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Cutaway of Toyota Prius showing (1) engine, (2) hybrid Synergy inverter, and (3) high voltage battery pack.
The Toyota Prius and Honda Insight hybrids have been around since 2001; the Honda Accord, Ford Escape and Lexus RX 400h hybrids since 2005; the Honda Civic, Toyota Highlander, Mercury Mariner, Chevrolet Silverado and GMC Sierra hybrids since 2006; and the Lexus GS 450h, Saturn VUE Green Line, Toyota Camry and Nissan Altima hybrids since model year 2007. In 2008, new hybrid models included the Saturn Aura Green Line, Lexus LS600h, Mazda Tribute, Chevrolet Tahoe, GMC Yukon and Dodge Durango hybrids. Today, there are dozens and dozens of hybrid models of various makes and models.
Although hybrid sales have historically represented only about 2 to 3 percent of annual new car sales, in recent years their sales have grown significantly as auto makers introduce more and more new hybrid cars and SUVs. In 2020, there were more than 6 million hybrids on the road in the U.S. The numbers are projected to grow even more as auto makers gradually shift toward more hybrid and Battery Electric Vehicles (BEVs).
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Hybrids are proving to be quite popular with motorists because of the improved fuel economy they deliver. By reducing the on time of the engine and using battery power as much as possible for low speed stop-and-go city driving, hybrid vehicles can achieve impressive fuel economy numbers.
Depending on the model year and version, a typical late model Toyota Prius has an EPA fuel economy rating of 58 mpg city and 53 mpg highway. The fuel economy of many late model hybrid SUVs is often 25 to 30 percent better than their conventional internal combustion powered counterparts. With todays high gas prices, a 25 to 30 percent improvement in fuel economy can save you quite a bit of money over time.
A hybrid vehicle is essentially the same as any other vehicle except for the extra high-voltage hybrid hardware. On a full hybrid such as the Toyota Prius or Ford Escape, this includes a unique continuously variable transmission, two electric motors, an integrated starter/alternator in the flywheel, various electronic control modules, and a high-voltage battery pack in the rear of the car. On some of the older partial hybrids, such as a Saturn VUE Green Line that lacks a full-electric mode, the only significant difference is the belt-driven starter/alternator that is used for start-stop driving and to recharge the hybrid battery in the back.
The main difference between all hybrids and other vehicles is that high-voltage hybrid battery, which is usually mounted in the rear of the vehicle. The voltage output of a hybrid battery depends on the vehicle. On a Honda Insight, the hybrid battery is 144 volts. On a first generation 2001-2003 Toyota Prius, the hybrid battery is rated at 276 volts, and 201 volts on a second generation Prius (2004 and up). On a Ford Escape, the hybrid battery is 300 volts.
The hybrid batteries used in older first generation vehicles were nickel metal hydride (NiMH). Most newer hybrid batteries are lithium-ion. A hybrid or EV battery consists of many individual cells wired together in modules.
Warning: High voltage hybrid batteries and electrical components can be deadly and must be treated with respect. If you think a shock from a spark plug wire is bad, a shock from one of these batteries can kill you in a split-second!
To protect the vehicle's occupants and service technicians from the high-voltage hazard, the hybrid power circuit is heavily insulated and is usually color-coded ORANGE. So if you see a heavy orange cable snaking under a vehicle or in the engine compartment, it is carrying the hybrid high-voltage current. The circuit may or may not be hot even when the engine is off, so treat all orange cabling with caution.
Hybrids still have an ordinary 12-volt battery for powering the ignition system, fuel pump, lights and other electrical accessories on the vehicle. The battery is often an AGM style battery rather than a liquid acid lead-acid battery. In the case of the Toyota Prius, the 12-volt battery also has an oddball post configuration that is smaller than standard posts, and the battery is located in the trunk.
No special precautions are required when replacing most maintenance or repair parts on the non-hybrid components in the vehicle. But if any repair work involves hybrid electrical or powertrain components, the hybrid battery must be disconnected prior to touching anything that might carry high voltage to prevent shock hazards.
High voltage wiring is usually color coded ORANGE. Do NOT touch!
High voltage cables in hybrid vehicles are well insulated and color-coded to warn you of their potential danger. Although most high voltage hybrid wiring cables are color-coded ORANGE, some older hybrids such as the Saturn Vue color-coded their high voltage wiring BLUE. On the Saturn, the system voltage is only 36 volts, not enough to kill you but enough to create a shock hazard. In all cases, avoid contact with these cables unless the high voltage hybrid battery in the back of the vehicle has first been disconnected.
The procedure for isolating the hybrid battery varies depending on the vehicle, but typically involves flipping a switch on the hybrid battery pack or disconnecting a battery cable or fuse. On a first generation Toyota Prius, the hybrid battery is disconnected by opening the trunk, removing the liner from the left front corner, and pulling straight back on a small orange handle to remove the battery connection plug.
Always refer to the vehicle manufacturer's recommended disconnect procedure. You can usually find this information in your Owners Manual.
Wearing protective rubber gloves that are rated to withstand up to 1,000 volts is also recommended for added protection. Also, do not touch any high-voltage components for at least 10 minutes after disconnecting the battery. This gives the high-voltage capacitors in the hybrid control system time to discharge.
Actually, hybrid vehicles are not as dangerous to work on as they might seem at first. If the key is OFF and the key is out of the vehicle (make sure the "READY" light if off), the hybrid system is powered down. The battery cannot shock anyone unless they go poking around the high-voltage battery connections with bare hands or uninsulated tools. Even so, there are some hidden dangers with these vehicles.
When the key is in the ignition (or the keyless entry fob is inside the car), and the Power button is pushed on a Toyota Prius, a Ready light on the dash comes on. This means the hybrid powertrain is active and is ready to go, even though the engine is not running and the car is not making any noise that would indicate it is on. So when the Ready light is on, the engine may suddenly start itself without warning if the hybrid battery is low and needs to be recharged. This presents no danger to the driver because the car will not go anywhere unless it is in drive or reverse, but it could be a hazard to someone if they were working under the hood and did not realize the Ready light was on. So always make sure the Ready light is out before doing any routine maintenance or repairs.
One question many people who own a hybrid or who are thinking about buying a new or used hybrid have is the durability of the hybrid battery. On a Toyota Prius, Ford Escape and other hybrids, the factory warranty on the hybrid battery and other hybrid powertrain components is eight years or 80,000 miles, or 10 years and 150,000 miles if you live in California. The actual design life of the battery, according to Toyota, is 15 years and more than 200,000 miles.
So far, the hybrid batteries have proven to be extremely reliable and trouble-free, which is a good thing because (1) replacement the hybrid batteries are hard to get and may only be available from a new car dealer (although there are some aftermarket companies who offer "reconditioned" hybrid batteries), and (2) they are very expensive. The list price for a brand new replacement hybrid battery may be $3000 to $4000 or more!
Hybrid batteries generate a lot of heat and require extra cooling. Most hybrids have some type of special venting for the battery pack, which may even include a separate cabin air filter (Ford Escape, for example). On the Prius, there is a cooling fan for the battery inside the right rear trim panel, and two battery temperature sensors in the hybrid battery compartment.
In theory, a hybrid battery should never run down. The control module should start and run the engine to maintain battery charge anytime the battery drops below a certain voltage. But if a vehicle is not driven very often, sits for weeks at a time in a garage, or has a problem that drains the battery or prevents the engine from running to recharge the battery, the hybrid battery can go dead. If this happens, a special jump start procedure or charging procedure may be required to get the vehicle moving.
On a Prius, there is a special jumper connection under the power distribution center cover in the engine compartment. A 12-volt battery charger can be used to boost the regular 12-volt battery enough to start the engine (Toyota recommends using their special 12-volt charger instead of a conventional 12-volt battery charger). Once the engine is running, it should be left running for at least 30 minutes to recharge the hybrid battery. No attempt should be made to recharge or jump start the high-voltage hybrid battery directly.
NOTE:. If the 12-auxiliary battery has failed and needs to be replaced on a Prius, the replacement battery must be the same type as the original. The Prius uses an AGM (Absorbant Glass Mat) gel cell battery which has somewhat different charging characteristics than a conventional lead-acid wet cell battery.
Hybrid vehicles have the same onboard diagnostics (OBD II) as every other vehicle built since 1996. You can access fault codes, freeze-frame data, sensor inputs and various self-tests with a scan tool, provided the scan tool is up-to-date with the latest software for the year/make and model of vehicle you are trying to diagnose. So you do not have to take a hybrid back to the new car dealer if the vehicle is out of warranty.
The software in professional grade scan tools has more capabilities than that in your typical DIY scan tool. Even so, for basic troubleshooting, an up-to-date DIY scan tool should be able to give you fault codes, voltage readings and sensor data.
If a problem occurs with something that is not part of the hybrid system, diagnostics is essentially the same as on any other vehicle. You troubleshoot the fault, do your diagnostic tests to rule out various possibilities, and hopefully zero in on the faulty component that needs to be replaced. Whether you have a P0300 random misfire code or an O2 sensor code, the diagnostics would be essentially the same as on any other gasoline engine.
As for faults that involve the hybrid system (things like the car won't start, it stalls or does not run right, battery problems, etc.), you need to be up to speed on hybrid technology, and have access to the factory service information (see OEM technical websites).
Craig Van Batenburg is one of the foremost hybrid trainers for the aftermarket. For more information, visit Craig's website at www.fixhybrid.com or call 800-939-7909. Denso is also offering hybrid training classes through their parts distributors, as are other aftermarket parts suppliers.
To access the onboard diagnostics you need a scan tool such as the MasterTech with up-to-date software that can access most of the PIDs and self-tests that are available on the Toyota factory scan tool.
If the Malfunction Indicator Lamp (MIL) is on, plug in your scan tool to read the code(s) and any freeze-frame data that might have been captured to help you in your diagnosis. Then, depending on what the code is, you will want to look at various related PIDs to see what is going on with the sensors and system controls. Problem areas that are going to require some hybrid know-how include any faults related to the high-voltage battery or charging system, any faults related to engine cranking, any faults related to the transmission, or any faults related to the hybrid control system.
Like other manufacturers, Toyota provides detailed diagnostic charts for all their fault codes. The charts include a description of the fault, the components involved, wiring diagrams and step-by-step checks that need to be made to isolate the fault.
Click Here for more information on Toyota Prius diagnostics.
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