I check the coolant tank levels regularly and recently noticed that the battery coolant level was about 1/2" lower than the top of the decal where it is supposed to be. I know it was previously exactly even with the top of the decal. The coolant level of the electronics tank is higher and about where it is supposed to be. After noting this change I marked the level of the battery coolant level with a piece of masking tank so that I could monitor if it was falling over a period of a few days. It seems to be holding the same level. All of the other cooling reservoirs, including the adjacent electronics reservoir were at their proper levels.
Because the battery tank reservoir has a tamper resistant bracket surrounding the tank cap and because I didn't want to confuse the situation I decided to take the car to the dealer I purchased it from new for an evaluation. I explained to the service writer that the battery coolant level was slightly low and asked them to perform a coolant pressure level check of the expansion tank as per the service manual and if there was a loss of pressure to then go on and perform a battery compartment pressure test.
I just got a call from the dealer informing me that they have determined that there is an internal leak between the battery coolant reservoir and the electronics cooling reservoir. I assumed that these were physically separate tanks but apparently there is one tank molding with some sort of barrier that forms the two tanks. According to the dealer somehow coolant is flowing from the battery coolant section into the electronics section and maybe back.
So the dealer says that a new expansion tank/reservoir must be ordered. It is not stocked by the dealer and is a special order item that must be paid for in advance. Their price for the tank is $134, more than twice the discounted price of gmparts online. The more troublesome part is the $481 labor charge to remove and replace the tanks. I was not able to speak with the tech but I assume that he observed coolant flowing from the battery reservoir to the electronics reservoir when he pressurized the battery reservoir. Since the proper level for the electronic reservoir is close to top of the tank there is not much room to accommodate much fluid flow from the battery reservoir.
Call me paranoid but I am a bit skeptical that a plastic tank with an internal presumably molded internal barrier to form two separate tanks somehow breaches that internal barrier after 8 years of proper operation. I could see a manufacturing molding failure that would render the item defective immediately but I don't see what would cause a failure after eight years.
Because of the GM strike the dealer cannot predict when they may be able to get a replacement dual battery/electronics expansion tanks. Therefore I asked that they just put things back together and return the car. The tech filled the battery coolant expansion tank/reservoir about a half an inch ABOVE the top of the decal proper level. This level is above the electronics reservoir. If there was some sort of failure in the internal tanks barrier I would think that coolant would flow from the battery section to the electronics section to form a equilibrium level for each. Now maybe my assumption that the tank failure is in the internal barrier, but maybe there is some other fault pathway.
As an aside, some years ago there was a flurry f activity on this forum about Service High Voltage System faults due to the a defective battery coolant level sensor at the bottom of the reservoir. WOPonTour developed a device that mimicked the electrical characteristics of a properly operating coolant level sensor at proper level. This replacement device simply always gave the signal that battery coolant level was OK, even if it was not. Therefore, one had to make sure to diligently follow the owner manual instruction to check coolant levels frequently. At that point I had not experienced the Service High Voltage System error but decided to order and install the WopOnTour device to avoid such condition due to a cheap and unreliable OEM sensor. No good deed goes unpunished, and a few weeks after installing the WopOnTour defeat plug I got the dreaded Service High Voltage System fault. I removed the WopOnTour device and reinstalled the original OEM sensor but had to take the car to the dealer for the required controller reprogramming to reset the fault I don't know if the WopOnTour device somehow triggered the fault or if there is some other explanation. The coolant level was correct and had not changed. It is now years later and I have not had a repeat of the Service High Voltage System fault despite having the risky OEM sensor in place. So the WopOnTour device sits in the glove box with the risky OEM sensor installed. My Volt is now just past the 8 year battery related warranty so I am afraid any faults that cause a need for module reprogramming will cost me a bundle. Does anyone have an informed opinion on whether I should reinstall the WopOnTour device with the idea that the previous Service High Voltage System fault was some other phantom cause and not related to the mimic device?
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