Two-Mode Hybrid Vehicles


TOPIC: GMC and Chevrolet 2008 model year two-mode hybrid vehicles

OBJECTIVE: Understand the design and operation of a 2008 model year or newer GM two-mode hybrid vehicle such as a Tahoe or Suburban

TASK: The fire, EMS and/or rescue team shall inspect a 2008 model year GM two-mode hybrid vehicle and be able to identify the major hybrid system components, explain the normal operation of the hybrid system, and identify the recommended emergency procedures for dealing with this vehicle at a real-world incident.

The 2008 model year GMC Yukon and Chevrolet Tahoe two-mode hybrid vehicles are gasoline-electric hybrid SUVs that were introduced in the fall of 2007. Each vehicle is equipped with a special V8 gasoline engine that can operate as a fuel-saving four-cylinder engine under certain conditions. To gain improved miles-per-gallon ratings, especially under city-driving conditions, the Tahoe and Yukon hybrids use a 300-volt electrical system coupled with a unique hybrid transmission. There are two very compact electric drive motors packaged within the transmission housing that can move the vehicle totally on electric power, called the electric mode, at up to 25 miles per hour. The SUVs also have a 42-volt and a traditional 12-volt electrical system.

The "two-mode" term comes from the two different combinations of power that can move the vehicle. For the Chevy Tahoe and GM Yukon, during low-speed, light-duty driving, the system works like a conventional single-mode hybrid electric vehicle, shutting off the V8 engine whenever possible, allowing one or both of the electric motors to move the vehicle.

During highway cruising, the two-mode system can also use one or both electric motors, but this time they function to provide a power boost to the vehicle along with the power from the gasoline engine. In addition, the V8 engine uses technology systems such as variable valve timing and cylinder shutdown to save fuel. For the driver of the SUV, the transition between the two modes is completely seamless.

GM engineers took great efforts to reduce the weight of numerous vehicle components, which keeps the Tahoe and Yukon Hybrid models in the same weight class as their gasoline-only twins. At a crash scene, therefore, responders will encounter an aluminum hood, aluminum liftgate with fixed glass and 18-inch cast aluminum wheels along with an aluminum front bumper beam.

The Option for Four Roof Airbags

Standard safety features that are important to responders include dual-stage frontal airbags, roof-mounted side-impact airbags that are rollover enabled and front seatbelt pretensioners. Worthy of note is the fact that there can be four roof airbags on these vehicles; two bags on each side. One roof airbag on each side is standard equipment. One stored-gas inflator is mounted horizontally along the roof rail to deploy the roof airbag that protects the front- and second-row occupants. If the optional third-row seating is ordered, a separate stored-gas inflator secured to the D-pillar will also be present. This roof airbag deploys to protect the third-row seat occupants. This is a total of four roof airbags and four high-pressure inflators. Strip the trim for sure!


Special emblems, called badging, are used to identify the 2008 GMC Yukon and Chevrolet Tahoe hybrid vehicles. There are emblems located on both front fenders, decals on both front doors, green "H" badging on both C-pillars, a green "H" on the lower right corner of the vehicle's liftgate, and hybrid decals on the rear window and the top of the front windshield. When the hood is opened, indications that a two-mode hybrid system is present include a hybrid label and a high voltage warning label on the power electronics cover.

For interior responders, identifying that you are dealing with a hybrid vehicle is evident when you see a large round tachometer that sits ahead of the steering column. It has a special "auto stop" indicator mark on the dial. To the left of this tach is a smaller economy gauge that is also unique to only these 2008 GMC two-mode hybrids. If you are inside with a patient and you see that the tachometer needle is pointing to the auto stop position, the hybrid vehicle you are in is still energized and can move on electricity only or the engine could start unexpectedly. This condition is referred to by responders as hybrid vehicle "sleep" mode. Chock the wheels and don't stand in front of the hybrid until the vehicle has been completely shutdown. Turn the ignition off right away!

300-volt Battery

For responders arriving at a hybrid Tahoe or Yukon incident, the 300-volt Nickel Metal Hydride (NiMH) hybrid battery is enclosed in a metal case located under the second row, rear seat sub-floor. The rear passenger's seat flips forward to allow for a plastic trim cover to be removed. Removing the panel exposes the orange service disconnect plug that can be used by responders as one means of powering down the high-voltage hybrid electrical system. The orange-colored, 300-volt high-voltage wiring runs from the battery to the engine compartment and is tucked inside the right, passenger's-side frame rail. Remember, just like with any hybrid we encounter, the two-mode hybrid battery has 300-volt electrical potential at all times.

High-Voltage Cables

Color-coded wiring on the 2008 GMC Yukon and Chevrolet Tahoe two-mode hybrid vehicles is used to identify the different levels of electrical power flowing through various electrical lines on these hybrids. Blue is used to identify "intermediate" voltage cables and the familiar traffic-cone orange is used for the high-voltage cables. GM's two-mode hybrids utilize 300 volts (both AC and DC) for hybrid system operation. A 120-volt AC accessory power outlet is provided at the left, rear corner of the cargo area and a 42-volt system supplies the electric power steering system.

Emergency Procedures: The 7-Step Lock Out/Tag Out

There is an Emergency Response Guide available for these vehicles. For your convenience, this GM training document can be downloaded at that University of Extrication section of the website or directly from GM. You'll be pleased to see that as you study this document, the original seven-step process for dealing with hybrid vehicles presented by the University of Extrication in 2002 will hold true for this latest technology hybrid vehicle.

Once the vehicle is identified as a hybrid, chocking or blocking the wheels and gaining interior access should be accomplished. Responders should then place the gear selector in "park" and turn the ignition key to the "off" position. During action to take away the 12-volt electrical power, responders who access the battery in the left front engine compartment area will find that each of the three positive battery cables has a bright-yellow label attached to it with lines showing where to cut through the three positive battery cables. GM calls them "First Responder Labels" and they are unique to the hybrids. Another great option for 12-volt shutdown is a quick-release lever that instantly releases each battery cable from the battery terminals. You'll have to see this to understand how innovative and easy it will be for responders. Airbag capacitor "drain" time is 10 seconds.

TASK: The Fire, EMS, and/or Rescue team shall inspect a 2008 model year GM two-mode hybrid vehicle and be able to identify the major hybrid system components, explain the normal operation of the hybrid system, and identify the recommended emergency procedures for dealing with this vehicle at a real-world incident.

RON MOORE, a Firehouse® contributing editor, is a battalion chief and the training officer for the McKinney, TX, Fire Department. He also authors a monthly online article in the "MembersZone" and serves as the Forum Moderator for the extrication section of the website. Moore can be contacted directly at