Leaky Power Electronics Coolant Seals Note: When filling the battery pack cooling system make sure to follow proper vac-n-fill procedures and the Hybrid/ EV Battery Pack Coolant Bleed Procedure located in GDS2 under HPCM2 control functions. On my personal GEN II 2004 Prius at around 100,000 miles I had to access the ABS module connections for a problem unrelated to anything high voltage. In order to access the ABS connector, I had to power down the HV system and remove the top cover to the power electronics. While I had the power electronics cover off on my Prius I took the opportunity to look around at the various components with a bright flashlight. To my surprise I found a few traces of coolant - inside the inverter assembly at the bottom of the area where the electronics are mounted! There were no symptoms of a problem or DTCs for any HV leaks - just a little coolant puddled up. Had there been more coolant in the inverter assembly's electrical component area or had the coolant that was present been more conductive, they would probably have been an issue. As with using the OEM recommended refrigerant oil (special dielectric Esther oil) on electric compressors to eliminate any chance of HV leakage, some OEMs are requiring either distilled water or DI water or strictly DI water. (Fig. 9 & 10) Important: Use of any coolants other than the GM recommended premixed coolant may set internal battery loss of isolation faults. If the proper GM premixed Dexcool is not used and there is a current loss of isolation code, you will need to remove the coolant from the hybrid battery cooling system and replace with the proper mix. Photos Dave Hobbs When filling the engine cooling system it is preferred to use the same premixed DEX-COOL®:part number 12378390 US, 10953456 Canada, coolant listed above. Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. Figure 9 and 10: 2009 Cadillac Escalade 2-Mode Hybrid. Surge tanks appear to be one but are actually separate. The one on the right (passenger side) is for the hybrid power electronics cooling system. On this application the HV battery is not liquid cooled. The power electronics located on right rear section of the engine compartment are liquid cooled however. Again, GM is being very emphatic that tap water NOT be used. Distilled water is OK in this case. May 2017 6 MACS Service Reports