Automobile Repair/Toyota/Prius

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General[edit | edit source]

This document refers to the Toyota Prius 2004-2009 (NHW20) and contains information only where details for the Prius differ from 'ordinary' vehicles. The Prius doesn't lend itself to home maintenance beyond the usual oil/coolant levels, tyres, etc. Almost everything electrical is computer controlled so special equipment is required to diagnose faults or change parameters. Also, worth mentioning, is the high voltage battery which can give a nasty jolt to anyone who takes liberties with it. Having said that, there are still steps that owners can take to minimise future problems and even some relatively minor problems that can be easily and safely fixed by the owner.

Electrical[edit | edit source]

Prius Has Two Battery Systems[edit | edit source]

Prius uses two battery systems: A high voltage battery system (several hundred volts! Note DANGER to untrained mechanics!) often referred to as the traction battery. This battery sits under the rear seat or under the floor in the rear cargo area. Its main function is to power the electric traction motor(s), start the gasoline engine and store regenerated power. A second battery, the auxiliary battery, powers computers that manage Prius systems, vehicle lighting, electric door locks, electric windows, the radio, etc. This is a 12-volt battery and electrical system much like traditional automobiles except it does not need to be powerful enough to run a starter motor. In earlier models, it is located in the rear cargo area and in the later models, in the engine compartment.

Discharged Auxiliary Battery[edit | edit source]

The 12 volt lead acid auxiliary battery, located underneath the cargo area near the right interior fender, is quite small and will discharge if the car isn't used for more than a couple of months. This battery powers the vehicle's entire 12 volt bus, including body and powertrain computers, window and lock actuators, exterior and interior lights, various accessories, and it initiates the hybrid control system when the driver switches the vehicle into ready mode. Note however, the auxiliary battery doesn't power the reciprocating engine's starting motor (motor generator 1 - MG1). The high voltage traction battery powers the starter.

A total loss of 12 volt power from a discharged or disconnected auxiliary battery can cause unforeseen problems. These are not insurmountable or even serious, but can be annoying. These issues need to be restored by clearing the error codes from the system (see below). You may also have to reset the clock and your radio station settings. A discharged battery can also make accessing the inside of the car difficult (especially the cargo area), as electrically actuated locks or latches may be inoperable. It's a good idea to use your manual key at least once a month for entry to avoid lock seizure, as this may be the only way to access the interior should the auxiliary battery become completely discharged.

Jumpstart Terminal

Jumpstarting the Prius[edit | edit source]

WARNING: Take great care to get the jumper leads the right way round! Connect Red to Red (+) and then Black to Black (-). Mixing up the polarity (+ to -) WILL cause extremely expensive damage to your Prius!

Prius can be jump started just like any other car from under the hood (bonnet), in the engine compartment. With earlier Prius models, access to the battery in the rear is not needed; there is a jump start terminal hidden under a Red plastic cover in the fuse box. In a later model Prius, the auxiliary battery is in the engine compartment under the hood, so connect the Red jumper cable to the positive battery terminal hidden under a Red cover. For any Prius model, see warning above.

Sufficient power from the donor vehicle is only required to start the hybrid control system, far less than a normal vehicle's starter motor would require. Connect the Red +12 volt jumper lead to the jump terminal or the +12 volt Red battery terminal, depending on the Prius model. Then connect the Black negative ground lead to any substantial bare metal part in the engine compartment. Once connected, just start the Prius as normal. The leads can then be disconnected. There is no need to leave them connected once the hybrid system has started even if the engine hasn't fired up. This sometimes confuses drivers of recovery vehicles who think the Prius has failed to start or has stalled. The Prius only needs to indicate READY. When READY is on, the Prius will now charge the auxiliary battery itself.

WARNING: Do not attempt to push start a Prius.

Recharging the 12 volt battery[edit | edit source]

Unlike normal cars, the Prius does not charge the 12 volt battery via an alternator spun by the engine. This vehicle charges its 12 volt battery from the hybrid system, so if you are unable to take the car out for a run for any reason, just switching the system on for a while will do the job. To maintain charge in the 12V battery if the car is not being used often, Toyota recommend putting the car in 'Ready' mode for one hour, once per week. If the battery is completely flat, the vehicle may need to be put in 'Ready' for 4 - 8 hours to fully recharge. Be aware that the engine will start occasionally to ensure the high voltage battery doesn't discharge - so this should only be done in a well-ventilated area, not a closed garage. Doing this will also wreck your miles per gallon record. Alternatively, the 12 volt battery can also be recharged using a standard car battery charger.

Notes about the high voltage battery[edit | edit source]

The high voltage battery will never discharge under normal operating conditions. The Prius strives to keep the battery within normal parameters while the car is being driven and it is physically disconnected via a relay when the car is shut down. It is charged automatically by energy reclaimed by the hybrid system when the vehicle is slowing down and by running the main engine when required. Barring faults, there are two ways to discharge the high voltage battery; run the car without fuel, thus preventing the engine starting, or putting it in Neutral. This is to be avoided except in emergencies and a specific warning is given in the vehicles handbook about this. If access is needed to the battery, there is a safety plug, close to the battery itself, which can be used to disconnect it completely for maintenance purposes. Having said that, there is no need for the average owner to go near that battery and it is not necessary to unplug it if you're not intending to use the car for some time.

The Diagnostic Trouble Codes[edit | edit source]

OBD-II Socket Pinout

The Diagnostic Trouble Codes or DTC's (also known simply as fault codes) are stored automatically whenever any of the sensors are outside of predetermined parameters. For instance, on the Prius, there are sensors at the braking systems master cylinder and also at the calipers. If the sensors show different readings, the system logs this as a fault. The fault codes are not generally cleared automatically; they are stored until reset manually with an OBD-II diagnostic tester. If you have such a tool and wish to interrogate the system and/or reset the fault codes, you can just plug it in and follow the instructions for the particular tool you are using. If you don't have an OBD-II tester, you can get away with a Special Service Tool (SST) 09843-18040 from Toyota. This is used to safely link particular terminals together in order to get the system to display the stored fault codes and respond to instructions given via the vehicles sensors. Warning lights on the dashboard that refuse to extinguish even when you are sure there is no fault can be due to stored fault codes.

To clear the stored fault codes using the SST[edit | edit source]

  1. Use the SST to connect terminals 4 and 13 of the OBD-II socket.
  2. Turn on the vehicles power (just press the power button).
  3. Press the brake pedal at least eight times within five seconds.
  4. Check that there are no warnings displayed on the dashboard - if warnings are still displayed, turn the power off and repeat from step 2.
  5. Unplug the SST.

Radio - poor reception[edit | edit source]

Poor reception on the Prius, including crackling, on both AM and FM is usually a grounding problem at the antenna end. The antenna is mounted on the roof, right at the back in the centre. The fix then is simply to gain access, unbolt the antenna, clean the antenna and the area of the roof where it makes contact, smear some dielectric grease on the contact point to prevent corrosion in the future (petroleum jelly will suffice) and put it all back together. Unfortunately, in order to gain access to the contact point in order to clean it, you'll need to remove the rear internal light assembly and drop the headlining at the back.

Auto (power) function of electric windows not working[edit | edit source]

Disconnecting the 12 volt battery in a Prius (or allowing it to go flat), will cause the automatic function of the power windows to fail. They need to be recalibrated before they will work properly again. Fortunately this is an easy task. Just use the switch to lower the window to about half way. Then use the same switch to raise the window. Hold the switch in the (up) position for at least a second after the window stops moving. If that fails, try disconnecting the battery again for a few minutes, reconnect it and try again, holding the switch for a couple of seconds after the window stops.