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48-Volts and beyond: Hybrid changes and DTC tips

Tuesday, January 1, 2019 - 09:00
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High voltage hybrids have been out close to two decades. More recently, 48-volt micro hybrid electric vehicles have been introduced in Europe and will soon be spreading to American roadways starting with select 2019 Dodge, Jeep and Mercedes models.

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Figure 1 - This GEN IV Prius (2016-Present) HEV has had the back seat removed  and the 12-volt High Voltage Interlock connector removed from the back or the HV service plug’s connector. Key on and connector removed – 12-volts. P0A0D sets.

All this greatly increases your chances of having to work on hybrids and EVs beyond the realm of preventative maintenance. To prepare you for today’s higher voltage hybrids as well as tomorrow’s lower voltage 48-volt systems, we’ll go over some common hybrid DTCs to help you diagnose faster by understanding how they work and why they set.

P0A0D – High voltage interlock circuit voltage high

The title of this common DTC is a bit misleading. The circuit that sets the DTC itself is NOT a high voltage circuit but rather a 12-volt circuit that is a watchdog for high-voltage components (Figure 1). On GM 2-mode HEVs for example, if you remove the hard plastic cover that hides the inverter and DC-DC converter assembly a shorting bar in that plastic cover is now pulled from a connector in the HV interlock circuit. A similar arrangement is on Toyota inverter cover plates  (Figure 2). A DTC P0A0D then sets and the high voltage contactors in the battery pack open (if they were closed) or won’t close if you then try to power up the vehicle. On Fords (Figure 3), this circuit will be attached to the major orange high voltage cables to insure those cables are fully seated/latched. On many hybrids, simply extending the high voltage battery pack’s service disconnect plug (Figure 4) prior to unlatching it will remove a shorting bar in the service plug from the terminals in this circuit. The whole point of this circuit is for safety.

Figure 2 - This GEN III Prius’ (2010-2015) inverter assembly cover has been removed to expose the HV battery pack’s cable connections. With the white plastic shorting bar connector remaining on the inverter access cover, a P0A0D will set and the system will not ‘boot up’ due the shorting bar not being in place.
Figure 3 - This 2010 Ford Fusion hybrid model  can set a P0A0D “High Voltage Interlock Circuit Voltage High” DTC if one of the 12-volt circuits (small wires) are not seated when the larger orange HV cable connections are not completely latched.

P0AA6 – Hybrid battery voltage system isolation fault

Battery packs are subject to leakage DTCs as are motor generators, HV electric AC compressors and high voltage cables. Full-voltage (over 60 volts) HEVs and EVs connect both of their HV battery pack’s cables directly to the inverter. The negative DC HV cable does NOT connect directly to chassis ground. However, if you have ever connected a DMM set to DC between chassis ground and either of the HV battery pack’s positive or negative cables you will read half of the HV battery pack’s voltage on Toyotas. On Fords, you’ll see something curious; a rhythmic sweeping of this voltage moving from 0 volts up to about 70 percent of the HV battery pack’s total voltage. This sweeping action takes just a couple of seconds. This “diagnostic” high-voltage circuit carries a harmless low current – about 2 mA. It is there to inform the vehicle’s electronics of the even the smallest amount of high voltage leakage. Similar in design intent to a bathroom/kitchen’s GFI (Ground Fault Interrupt) they cause the disconnection of high voltage power for safety’s sake. The down side is a no start condition will occur on HEVs that use their MGs for starting the gas engine. Isolation fault detection use high ohm (typically around 150K ohms) resistors in parallel between the high voltage circuit and chassis ground (Figure 5).

Figure 4 - Newer Honda HEVs are now using orange service plugs like other OEMs. The larger contacts complete the high voltage connections within the battery pack. The smaller contacts are for the high voltage interlock (safety) circuit which is actually a 12-volt circuit. If the service plug is NOT inserted AND latched properly, the high voltage interlock circuit voltage goes to 12-volts to indicate a problem.
Figure 5 - SMRs 1,2, & 3 (a.k.a. contactors) close in the order of their number to allow the power resistor to 1st feed limited current to the inverter on battery pack power up to eliminate inrush current damage to inverter.  Whenever contactors are closed the high ohm resistors tied between each of the high voltage circuits and ground. This allows only a couple of mA of high voltage to flow to ground while allowing the battery smart unit to monitor for high voltage leakage.
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