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Discussion Starter #1
(Apologies for being long-winded) Hey everyone. Thought I'd throw an issue out there to see if there is any commonality/frequency, or if I just happened to have Murphy's target on my back today. It was roughly low 40s this morning so I decided to fire up the heater to warm the cabin up a little prior to departure. Didn't run very long in precondition mode before I left so I wasn't overly concerned about the initial lack of heat output. However, after driving for about 10 minutes it became obvious that there was no heat coming from the vents. I started watching the power meter and noticed that there was no variation in the power being consumed (for example, at stop, the car uses about 0.5kw with the HVAC off; when the heater is enabled, set at HI and MAX, the car should at least initially show between 4 and 5kw). This suggested to me that the electric heater was not even coming on. Ok, no big deal; so I switched to HOLD mode to see if I could get some heat from the engine. The engine started and warmed the coolant to about 130F before it started throwing any heat. A little unusual unless the car is designed to do that. However, the odd part was that there was a little "kick" in the engine when the heat started working, as if the water pump had just kicked in (as far as I know it is not clutched like a traditional air conditioner compressor; wouldn't make sense to be so although I believe that there is an electric pump in that mass of plumbing somewhere that could factor into this as well). I would have just ignored it if this had not happened multiple times, including one trip where there was no detectable heat from the engine even after reaching operating temperature (~170F or so, not highway speed temp, perhaps, but still should be giving off heat). Also of note is that when the heat does work in HOLD mode, shutting the engine off results in a not-so-gradual decrease in the vent temps until no heat is given off at all. Given that the car sat for at least a year on the dealership's lot, I can write off the issue with the ICE heat as a sticky valve due to lack of use. However, the electric heat worries me. I have an appointment with the dealer on Monday to have a recall job done, so I am planning on having them check on these two anomalies at that time. Anyone else run into this on the second gen? The amusing thing is that the first gen I leased had a reputation for HVAC issues, but I never had a single problem with mine. I get a second gen that, as far as I'm aware, doesn't have a bad reputation with regards to the HVAC and suddenly I have a problem. Gotta love it. Cheers!
 

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Definitely tell your dealer about this. There could be several things wrong. The most obvious issue would be that the electric coolant heater is not working, although I thought a DTC would be thrown in that case (because the battery can't be warmed up to operating temperature). There could also be a coolant level issue, but I would expect it to still consume energy while attempting to warm up the coolant, which pretty much narrows it down to a broken heater.

The "kick" in the engine could be the mixture changing. I know mine runs fairly rough until the coolant reaches somewhere around 90 *F, and then the mixture suddenly changes (likely the change from open loop to closed loop) and there's an audible difference in the engine which is slightly reminiscent of an A/C compressor being disengaged or engaged on a normal ICE vehicle.

The behavior you observed with the ICE heat and cabin heat not correlating is likely due to the fact that there are two separate coolant loops (engine and battery) which can be connected in some cases, as well as (I believe) exchanging heat between them.
 

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I had a heater failure in my 2017 a month ago but received a "service heater" warning. It was a high voltage electrical issue but I tried the same thing the OP tried---starting the engine. Still, no heat. The entire Heating side of the HVAC was locked out. According to my work order the repair was to "follow instructions for PIC66266" which required checking the high voltage connections at the heater and battery. It's been working fine since then.

I am still surprised that there was no fallback to heat the heater core with engine coolant when the electric water heater was out of order.
 

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Discussion Starter #4
Symptoms sound similar, but for whatever reason mine is not triggering the service heater warning. I did check the coolant levels, and all three bottles are at the same level they were at when I bought the car (right at the full/max level), so I don't suspect a coolant loss yet. It doesn't seem to be a sensor issue either as I can set the thermostat to HI and still get nothing when on battery. When the engine is running and (what I suspect at any rate) the coolant diverter valve isn't sticking, the heat works as expected at that same setting. Hopefully I will find out Tuesday. I did stop by and add the heater issue to my list of issues so they are expecting that in addition to the recall and basic maintenance work I've requested.
 

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Symptoms sound similar, but for whatever reason mine is not triggering the service heater warning. I did check the coolant levels, and all three bottles are at the same level they were at when I bought the car (right at the full/max level), so I don't suspect a coolant loss yet. It doesn't seem to be a sensor issue either as I can set the thermostat to HI and still get nothing when on battery. When the engine is running and (what I suspect at any rate) the coolant diverter valve isn't sticking, the heat works as expected at that same setting. Hopefully I will find out Tuesday. I did stop by and add the heater issue to my list of issues so they are expecting that in addition to the recall and basic maintenance work I've requested.
A stuck valve will throw a DTC; I remote started my car last winter about 2-3 seconds after shutting it down from inside, and it threw 8 DTCs because there were valves and relays stuck in the "shutting down" state when the vehicle started up again and it confused the computer. The 15-below temperatures probably made them react more slowly than it was anticipating.

I think it's just that the diverter valves don't open unless the temperature is in a certain window. I am still baffled that a non-functioning heating element is not throwing a DTC in your case.
 

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My 2017 Volt had no electric heat due to a connector on the electric water pump that circulates heater coolant, which was never seated during assembly. There were no codes or service lights and I didn't notice it until the weather got cold, about this time of year, one year ago. In charge depleting mode, when you set the climate control on max heat, it should roast you out of there if things are working right. See your friendly dealer.
 

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At one point, I had forgotten to run the compressor/heat pump. Is there a chance that either "eco" or "max" wasn't lit on the day you were having issues?
 

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Discussion Starter #8
(long winded again...) For electric heat, either ECO or MAX has to be lit. In all cases, MAX was illuminated. The A/C runs without issue so I know the setting is correct. I had the car in for service yesterday and had all issues addressed. The service writer even stated that they tested the heat on HI and had to turn it down because it was too warm. However, enter this morning. I get in the car (which is garaged, interior temp was about 64F), head to work and run the HVAC on MAX and 74. Initially everything seems normal, feels like some heat is coming out of the vents and the power consumption meter shows a few KW above normal. I get about 2/3 the way to work, and the vent temps feel like they are falling. Now I know the heater output is not normally very warm when there isn't a big difference in the set point and the interior temperature, so I figured the best way to verify was to crank the temp back to HI, which should force the heat to operate at full output. Now I'm back to square one. At a stop, the power consumption was back to 0.5KW and the vent temps were cold, although the fan was operating at full speed. Based on this and what the dealer told me, it sounds like their tests were based off of engine heat rather than electric. This would make sense because they were checking the coolant flow valve due to symptoms of a sticking valve (probably due to lack of use, but was working correctly when I dropped the car off). The explanation I received was that they had to upload the computer calibrations to GM to check since they could not figure it out locally. GM gave them some instructions, and had them reboot the computers (after all, what is this thing but a ~3500lb computer on wheels, right?). They claim that the heat worked fine after the reboot, but I have my doubts about it being electric-based heat. Not upset at the dealer, particularly since there aren't a lot of Volt-certified dealers in the southeast (try finding one that is Bolt-certified; got a better chance of winning the lottery...). However, I will have to take it back to have them check it again.
 

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Definitely tell your dealer about this. There could be several things wrong. The most obvious issue would be that the electric coolant heater is not working, although I thought a DTC would be thrown in that case (because the battery can't be warmed up to operating temperature). There could also be a coolant level issue, but I would expect it to still consume energy while attempting to warm up the coolant, which pretty much narrows it down to a broken heater.

The "kick" in the engine could be the mixture changing. I know mine runs fairly rough until the coolant reaches somewhere around 90 *F, and then the mixture suddenly changes (likely the change from open loop to closed loop) and there's an audible difference in the engine which is slightly reminiscent of an A/C compressor being disengaged or engaged on a normal ICE vehicle.

The behavior you observed with the ICE heat and cabin heat not correlating is likely due to the fact that there are two separate coolant loops (engine and battery) which can be connected in some cases, as well as (I believe) exchanging heat between them.
Here's a good info page on the coolant loops. Note that lack of cabin heat does not mean the battery can't warm itself, they are separate heaters on separate loops (4 total in the car, actually). I don't believe those two can connect to each other, unless they changed the design for gen2. There is some radiator sharing of power electronics and battery loops.
 

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Here's a good info page on the coolant loops. Note that lack of cabin heat does not mean the battery can't warm itself, they are separate heaters on separate loops (4 total in the car, actually). I don't believe those two can connect to each other, unless they changed the design for gen2. There is some radiator sharing of power electronics and battery loops.
I was under the impression that the Gen 2 connects the engine loop with at least one of the battery loops under certain conditions. My engine coolant temperature (as shown on the DIC) will go up to well above ambient temperatures (85+ on a 70 degree day) if I'm driving at high speeds in EV mode a lot or using a lot of heavy acceleration...I don't know why that would happen unless the loops were connected.
 

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I was under the impression that the Gen 2 connects the engine loop with at least one of the battery loops under certain conditions. My engine coolant temperature (as shown on the DIC) will go up to well above ambient temperatures (85+ on a 70 degree day) if I'm driving at high speeds in EV mode a lot or using a lot of heavy acceleration...I don't know why that would happen unless the loops were connected.
Interesting... well, I've never seen a diagram for gen2, so I don't know for sure what is going on. I do think they intentionally kept those loops isolated in gen1 so that a valve malfunction didn't lead to a ruined $$$ battery pack (you can ruin lithiums quickly at 200F!), it'd be curious to know if/how gen2 is different.
 

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Interesting... well, I've never seen a diagram for gen2, so I don't know for sure what is going on. I do think they intentionally kept those loops isolated in gen1 so that a valve malfunction didn't lead to a ruined $$$ battery pack (you can ruin lithiums quickly at 200F!), it'd be curious to know if/how gen2 is different.
I suppose it's possible that the battery loops are completely isolated, but there's a coolant loop in the transmission that is/can be connected to the engine coolant loop. I'd imagine the drive motors generate plenty of heat as well.
 

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I suppose it's possible that the battery loops are completely isolated, but there's a coolant loop in the transmission that is/can be connected to the engine coolant loop. I'd imagine the drive motors generate plenty of heat as well.
I don't believe there is a coolant loop in the transmission. Maybe an oil line that shares part of the engine coolant radiator but not engine coolant. There are only 3 coolant loops, engine, HV electronics, and HV battery. They are all separate with 3 radiators and 3 tanks.
 

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I don't believe there is a coolant loop in the transmission. Maybe an oil line that shares part of the engine coolant radiator but not engine coolant. There are only 3 coolant loops, engine, HV electronics, and HV battery. They are all separate with 3 radiators and 3 tanks.
I don't think that's true. Gen1 has 4 loops (and 4 rads, though one is shared, but then you have one for A/C alone), and although it's hard to find info on gen2, this link does mention a drive unit cooling loop (at the very bottom when discussing drive unit fluid volume.
 

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OP- throw in a paragraph break or two every few lines. Difficult to read.
 

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Discussion Starter #17
Ok, for those who might be interested, I finally received a resolution for my heating problem. After much digging, and apparently several calls to GM TAC, the problem was pinned down to a failed heater coolant heater (great name eh?). The part was replaced under warranty and my heat now works properly. Not sure what the vent temps get to, but it feels equivalent to the output of a typical heat pump (maybe 110-120F, but that is just a guess).
For those who may have this issue, be forewarned. The part is in a restricted status and requires GM TAC to authorize warranty replacement. No idea on actual/retail cost.
 
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