This article was originally published May 1, 2019. Some of the information may no longer be relevant, so please use it at your discretion.
Cooling systems. Could there be a more boring topic? That is exactly what I would be thinking if I were reading this article right now, but I would be wrong.
Hybrid and electric vehicle cooling systems are anything but boring. They will challenge you, frustrate you, and make you yearn for the good old days before all of this started. Unless you have had no internet access, you have probably read the entire automotive industry is moving towards the electrification of their vehicle lineup. It is time to get the proper training and tools to service these complex cooling systems.
Two years ago, our automotive technology department received a grant to develop and provide hybrid and electric vehicle training to teachers of automotive programs at high schools and other colleges in our state. The purpose of the training was to help prepare the next generation of service professionals for jobs in the electrified automotive industry of today and tomorrow. We purchased three new electrified vehicles for this training:
- A Hybrid-Electric vehicle (HEV)
- A Plug-In Hybrid-Electric Vehicle (PHEV)
- A Battery Electric Vehicle (BEV)
As part of the curriculum development process, I began exploring the technology of each of these vehicles. My exploration included completely removing and disassembling all of the high voltage components and documenting these efforts on video for my students. One technology I had not thought much about is their cooling systems. As you will see, some of these cooling systems are complex with multiple coolant loops, switching valves, one-way valves, chillers, heaters, pumps, and dozens of hoses. PHEVs have the most complex systems, followed by HEVs and then BEVs.
All of these cooling systems require special procedures for diagnostics, service, maintenance, and repair. For this article, we will concentrate on the liquid cooling systems. However, some hybrid and electric vehicles use air cooling for some of their components.
Hybrid-Electric Vehicle (HEV) cooling systems
The first vehicle I explored for our training project was a 2017 Toyota Prius HEV. We picked the Prius for training purposes because it has been the top selling hybrid in the U.S.A. for the last 18 years. Any HEV will have a complex cooling system due to the fact that an Internal Combustion Engine (ICE) is still involved in propelling the vehicle. The Prius is a series-parallel hybrid; this hybrid type has the most complex cooling system when compared to series hybrids and parallel hybrids. The Prius has five coolant loops as shown in Figure 1.
Prius Internal Combustion Engine (ICE) Cooling
The 2016-2019 Prius has four parallel coolant loops that are connected to the upper section of the radiator just for the ICE. This cooling system has 11 major components and 14 coolant hoses.
- ICE Cooling Loop 1, for ICE Cooling
- ICE Cooling Loop 2, for expansion and Air Bleeding Loop:
- ICE Cooling Loop 3, for Exhaust Gas Recirculation (EGR) Cooling and Throttle Deicing Loop
- ICE Cooling Loop 4, for Exhaust Heat Recovery for Fast Warm Up Loop
Prius Power Electronics (PE) and Transaxle Cooling
There is a single coolant loop connected to the lower section of the radiator for the high voltage electronics and transaxle. This cooling system has five major parts and six coolant hoses.
Prius High Voltage (HV) Battery Cooling/Heating
The HV battery on the Prius is air cooled/heated with a single cooling fan pulling in air from the passenger compartment and pushing it out the one-way pressure relief vents in the rear quarter panels.
Plug-In Hybrid-Electric Vehicle (PHEV) Cooling Systems
The second vehicle I explored for our training project was a 2018 Chevrolet Volt PHEV. We picked the Volt for training purposes because it has the longest range (53 miles, 85.3 km) of any PHEV sold in the U.S.A. As you may know, GM recently stopped production of the Volt and has made a commitment to move to a Battery Electric Vehicle (BEV) lineup.
The 2016-2019 Volt has the most complex cooling system I have ever seen in a vehicle. The Volt has seven coolant loops, 25 major components, 31 coolant hoses, and three electric coolant pumps. The incredible complexity of the Volt cooling systems is a result of having almost all the same basic components found in the Prius HEV, plus four additional liquid cooled components:
- The On-Board Charging Module (OBCM): Located in the truck area, this module is used when an Alternating Current (AC) charge cord is plugged into the vehicle to charge the HV battery. This module does not develop heat unless the charge cord is plugged in. Although the vehicle is typically powered off while the charge cord is plugged in, it is normal for coolant pumps, fans, heaters, or the air conditioning (A/C) compressor to run to maintain battery temperature.
- The Accessory Power Module (APM): Also located in the truck area, this module provides power for the 12V system and charges the 12V battery. It develops more heat when the demand for current in the 12V system increases.
- The HV Reserve Energy Storage System (RESS): RESS is GM’s name for the HV battery in their vehicles. It develops heat while both charging and discharging. For optimum performance, the HV battery must be heated in cold weather and cooled in hot weather.
- HV Cabin Coolant Heater Control Module: This module is used to heat the passenger compartment with the ICE off.
Volt Internal Combustion Engine (ICE) Cooling
The 2016-2019 Volt uses three parallel coolant loops that are connected to the ICE radiator. This cooling system has seven major components and 11 coolant hoses. Additionally, the Volt uses a heated electro-mechanical thermostat that is controlled with a Pulse Width Modulated (PWM) signal from the Engine Control Module (ECM).
- ICE Cooling Loop 1, for ICE Cooling
- ICE Cooling Loop 2, for EGR Cooling
- ICE Cooling Loop 3, for Expansion and Air Bleeding
Volt Power Electronics (PE) and Transaxle Cooling
There is a single coolant loop connected to a PE radiator for the high voltage electronics and transaxle. The PE radiator is actually the upper portion of the A/C condenser. This coolant loop has six major parts and 10 coolant hoses.
Volt High Voltage (HV) Battery Cooling/Heating
The HV battery on the Volt uses a single coolant loop with an external coolant chiller (a mini-evaporator connected to the A/C system), and a 1.5 kW internal heater, coolant hoses, cooling manifolds, and cooling plates. These cooling plates have tiny coolant passages. Never use stop leak or used coolant, or cooling passage restrictions can occur.
Volt High Voltage Cabin Heating
Cabin heating uses two coolant loops. The Volt uses a HV 7.5kW electric heater in the right front fender to heat the coolant before it is fed to the heater core. In certain low temperature conditions, the ICE can be activated to help add heat to the coolant for additional passenger comfort.
Battery Electric Vehicle (BEV) cooling systems
The third vehicle I explored for our training project was a 2017 Chevrolet Bolt EV (BEV). We picked the Bolt EV for training purposes because at the time, it had the longest range (238 miles, 383 km) of any electric vehicle for less than $40,000. Obviously, there is no ICE to complicate things, but without waste heat created by an ICE, BEVs need a way to heat the coolant for the heater core in the passenger compartment. This means that BEVs have an extra coolant loop that most other vehicles do not have. The Bolt EV has three coolant loops as shown in Figure 2.
Bolt EV Power Electronics (PE) Cooling
There is a single coolant loop connected a dedicated PE radiator for the HV electronics. This cooling system has seven major parts and 10 coolant hoses.
Bolt EV High Voltage (HV) Battery Cooling/Heating
The HV battery on the Bolt EV has an external 2.5 kW heater, external coolant chiller (a mini-evaporator connected to the A/C system), and internal cooling manifolds, cooling plates, and coolant hoses.
Bolt EV High Voltage Cabin Heating
The Bolt EV uses a HV 7.5 kW electric heater to heat the coolant before it is fed to the heater core to heat the air in the passenger compartment.
Cooling System maintenance procedures
In North America, HEVs have been around for almost 20 years, PHEVs and BEVs have been around for almost nine years. I am sure their coolant hoses have hardened, their coolant has degraded, and their heat sinks have started corroding, but they continue to function well. If I operated a service center, I would look into the maintenance guides for these vehicles and selling the required services at the correct intervals.
General Cooling System information
ICE and PE Coolants
Each vehicle manufacturer has their own coolant recommendation based upon the materials in which the coolant must flow as well as the operating conditions in which it must exist. Coolants for HEVS, PHEVs, and BEVs are typically the same coolant used in the ICE, but with the following additional precautions and warnings:
- Only use new, pre-mixed 50/50 Coolant upon refilling the cooling system. Failure to use new coolant, the correct type of coolant, or the correct 50/50 ratio of coolant to distilled or de-ionized water can cause:
- Cooling fin corrosion inside the heat generating Power Electronics components leading to poor heatsink performance, overheating, and eventual premature failure.
- Restriction of the passages inside the HV Battery cooling/heating plates. This can lead to overheating, setting Diagnostic Trouble Codes (DTC), and HV system shutdown
- Loss of HV isolation at a battery coolant heater element. This will set DTCs and shut down the entire HV system.
Scheduled maintenance
Toyota
The pink colored Super Long-Life Coolant (SLLC) is scheduled to be replaced in the:
- ICE, every 10 years or 100,000 miles (160,000 km) and then every five years or 50,000 miles (80,000 km) afterwards.
- PE, every 15 years or 150,000 miles (241,400 km) and then every five years or 50,000 miles (80,000 km) afterwards.
General Motors (GM)
The orange colored DEX-COOL coolant is scheduled to be replaced every five years or 150,000 miles (241,400 km).
Nissan
The blue colored premixed Nissan Long Life Coolant used in their BEVs is scheduled to be replaced every 15 years or 125,000 miles (200,000 km).
Tesla
The purple colored Tesla G-48 ethylene-glycol Hybrid Organic Acid Technology (HOAT) coolant in their BEVs is scheduled to be replaced every eight years or 100,000 miles (160,000 km).
Note: Any damage caused by opening the battery coolant reservoir is excluded from the warranty.
Service procedures
Pressure testing
Pressure testing the cooling systems for leaks is permitted. However, keep in mind that some low temperature PE cooling system pressures as low as 5 psi (35kPa) may be much lower than the 20 psi (140 kPa) for the high temperature ICE cooling system. Over-pressuring a system could cause coolant leaks and possible component damage.
Note: If the PE or HV Battery coolant levels are low, the vehicle may not be safe to drive. A leak test and visual inspection must be performed. Some batteries have an inspection plug to check for coolant leaks.
Vacuum fill or air bleed?
After draining the coolant, a vacuum fill procedure is required for refilling the PE and RESS cooling systems on the Chevrolet Volt and Bolt EVs. This method works well at removing the air from these complex systems before pulling the coolant into the system. The Toyota service information only recommends a traditional air bleeding procedure for their PE cooling systems.
Many Toyota HEV have air bleed valves to help remove trapped air. I believe the vacuum fill method would work great for the Toyota HEVs.
Scan tool usage
After the vacuum fill or traditional air bleeding, an electric water pump must be activated with a scan tool to circulate the coolant and remove air bubbles. If all of the air is not removed from the inverter coolant loop of a Prius, inverter damage can occur and trigger up to 17 non-recoverable trouble codes.
Hose clamps and hoses
Some original spring style hose clamps are glued in place on the hose. When attempting to remove the hose clamp, it is critical that the clamp is not rotated or repositioned, so that is tears or damages the hose. You must replace the hose if the clamp needs replacing.
Active shutters
Newer vehicles have active grille shutters to control airflow through the radiator and improve aerodynamics at higher vehicle speeds. A scan tool can test the function of the shutters, and for DTCs, if an overheating issue is encountered.
Summary
You may have noticed that there was not much information on Tesla coolant system loops. That is because Tesla will only allow access to their service information if you live in the state of Massachusetts. Our school in Utah has a Model S P100D, and I cannot disassemble anything on it without risking a loss of warranty coverage. With more Tesla vehicles on the road today, that will need to change soon, but it will not happen until Tesla trained technicians are available to service these vehicles.
We have covered a lot of material in this article, but there is much more to learn. Hopefully you have learned enough from this article to determine if you need more training on this and other hybrid and electric vehicle topics. These complex cooling systems are critical to the proper operation of these systems.
My best advice for you when working on one of these vehicles is to purchase a short-term or a long-term subscription to the factory service information through NASTF and follow the exact procedures recommended by each vehicle manufacturer. Best wishes!
