4 GEARS March 2007

Welcome back: In our final part of our series on the GM Parallel Hybrid Truck

(PHT), we’ll look at some service fea-tures and general service procedures. Hybrids are here to stay, so the sooner you learn to work on these systems safely, the sooner you and your shop can take advantage of this market.

Let’s examine the tools you’ll need to work on the PHT and other hybrid applications. They include:• A quality scan tool capable of

interfacing with the various mod-ules within the system.

• A quality, high-impedance, digital multimeter. This isn’t the $4.99 version that you see in hardware stores, but a quality tool designed with the features and safety stan-dards necessary for electronics and commercial electrical diagnosis.

• A pair of 1000-volt isolation gloves.

• Eye protection and rubber-soled shoes.

The PHT has a high-voltage sys-tem, which can injure or kill you if you don’t understand and respect the system safety requirements. The PHT as well as other GM hybrid systems contain the following safety systems:• Automatic Battery Disconnect• Isolation Fault Detection• High Voltage Circuitry Interlock• Manual Disconnect• Uniquely colored conduit for volt-

age level identification

Since the type of voltage that the

hybrid system uses varies so much based on the vehicle manufacturer and the type of system used, GM instituted a standard for voltage level identifica-tion. These systems were broken down into 3 categories:Ø 8-volts DC to 30-volts DC (15

VRMS AC) — Low voltage sys-tem

Ø 30-volts DC (15 VRMS AC) to 60-volts DC (30 VRMS AC) — Intermediate voltage system

Ø 60-volts DC (30 VRMS AC) and above — High voltage system

GM hybrid applications in produc-tion now and in the near future will typically use all three systems. In fact, it may become common to work on a system in the next generation 2-mode transmission that will have the capabil-ity of 300-600 volts! So safety is your number one concern.

To make it easier to identify the different GM electrical systems, the wires will be cov-ered with a color-coded conduit:Ø Low voltage system

— Standard conduit, standard wiring

Ø Intermediate voltage — Blue conduit

Ø High voltage — Orange conduit

Battery Disconnects and Service Precautions

The PHT truck application has a service disconnect switch located on the passenger side of

the energy storage box (ESB). The ESB is located under the rear seat. To reduce the risk of shock or burns, turn the engine and the service disconnect switch off whenever you’re performing service work on the ESB system, trans-mission or any system under the hood. The service disconnect switch should also be turned off if the vehicle won’t be used for more than 30 days.

Turning the service disconnect off doesn’t protect you from the shock hazard produced by the 3-phase electri-cal system, nor the 120-volt system if it’s in use, so make sure the engine and ignition key are off when you’re per-forming any service procedures. Make sure the APO switch is also off. If that isn’t possible, here are some basic pre-cautions you must follow:

by Steve Garrett

The Shock of The Shock of Your Life, Your Life, PART 3 OF 3PART 3 OF 3

PHT Truck

PHT Stator Location

Service Disconnect Switch

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6 GEARS March 2007

• Pay attention to the orange, 3-phase cables while making any electri-cal measurements. Probing these cables can cause serious injury as well as damage the cables.

• Remove all jewelry, watches, rings, necklaces and metal belt buckles.

• If you’re performing electrical tests on the intermediate or high volt-age systems, always wear isolation gloves. These special gloves are designed to provide protection for up to 1000 volts in case of an acci-dent. Keep in mind that isolation gloves aren’t something you buy just once. Isolation gloves have an expiration date printed on them. The gloves may deteriorate with age so replace them when the date has expired.

• Perform all service work on the intermediate and high voltage sys-tems using the “one arm method.” You’ve been telling people for years that you could fix their cars with one arm tied behind your back: Now’s your chance! Never place both hands on the vehicle when working with the intermedi-ate or high voltage systems. Most manufacturers insist that techni-cians use only one hand when making measurements or repairs on high voltage and high current flow circuits. Placing your other hand on the vehicle may inadver-tently complete the path to ground, resulting in injury or death.

• Always check for power with your digital meter before attempting to a repair a circuit or system.

• Remember, disconnecting the 12-volt vehicle battery doesn’t disable the 42-volt system.

Battery Service The PHT application uses three,

12-volt Panasonic HV1255 Valve Regulated Lead Acid (VRLA) batteries. The batteries are located in the ESB, and are designed to provide a useable life of 36,000 amp hours. The batteries normally operate at about a 50% state of charge (SOC). Your scan tool will provide the state of charge rate as a data parameter. A state of charge greater than 80% generally means one or more batteries is starting to fail. If a battery has failed, you’ll need to replace it. The

replacement batteries will arrive with about an 80% SOC. Before you put them into the truck, charge the battery to 100% SOC. Here’s how:• Install a scan tool.• Turn the service disconnect switch

on.• Connect a manual battery charger

to the underhood jump-start termi-nal and ground.

• Set the charger to 40 amps.• Key on, engine off.• With the scan tool, select “SGCM

service charge” override and turn the function on. This will initiate the battery service charge process.

• A message will appear on your scan tool showing when the ser-vice charge is complete.

Connections and FusesAs system voltages rise, the poten-

tial of component and terminal damage also rises. The arcing potential of a 42-volt electrical system is 50 to 100 times greater than that of a 12-volt system.

Arcing temperatures will exceed 1800ºF. This means a service proce-dure as simple as replacing a fuse can be hazardous if not performed cor-rectly.

The fuses may look the same but they aren’t. The 42-volt fuse is designed to flash open under excessive current load. If you put a 12-volt fuse in its place, it could allow the current to arc across the fuse, effectively leaving the circuit unprotected.

Disconnecting and connecting an intermediate- or high-voltage system is very dangerous if the power isn’t removed from the circuit. Severe arcing and terminal damage can result and the chance of injury rises dramatically.

Programming Calibrating the PHT control mod-

ules is basically like any other GM vehicle reprogramming process. The only difference deals with the EHPS (Electro-Hydraulic Power Steering) system. The EHPS must be discon-nected when updating modules that use the class 2 data bus. Modules on the GMLAN high-speed (CAN) data bus will set U-codes while being updated. Clear those U-codes before returning the vehicle to the customer.

As you can see, working with hybrid vehicles will take some getting used to. Your service manual is your friend when it comes to working on these vehicles: Use it. The number of variables between different systems is astounding. The GM PHT is one of the simplest systems on the market, so it’s a good one to start with.

We’ve spent a lot of time over the last three issues discussing this system. If you’re interested in getting into this type of repair (an important step toward remaining successful in the transmis-sion business of the future), get a copy of Understanding General Motors Electric Vehicles, available from the ATRA BookStore. This book really lays out the different systems in a simple format, and it’ll make it easier to under-stand our technical information and seminars when we talk about hybrids, and One- and Two-Mode transmissions. Until next time, remember: There’s fire in that wire!

The Shock of Your Life, Part 3 of 3

42 Volt Battery Pack

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