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There are more than seven million hybrids on US roads, and they all need maintenance. Many hybrids carry more service items than conventional cars because of their hybrid systems, which is business shops should not be leaving on the table.
Before You Start
About 90 percent of hybrid repairs do not involve the high-voltage system. Most service items concern mechanical systems and the 12-volt battery, but you must still know which systems are live while you work.
Start with the key. Hybrids generally use proximity keys, and bringing the fob near the vehicle can trigger a system start. If you do not want to take that chance, keep the fob away from the bay or put it in a pouch that blocks radio signals. You can also use a scan tool to enable or disable specific modes, or even to start the engine for diagnosis or an oil level check.
If the high-voltage system has to be shut down, most models have a service plug that disconnects the high-voltage battery from the system. Always consult the service information before pulling that plug, because procedures differ between manufacturers: some require the 12-volt battery to be disconnected first, others require the system to be powered down with a scan tool. Never assume, as hybrid systems vary considerably from one model year to another.
Cooling Systems
Most hybrids have a separate cooling circuit for the inverter and motor/generator inside the transaxle, usually with its own electric pump, heat exchanger and specific coolant. Most hybrids have two coolant reservoirs under the hood, and both systems must be kept at the correct level. Hybrid battery systems generate a lot of heat: the traction motor heats up as it runs, the inverter produces heat while converting DC to AC, and the battery heats while charging. The coolant in these circuits is often specialised and must be non-conductive. Most systems use at least two electric pumps to circulate coolant, with valve-controlled flow cooling the motor, high-voltage battery and inverter at different rates. When changing coolant, a scan tool is needed to operate the valves so the system can be filled and bled.
Some hybrids include a coolant retention tank in the engine cooling circuit that keeps coolant at about 82 C (180 F) for up to three days. When the engine starts, the warm coolant is released into the cooling loop through valves and a pump, and draining, flushing or bleeding again requires scan-tool operation.
Engine Oil
Hybrids run the engine less at low speed and in stop-start driving, so oil changes can be less frequent, but the manufacturer's interval still applies. Engine starting is the single biggest source of wear, and hybrids start and stop the engine more often than conventional cars. Plug-in hybrids are hardest on oil: on some trips the engine runs only very briefly, and repeated short journeys let condensation accumulate in the oil, in severe cases turning the oil on the dipstick milky. The only real fix is an updated calibration for the engine and hybrid system that runs the engine to normal operating temperature and evaporates the water out of the oil. These vehicles must get quality oil, and some newer hybrids call for 0W-20 or 0W-16; the wrong viscosity can damage the engine.
Brake Fluid
Hybrid braking systems include regenerative braking components. On most hybrids, caliper pressure under normal conditions is not a direct result of the driver pressing the pedal: pedal force triggers a pressure sensor that generates a braking request, and hydraulic pressure at the wheels is produced by the pump and accumulator in the ABS hydraulic unit or by a pedal simulator. When changing brake fluid, a scan tool must be used to open and close the valves in the hydraulic unit and pedal simulator to purge air. Some newer hybrids specify DOT 4+ or low-viscosity DOT 4 brake fluid, which is thinner than DOT 3 and ordinary DOT 4 and is designed for the high-pressure pumps and valves in hybrid braking systems.
Transmission Fluid
In every hybrid automatic transmission, the fluid does not contact the motor drive section. There have been isolated cases of transmission fluid leaking into the motor housing and damaging insulation, and any leak must be taken seriously. Some models require a dedicated CVT fluid, others a specific grade compatible with conventional transmission fluid specifications. Always verify the manufacturer's specified fluid: grades differ in viscosity, so do not assume.
Air Conditioning
Some hybrids use an electrically driven air-conditioning compressor rather than a belt-driven one. The motor windings sit in the compressor lubricant, and the oil used in these compressors has high dielectric, non-conductive properties. If the wrong oil gets into the compressor, winding insulation is damaged or weakened, and the worst case is that the oil conducts high voltage to the compressor housing or connected components, creating a shock hazard when a person touches the compressor, engine or other parts and completes a path to ground.
