounts for all of it, but GM's new fancy-shmancy Electrical Power Management system might help.

I'll quote the description/operation section out of the manual. It's long, but kinda interesting.

Charging System Description and Operation (Generator/Battery Control Module) Electrical Power Management (EPM) Overview The Electrical Power Management (EPM) System is designed to monitor and control the charging system and send diagnostic messages to alert the driver of possible problems with the battery and generator. This EPM System primarily utilizes existing on-board computer capability to maximize the effectiveness of the generator, to manage the load, improve battery state-of-charge (SOC) and life, and minimize the systems impact on fuel economy. The EPM System performs 3 functions:

. It monitors the battery voltage and estimates the battery condition.

. It takes corrective actions by adjusting the regulated voltage.

. It performs diagnostics and driver notification.

The battery condition is estimated during key-off and during key-on. During key-off the SOC of the battery is determined by measuring the open-circuit voltage. The SOC is a function of the acid concentration and the internal resistance of the battery, and is estimated by reading the battery open-circuit-voltage when the battery has been at rest for several hours.

The SOC can be used as a diagnostic tool to tell the customer or the dealer the condition of the battery. Throughout key-on the algorithm continuously estimates SOC based on adjusted net amp hours, battery capacity, initial SOC, and temperature.

While running, the battery degree of discharge is primarily determined by a battery current sensor, which is integrated to obtain net amp hours.

In addition, the EPM function is designed to perform regulated voltage control (RVC) to improve battery SOC, battery life, and fuel economy. This is accomplished by using knowledge of the battery's SOC and temperature to set the charging voltage to an optimum battery voltage level for recharging without detriment to battery life.

The Charging System Description and Operation is divided into 3 sections. The first section describes the charging system components and their integration into the EPM. The second section describes charging system operation. The third section describes the instrument panel cluster operation of the charge indicator, driver information center messages and voltmeter operation.

Charging System Components

Generator The generator is a serviceable component. If there is a diagnosed failure of the generator it must be replaced as an assembly. The engine drive belt drives the generator. When the rotor is spun it induces an alternating current (AC) into the stator windings. The AC voltage is then sent through a series of diodes for rectification. The rectified voltage has been converted into a direct current (DC) for use by the vehicles electrical system to maintain electrical loads and the battery charge. The voltage regulator integral to the generator controls the output of the generator. It is not serviceable. The voltage regulator controls the amount of current provided to the rotor. If the generator has field control circuit failure, the generator defaults to an output voltage of 13.8 volts.

Generator Battery Control Module The generator battery control module is a class 2 device. It communicates with the powertrain control module (PCM), instrument panel cluster and the body control module for electrical power management (EPM) operation. It is a serviceable component that is connected to the negative battery cable at the battery. It directly controls the generator field control circuit, charge indicator control, input to the generator. It continuously monitors the generator field duty cycle signal circuit and the battery voltage. If the generator battery control module loses communication with the PCM, the default voltage will be set to 13.8 volts and the module will set U1016. If the generator battery control module loses communication with the body control module (BCM), the module will set U1064.

Powertrain Control Module (PCM) The PCM provides information over the class 2 serial data circuit to the generator battery control module. The generator battery control module monitors the following data parameters provided by the PCM:

. Intake air temperature

. Fuel grams per second

. Throttle position

. Engine cooling fan speed

. Engine coolant temperature

. Exterior Environment - Outside Air Temperature

The generator battery control module uses these data parameters for different charging system modes depending on the required voltage needed.

Instrument Panel Cluster (IPC) The instrument panel cluster (IPC) provides a means of customer notification in case of a failure. There are two means of notification, a battery charge indicator and a driver information center message of SERVICE CHARGING SYSTEM FAILURE and CHARGING SYSTEM FAULT.

Charging System Operation The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 9 modes of operation and they include:

. Charge Mode

. Fuel Economy Mode

. Voltage Reduction Mode

. Start Up Mode

. Headlamp Mode

. Battery Sulfation Protection Mode

. Windshield Wiper Voltage Boost Mode

. Fuel Pump Voltage Boost Mode

. De-Ice Voltage Boost Mode

The generator battery control module monitors the generator performance though the generator field duty cycle signal circuit, the generator field control circuit, and the battery positive voltage circuit. The generator battery control module controls the generator through the generator field control, charge indicator control, circuit. The signal is a 5-volt pulse width modulation (PWM) signal of 128 Hz +/- 5 percent with a duty cycle of

0-100 percent. The duty cycle sent by the generator battery control module is limited between 36-90 percent. When the engine is turned OFF, the module will send 0 percent duty cycle, low voltage. When there is loss of class 2 communication with the powertrain control module (PCM), the module will send 100 percent duty cycle, 13.8 volts. The following table shows the commanded duty cycle and output voltage of the generator:

Commanded Duty Cycle Generator Output Voltage

10% 11.0 V 20% 11.56 V 30% 12.12 V 40% 12.68 V 50% 13.25 V 60% 13.81 V 70% 14.37 V 80% 14.94 V 90% 15.5 V

The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the generator battery control module. The signal is a 5-volt PWM signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-99 percent. Between 0-5 percent and 100 percent are for diagnostic purposes.

Charge Mode The generator battery control module will enter Charge Mode when at least one of the following conditions is met:

. The electric cooling fans are on high speed.

. The rear defogger is ON.

. The battery state of charge is less than 80 percent.

. The battery current is not between -8 and 15 amps.

. The estimated ambient air temperature is less than 0°C.

. DTC B1516 is set.

Once one of these conditions are met the generator battery control module will set the targeted generator output voltage to the nominal optimum battery voltage which is from 13.9-15.5 volts, the voltage set point is based on the batteries state of charge and estimated battery temperature. The battery voltage ramps up to the targeted set point at a rate of 20 mV per second.

Fuel Economy Mode The generator battery control module will enter Fuel Economy Mode when all of the following conditions are true:

. Estimated ambient air temperature is equal to or greater than

0°C (32°F).

. The calculated battery current is less than 15 amperes and greater than - 8 amperes.

. The battery state of charge is greater than or equal to 80 percent.

. The rear defoggers are turned OFF.

. The electric cooling fans are on low speed or OFF.

The targeted generator output voltage is 13 volts. The generator battery control module will exit this mode once the criteria are met for Charge Mode or it will boost voltage to a pre-determined set point for the fuel pump, headlamps, or windshield wipers.

Voltage Reduction Mode The generator battery control module will enter Voltage Reduction Mode when the calculated ambient air temperature is above 0°C (32°F); the calculated battery current is less than 2 amperes and greater than -7 amperes, the generator field duty cycle is less than 99 percent. the rear defoggers are turned OFF, and the electric cooling fans are on low speed or OFF. Its targeted generator output voltage is 87 percent of the Charge Mode set point but limited to 12.9 volts. The generator battery control module will exit this mode once the criteria are met for Charge Mode or it will boost voltage to a pre-determined set point for the fuel pump, headlamps, or windshield wipers.

Start Up Mode After the engine has started the generator battery control module sets a targeted generator output voltage of 14.5 volts for 30 seconds.

Headlamp Mode The generator battery control module will enter the Headlamp Mode when the headlamps, low or high beams, are turned ON. The voltage will ramp up or down to 14.5 volts at a rate of 10 mV/second. The module will exit this mode once the headlamps are turned OFF and enter Charge Mode, Fuel Economy Mode, or Voltage Reduction Mode.

Battery Sulfation Mode The generator battery control module will enter this mode when the interpreted generator output voltage is less than 13.2 volts for 45 minutes. Once in this mode the generator battery control module will set the targeted output voltage to the nominal optimum battery voltage, see Charge Mode, for

3 minutes. The generator battery control module will then determine which mode to enter depending on vehicle conditions.

Windshield Wiper Voltage Boost Mode When the generator battery control module is in Fuel Economy Mode or Voltage Reduction Mode, the module will boost battery voltage to 14.5 volts when the windshield wipers are ON, intermittent, low, or high speed, after 8 seconds. The voltage will ramp to 14.5 volts at a rate of 50 mV/second. The module will exit this mode once the Windshield Wipers are OFF for 5 seconds and the module will enter Charge Mode, Fuel Economy Mode, or Voltage Reduction Mode.

Fuel Pump Voltage Boost Mode When the generator battery control module is in Fuel Economy Mode or Voltage Reduction Mode, the module will immediately boost battery voltage to 13.4 volts when the instantaneous fuel flow is greater than 21k grams/second and the throttle position sensor pedal position is greater than 90 percent. The module will exit this mode once the instantaneous fuel flow is less than 5k grams/second and enter Charge Mode, Fuel Economy Mode, or Voltage Reduction Mode.

De-Ice Voltage Boost Mode The generator battery control module will enter De-Ice Voltage Boost Mode when the estimated ambient air temperature is less than or equal to -1°C and the engine coolant temperature is less than or equal to 75°C. The module will be in Charge Mode if the above conditions are true. Once the engine coolant temperature becomes greater than 75°C, the module will remain in Charge Mode or enter Fuel Economy Mode or Voltage Reduction Mode based on the vehicle conditions.

Instrument Panel Cluster (IPC) Operation

Charge Indicator Operation The instrument panel cluster (IPC) illuminates the charge indicator in the message center when the one or more of the following occurs:

NOTE: The generator battery control module is not set up to set a DTC if the battery voltage is too high or too low. Check with the powertrain control module (PCM) to see if they set a DTC when the battery voltage is too high or too low.

. The IPC determines that the system voltage is less than 11 volts or greater than 16 volts. The IPC receives a class 2 message from the body control module (BCM) indicating there is a system voltage range concern. NOTE: Verify with the BCM DRE that this is true.

. The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.

. The ignition is ON, with the engine OFF.

. The generator battery control module determines there is a fault and sends a class 2 message to the IPC to illuminate the charge indicator.

Charging System Failure The generator battery control module will send a class 2 message to the IPC for the CHARGING SYSTEM FAILURE message to be displayed. It is commanded ON when DTC B1487 sets. The message is turned off when the conditions for clearing the DTC have been met and after an ignition cycle. NOTE: This will be true for MY2005 and MY2006.

Service Charging System The generator battery control module will send a class 2 message to the IPC for the SERVICE CHARGING SYSTEM message to be displayed. It is commanded ON when DTC B1390, B1488, B1492, or B1516 sets. The message is turned off when the conditions for clearing the DTC have been met and after an ignition cycle. NOTE: This will be true for MY2005 and MY2006.

Voltmeter Operation The IPC displays the system voltage as detected at the ignition 1 input of the IPC. When the engine is ON, the gage should be between 10-16 volts. The voltmeter will be noticeably different than previous model year vehicle as far as voltage fluctuations. If there is a concern with gage operation ensure to compare to a known good like vehicle.

Good Lord! No wonder they want $40k for the truck!

I don't know...all it has extra is the current sensor sitting on the negative cable and no doubt just a bunch of new software in the computers. But they will gladly charge you extra for it!

Ian

Very good stuff. Thanks all for your help.

"Ken Williams" wrote in message news:R4rte.1667$gt5.1006@fed1read02...

Maybe the rear axle ratio: In 2005, GM introduced a new GU5, 3.23 ratio rear axle configuration on its 2005 trucks. The standard rear axle ratio on the

1999 - 2004 1500 series trucks was the GU6, 3.42. Less power, but better gas mileage.

This may not apply to the Tahoe, because I believe that it was only the 1500 series trucks that dropped the rear disk brakes and went back to rear drums in 2005. New 17 inch rims (combined with the new 3.23 rear axle) were put on the 1500 series truck to accommodate the oversized rear brake drums. However, if the 2005 Tahoe has 17 inch rims, it may also have the new 3.23 rear axle.

Look for a white sticker on the bottom of the glove compartment to find the codes for full-size trucks:

GU4 Rear axle, 3.08 ratio

GU5 Rear axle, 3.23 ratio

GU6 Rear axle, 3.42 ratio

GT4 Rear axle, 3.73 ratio

GT5 Rear axle, 4.10 ratio

G80 Differential, locking, heavy duty

G86 Differential, limited slip, heavy-duty, rear