GM has introduced two full-sized utility hybrid vehicles for 2008, the Chevrolet Tahoe Hybrid and the GMC Yukon Hybrid (fig. 1). These are the world's first and only full-sized hybrid SUVs, and offer a fuel economy improvement of 30% combined and 50% city. They also provide room for eight passengers, offer four-wheel drive, and can tow up to 6,200 pounds.

To maximize its efficiency, the 6.0L V-8 engine is equipped with GM’s Active Fuel Management™ cylinder deactivation technology, which switches between four- and eight-cylinder power in certain driving situations (fig. 2).

2-MODE OPERATION

The 2-mode hybrid technology is optimized for both city and highway driving. The driver can monitor hybrid operation on the display screen of the standard navigation system (fig. 3).

In the first mode, at low speed and light loads, the vehicle can operate in three ways:

- electric power only

- engine power only

- a combination of engine and electric power.

Leaving the engine shut off for extended periods of time and moving under electric power at low speed is key to reducing fuel consumption in heavy stop and go traffic

In this mode, one electric motor acts as a generator and the other as a motor.

The second mode is used primarily at highway speeds. In this mode, the gasoline engine always runs. Active Fuel Management, cam phasing, and late-intake valve closure, allow even more efficient engine operation. In this mode, depending on need, both electric motors function either as generators or as motors.

HYBRID TRANSMISSION

Two 30 kilowatt electric motors, two planetary gearsets and two clutches fit into roughly the space of a conventional automatic transmission (fig. 4). One gearset is for the gasoline engine and the other is to multiply the power of the electric motors.

A sophisticated underhood controller (fig. 5) determines when the vehicle should operate in either mode of the 2-mode drive system. Input from the controller determines the necessary torque for the driving conditions and sends a corresponding commands to the engine and electric motors.

BATTERY PACK

The 300-volt battery pack is located beneath the second-row seats (fig. 6). It provides power to start the engine, it operates the electric motors, it provides power for the air conditioning and power steering systems, and operating through a converter, it provides 12 volts DC to operate normal accessories such as lights and radio.

A Control module

B Electric motors

C Battery pack

The engine provides power to charge the battery pack when needed -- there is no need to plug in the battery pack to keep it maintained.

The battery pack is disabled when access panels are removed, for safety. Special training is required to service this system (see the accompanying sidebar for details).

ELECTRIC ACCESSORIES

Electrically driven accessories eliminate the need for drive belts and allow the accessories to work during Autostop mode.

The electric air conditioning compressor is operated by the 300-volt battery. If necessary, the gasoline engine may run if the battery is low or coolant temperature drops.

An electric heater coolant pump circulates coolant when the engine is off.

Electric power steering operates from its own 42-volt power source.

BRAKING

Both friction brakes and regenerative braking are used. Regenerative braking, under light requirements, is used to charge the battery pack. Under moderate braking, both friction and regenerative braking are used. And under panic/heavy effort braking, only the friction brakes are used.

WEIGHT REDUCTION

To offset the added mass of the battery pack and other hybrid components, the vehicle uses lightweight aluminum hood and tailgate, aluminum bumper beam and low mass aluminum wheels. Aluminum suspension control arms are lightweight. Even the seats are 20 pounds lighter. The spare tire is replaced by an inflator kit. As a result, the weight of the Hybrid is similar to that of a non-hybrid vehicle.

IMPROVED AERODYNAMICS

Finally, the Hybrid trucks benefit from improved aerodynamics for improved economy. Grille, fascias, rear spoiler and running boards are unique to the Hybrid. Tow hooks and luggage rack are eliminated and the suspension is lowered to improve airflow.

ADDITIONAL HYBRID INFORMATION

For a comprensive look at all of GM's fuel and hybrid technology, go to the www.chevrolet.com website and click on the Fuel Solutions heading. Here, you'll be able to learn about GM's hybrid vehicles, as well as electric vehicles, fuel cell technology and more.

- Thanks to Keith Newbury

According to Bulletin 07-00-89-047, General Motors Service Operations is piloting Saturday service support in both the Technical Assistance Center (TAC) and Techline Customer Support Center (TCSC).

During the pilot, TAC and TCSC will closely monitor volumes, types of calls, service levels, peak hours, technician ability, as well as vehicle service success rates associated with these additional hours of operation.

At the conclusion of the pilot, a decision regarding Saturday service support will be made, considering the facts collected during this time.

Model Year Coverage Changes

TAC will begin to support only 1999 and newer vehicles both during the regular weekdays and Saturday. An analysis indicated that, on older vehicles, TAC consultants are used as a look-up reference for information readily available at the dealership level. Focusing on the nine previous years will allow TAC to direct their expertise to the types of requests which truly require TAC support.

Contacting TAC or TCSC

Use all available printed and electronic resources before contacting TAC or TCSC. Refer
to Bulletin 01-00-89-011E for the correct procedure to contact Technical Assistance. When calling TCSC, have all pertinent information available, including VIN and error code
(as applicable).

- Thanks to TAC and TCSC

Here are some tips to help you understand how to read diagnostic steps when you are performing circuit or system tests found in SI.

Circuit and System Testing is arranged to allow you to perform each test step, in sequence, until a fault is detected. If the result of a test step is achieved, the normal flow is to proceed to the next step. If the result is NOT achieved, the repair arrow bullet (⇒) will identify what actions need to take place.

Intrusive diagnostics are performed to locate the system fault. For instance, system harness connections are disconnected from the module or component to test individual circuit functions. The module or component will be used to assist in verifying the circuit function. When a test does not pass, the repair steps (⇒) will indicate what circuit faults to test. For example, short to voltage, short to ground or open/high resistance.

When testing for individual circuit faults, you are expected to include terminal inspections such as connection surfaces and terminal tension at both the harness and component or module. Additionally, you can use the links in Electrical Information Reference: Testing for Intermittent Conditions and Poor Connections or Circuit Testing.

The control modules and components will also be diagnosed during these test steps. Always retest a control module or component before replacement. For example, re-connect all components and modules and retest the system to verify the condition still exists before replacing modules or components.

- Thanks to Dave Nowak

Please note these revisions to the Saturn Astra information in the October 2007 TechLink.

Automatic Transmission Fluid

Use only T-IV Automatic Transmission Fluid (GM p/n U.S. 88900925, Canada 22689186)

Power Steering Fluid

GM Power Steering Fluid (GM p/n U.S. 88901975, Canada 88901976)

2007 full-size utility models built before October 2006 were built with 1st design TPM sensors with the valve stem integral to the body of the sensor. The 1st design TPM sensor can be visually identified by the aluminum valve stem and retaining nut (fig. 7).

2007 full-size utility models built after October 2006 were built with 2nd design TPM sensors. The body of the sensor is attached to the valve stem with a small Torx fastener. The 2nd design TPM sensor uses a valve stem made of black rubber and looks much like the traditional rubber valve stem (fig. 8). Service parts are available to service the 2nd design sensor and valve stem individually.

TIP: The 1st design TPM sensor is no longer available from GMSPO and must be serviced with the 2nd design TPM sensor. The 1st and 2nd design sensors are physically and electronically interchangeable.

TIP: Owners of utility trucks built before October 2006 (1st design TPM sensors) may object to the visual difference between the 1st and 2nd design parts on the same vehicle. In these cases, it is acceptable to use an alternative TPM p/n 15268606 which is also electrically and visually compatible with the 2007 full-size utility truck. This TPM sensor is not released with application for full-size utility trucks, but the sensor will operate correctly in this application and will closely match the 1st design part appearance.

- Thanks to Dave Roland

Starting with Tech 2 Version 27.003, the Tech 2 now has a new function for Tire Pressure Monitoring called TPM Copy and Paste. This function can be used to copy all the information that is related to the tire pressure monitoring system (including tire sensor IDs) and paste the information into a new or freshly SPS-re-programmed remote control door lock receiver (RCDLR).

Copy and Paste can be used on the following vehicles:

- Thanks to Mark Haning

The 2008 Saturn ASTRA entertainment system is comprised of two main components,
a Radio, or Entertainment Head Unit (EHU), and a Driver Information Display. These two components utilize a Security Code (or Immobilizer Code) to enable the theft deterrent feature. The Security Code in the Radio and the Driver Information Display must match in order for the entertainment system to operate correctly.

The Radio is the master of the Security Code, and all programming of the Security Code is done through the Radio. Each vehicle has its own unique Security Code, which is the same code used for the Entertainment System Security Code. The Security Code can be obtained in DealerWorld under the Parts tab. Click on KeyCode Look Up. In Canada, the security code can be obtained in the GM Parts Locator and Information Services application.

TIP: KeyCode Look Up is restricted to authorized personnel only. Check with your dealership PSC for access information.

De-Coupling Radio and Display

When replacing either the Radio or the Driver Information Display, it is important to
de-couple the two components by clearing the Security Code before either component is removed from the vehicle. To clear the Security Code, connect a Tech 2 loaded with 2008 Saturn Astra software. Navigate to the Entertainment System and select the EHU. From the Programming menu, select Reset ECU, and this will perform the Deactivation Theft Protection function.

To successfully de-activate the theft protection and de-couple the Radio and Driver Information Display, the Security Code must be entered (fig. 9). When de-coupled, the Entertainment System will function, with the DIC showing Headunit SAFE or Radio SAFE and Display Safe messages periodically. A beep will repeat about every five seconds, also indicating the Theft Protection has been disabled.

Re-Coupling Radio and Display

After installing the new component, the Radio and Driver Information Display must be coupled with the proper Security Code. To couple the Radio and Display, navigate to the Entertainment system and select the EHU. From the Programming menu, select Program ECU. Any software updates, configurations and the Security Code are then programmed. Be sure to select the correct tuner frequencies in the Variant Configuration section. When prompted, enter the proper Security Code twice to couple the Radio and Display to activate the theft protection feature.

TIP: Be sure to enter the proper Immobilizer Code (matched to the vehicle VIN) for the Entertainment Security Code. There is no way to retrieve the Security Code from the Radio or Display. DealerWorld (GM Parts Locator in Canada) contains the only record of Security Codes applied to vehicles by VIN.

- Thanks to Nohr Tillman and Mark Stesney

On the Acadia, Outlook and Enclave, the positive crankcase ventilation (PCV) fresh air tube is difficult to remove from the outlet duct. It is very easy to damage the outlet duct to PCV tube interface point when attempting to remove the tube. Once the Fuel Injector Sight Shield is removed, the tube can be easily disconnected from the camshaft cover (fig. 10) and repositioned out of the way (fig. 11), without having to remove it from the outlet duct assembly.

TIP: Refer to SI document 1860669.

- Thanks to Jonathan Johnson

Bulletin 07003 has been issued to explain a modification to improve side impact protection on the 2007 LaCrosse (Allure in Canada).

This modification requires removing the front door panels and modifying components on its backside, with the panel on a work bench.

If care is not taken, it would be possible to drill holes through the trim panel (fig. 12), damaging the appearance of the upholstery.

Several cautionary notes about drilling have been added to the procedure.

Step 5. Do not use a drill bit to remove the beltline flange.

Step 6. Use a 13.5 mm (17/32 in) drill stop to ensure that you do not drill through the door trim panel.

- Thanks to Chris Anderson

This information applies to the Equinox, Torrent and Vue equipped with All Wheel Drive.
An incorrect service procedure when installing the rear differential clutch drum assembly can cause a ticking noise.

When installing the differential clutch drum assembly to the pinion shaft of the differential assembly, one of the first steps is to align the clutch with the pump and pump bushing.
This is done by placing alignment tool J 46607 in the splines of the clutch. Twist the alignment tool back and forth to align the pump and bushing. With a properly aligned
clutch, the groove on the J 46607 will be flush with the drum (fig. 13). Remove the tool by pulling straight out.

Perform this step correctly, as shown in SI. Do not attempt to force or hammer the alignment tool into the clutch drum. Do not use the pinion shaft to perform the alignment function by installing the clutch drum onto the shaft without using the alignment tool. Using the alignment tool correctly assures that no damage occurs to the internal components of the clutch drum. If damage occurs to the clutch, the pump, or the pump bushing, a ticking noise will result.

TIP: Changes are being made to the assembly procedure in SI to reflect this information.

- Thanks to Chuck Krepp

According to bulletin PIC3608D, some owners of the following vehicles may comment that the outside temperature display reading is inaccurate at times.

The outside air temperature algorithm uses outside air temperature, vehicle speed and time the engine was turned off.

If the engine was off long enough or the current outside air temperature is lower than the stored value from the previous ignition cycle, a new outside temperature reading is taken and displayed. The outside air temperature continues to update every second as long as the outside air temperature is the same as or lower than the previous reading.

If the engine was not off long enough or the current outside air temperature is higher than the stored value, the last stored value will be displayed. Vehicle speed has to exceed 18 mph (29 km/h) for up to 5 minutes in order for the displayed outside air temperature to be updated. If the speed drops below 18 mph (29 km/h), the outside air temperature will display its last value.

- Thanks to Joe Kayfish

According to bulletin PIC4291, the following condition could affect a 2007 Cadillac DTS equipped with a US8 or US9 radio.

Under some conditions, the radio may lock up during vehicle start up. The radio will no longer function, there will be no radio display, no chime function, and limited or no CD functionality. The radio back lighting may or may not illuminate. Once a radio has entered this state, it will remain locked up. It will continue to draw power until power is removed from the radio, which may result in a dead battery.

Do not replace the radio for this concern. Check the radio software ID.

-  If the software ID is 25814799, proceed with the normal diagnostic procedure.

-  If the software ID is 25774458, program the radio with the updated software and/or calibration files using the TIS Service Programming System (SPS) application.

- Thanks to Steve Falko

Some owners of a 2007 Silverado or Sierra may comment that the A/C compressor is inoperative, SES MIL, and/or the engine does not continue to run. The ECM may have set one or more of the following DTCs: P0341, P0452, P0532 and P0641.

The A/C pressure sensor 5 volt reference circuit 2700 may short to ground at the A/C hose (fig. 14). To correct this concern, repair the wiring and secure the harness to provide the condition from reoccurring.

A  Short to ground

B AC pressure sensor

- Thanks to Jim Will

Some owners of a 2003-06 full-size utility or 2003-07 classic pickup may comment of a SIR MIL and if equipped the temperature/compass display on the inside rearview mirror is inoperative.

This concern may be caused by an open 15 amp air bag fuse. Circuit 1139 (yellow) may be shorted to ground at the left I/P junction block mounting bracket (fig. 15).

To correct the concern, repair the wiring as necessary and insulate the harness and mounting bracket with foam tape to prevent a reoccurrence.

- Thanks to Jim Will

The owner of a 2008 Saturn VUE may comment that the compass does not display in the driver information center (DIC).

The compass has been deleted from this vehicle, and no repairs should be made to try to repair this.

The owner manual page 3-47 and page 4 of Getting to Know Your VUE brochure state that the compass displays in the DIC. Due to the timing of the compass delete, these materials were already printed and could not be changed.

- Thanks to Ron Erman

This is information about installing a power take off (PTO) unit on 2007-08 Silverado and Sierra 3500 Chassis Cab Models with 6.6L RPO LMM.

Packaging problems have been found when attempting to install a Series 270 Chelsea unit on these vehicles, due to the structure of the exhaust system and the Diesel Particulate Filter (DPF).

IMPORTANT: Absolutely no modification to the LMM exhaust system should be made or attempted.

The Series 250 Chelsea PTO (right side) model is the only verified compatible configuration. Details on this requirement can be found in the Body Builders Book for this model, under General Instructions. The Body Builders Book is avaiable on the GM Upfitter Integration website: www.gmupfitter.com.

- Thanks to Jim Will

Some customers may comment that the volt gauge fluctuates between 12 and 14 volts on their 2005-08 full size pickup or utility vehicle. Starting with the 2005 model year, these vehicles are equipped with a Regulated Voltage Control (RVC) system. RVC reduces the targeted output voltage to 12.6-13.1 volts when in Fuel Economy Mode to improve fuel economy.

The generator may exit Fuel Economy Mode if additional voltage is required. This causes the voltmeter to fluctuate between 12 and 14 volts (fig. 16), as opposed to non-regulated systems which usually maintain a more consistent reading of 14 volts.

This fluctuation with the RVC system is normal system operation. No repairs should be attempted. Refer to the owner manual Section 3 (Instrument Panel) Voltmeter Gage for more information.

TIP: Refer to PIT4426 for concerns with keeping a camper or trailer battery properly charged.

- Thanks to Jim Will

This information expalins how to address leaks between the front tonneau cover and the midgate on the 2002-08 Escalade EXT and Chevrolet Avalanche.

TIP: Water in the drain troughs on either side of the bed is normal, and is part of the water management system.

Determine where the leak is taking place. At the location where water leakage is observed, check that the seals are in good condition, not twisted, not deformed out of position or damaged in any way that would allow leakage to occur.

Check the seal clearance where the leak is observed. At the midgate to front cover 1 interface, the thickness of a sheet of paper (about 0.1 mm) is the maximum thickness that can be placed in the seal interface (fig. 17).

If the seal gap exceeds the suggested amount, adjust the striker on the affected side by loosening the two retaining bolts (fig. 18), sliding the striker slightly forward, and tightening the bolts. Check the gap and repeat the process until the correct gap is achieved.

The remaining strikers on the same side of the vehicle may need to be repositioned. See accompanying dimensions for readustment (fig. 19 and 20).

Once strikers are adjusted, install the tonneau covers. To ensure proper location, insert each panel in order, always latching the driver's side latch first (fig. 21), then the passengers side, on each panel.

To verify that cover to cover seal interfaces are correct, sight each joint across the vehicle (fig. 22). The interfaces should be parallel and uniform. If not, remove the covers and adjust the passenger side striker positions to align seal interfaces. Once adjustment is complete, install tonneau covers.

Check for leakage by running water onto the tonneau covers. Repeat adjustment steps if leakage is still observed.

- Thanks to Rich Pyszel

Some owners of GM vehicles equipped with the power liftgate option may comment that the power liftgate is inoperative in very cold weather. The concern seems to go away when the temperature is warmer.

The Power Lift Gate Module (PLGM) software will not allow the liftgate to operate at extreme temperatures, either when very hot or very cold. Do not replace any parts for this concern; this is a normal design of the power liftgate system. The power liftgate will release but has
to be manually opened and closed when outside its temperature range limits.

- Thanks to Ron Erman and Grant Lumsden

This information applies to 2007-08 Express Van and Savana Van 2500/3500, Silverado Classic and Sierra Classic 2500 equipped with a 4L80E/4L85E Transmission RPO MT1 or MN8 and two wheel drive, with transmission fluid coming out of the transmission extension housing seal.

Until November 30, 2007, GM Powertrain Division is requesting that additional parts be replaced and returned for analysis of extension housing seal leaks. If a fluid leak is diagnosed as coming from the transmission extension housing seal (use fluid dye if necessary), replace the extension housing and seal together, without removing the seal from the housing. Also replace the slip yoke to facilitate analysis of the yoke and seal inner diameter.

After November 30, 2007, disregard this request and repair the fluid leak condition in the normal manner.

- Thanks to Chuck Krepp

On a 2008 Cadillac CTS, a DTC B1009 EEPROM Checksum Mismatch may be found stored in the Memory Seat Module (MSM) while scanning for DTCs.

A software concern exists that may cause the DTC to set when the vehicle goes to sleep or when the vehicle’s battery power supply is interrupted. No repairs should be attempted to correct for this DTC if stored in the MSM.

- Thanks to Michael Ciarkowski