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Paging WOT, MM acceleration issue, BAD

6K views 46 replies 20 participants last post by  PilotAlan 
#1 ·
I don't drive in the mountains a lot, but today I was. Ran into a very concerning issues. Tried searching, nothing on topic.

Setup: Two different drives, car shut off for three hours in between. 2015 Volt. Both times on the interstate, in MM, about 40% of battery remaining, ICE running, doing about 65. With my commute, the last part of every day in on gas, on the interstate, 65-75mph, so I am used to what the car normally feels like.

Going uphill, needed to pass, floored the gas. Saw kW reading go up to about 80 and just hung there. Car not accelerating much. Not nearly what I'm used to. Managed to get around, but concerned.

On the way back, I was going uphill, MM, ICE running, 65mph, about 40% of battery remaining. Floored it, again maxed out at about 80kW, not accelerating much. Held my foot to the floor for about 10 seconds, no change.

Tapped the mode button to Normal (foot still flat to the floor). The instant it dropped to normal mode, the car surged forward and was accelerating MUCH harder, the kW jumped to 110kW, had to get off the gas pedal after just a couple seconds.

Anyone seen this? Known issue? Something wrong with my car?
I'm not happy. In the mountains, uphill is NOT the time for my car to decide that it doesn't want to give me full power.
 
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#2 ·
Hmm...have you ever used mountain mode before under those same conditions? The owners manual does say acceleration will be less responsive in MM...I wonder if that is what you experienced.

But the manual doesn't say anything about switching out of mountain mode before/during a climb to access full power, so perhaps what you experienced is some kind of bug. Guess we'll wait for ***.
 
#3 ·
I went through mountains without first engaging mountain mode and got reduced power warning and a top speed of 25mph,

Also not being able to pass in mountains, is well not really an issue, any car that has less than 100hp doesn't do much accelerating up a mountain.

I pulled some trailers through the mountains with a 120hp 1982 diesel suburban, it too couldn't get much above 25mph.

So consider yourself lucky, the car will not allow more than about 40kw from the engine alone and that can result in very low speeds if you deplete the battery.
 
#5 ·
Steverino, the OP stated he had about 40% charge. I would assume that when he engaged MM with 40% charge would not matter: he had what MM is supposed to provide.

I think that what we need to know is whether or not MM limits the amount of power released by the battery. Anybody???
 
#6 ·
I don't have a precise answer but I can tell you that when coming over the mountains back to San Diego I had plenty of power with the ELR in HOLD. Passing many vehicles which were power limited.

One puzzle mentioned above by jbakerjonathan -- and what might be bothering the OP -- is that if the battery has a SOC of 40% then moving into MM shouldn't really be any different than being in Normal. So shifting to Normal shouldn't increase the available power. That could be a bug.

If so there might be an easy workaround. I never use MM in the ELR. I use HOLD instead. Works like MM but you have much more control and, if you have a lot of climbing to do, you can reserve a lot more of the battery. Wonder whether that would change the behavior in the OP's car?
 
#7 ·
Let’s simplify the scenario. You were driving your 2015 Volt in the mountains in Range Extending Mode on the freeways at 65-75 mph with no problems. You attempted to pass while driving uphill, but flooring the accelerator added speed very slowly. Without taking your foot off the accelerator, you switched to Normal. The instant the ICE turned off and the car returned to Electric Mode, the car rapidly achieved the passing speed you sought.

Perhaps while driving at 65-75 in the mountains, your car had been borrowing from the MM-maintained buffer when needed, then recharging it. Perhaps both times you sought to pass at high speed, the ICE output was both powering the car and recharging the battery to the MM-maintained level (i.e., the electric motor was not using the MM buffer charge). When you then demanded more uphill power (passing speed of 80-85 mph?) by flooring the accelerator, the ICE didn’t have that much more to give (you did actually accelerate). Perhaps flooring the accelerator under high power demand conditions doesn’t allow the MM programming to switch from recharging the battery to using it (or perhaps it does, and this was a glitch you experienced - did you try pumping the accelerator, or did you just floor it and keep it there?).

Switching to Normal would then return the car to CD mode, freeing up the existing power being "held" by MM for normal ev use, which was sufficient to provide the acceleration you sought.
 
#10 ·
Let’s simplify the scenario. You were driving your 2015 Volt in the mountains in Range Extending Mode on the freeways at 65-75 mph with no problems. You attempted to pass while driving uphill, but flooring the accelerator added speed very slowly. Without taking your foot off the accelerator, you switched to Normal. The instant the ICE turned off and the car returned to Electric Mode, the car rapidly achieved the passing speed you sought.
Exactly correct.
Perhaps while driving at 65-75 in the mountains, your car had been borrowing from the MM-maintained buffer when needed, then recharging it. Perhaps both times you sought to pass at high speed, the ICE output was both powering the car and recharging the battery to the MM-maintained level (i.e., the electric motor was not using the MM buffer charge).
Unlikely. I was pulling 40-50kW in cruise before stepping on the gas, with lots of yellow in the demand indicator. The car was drawing from the buffer. I'm pretty certain the car was in torque split mode, as I had been cruising at 70 for several miles prior.
Perhaps flooring the accelerator under high power demand conditions doesn’t allow the MM programming to switch from recharging the battery to using it (or perhaps it does, and this was a glitch you experienced - did you try pumping the accelerator, or did you just floor it and keep it there?).
That's exactly what I'm wondering.
I did not lift and reapply, I didn't want to confuse the powertrain.
The second time (suspecting something was up), I applied full throttle and waited, and waited, long enough to then come up with the idea of switching to EV mode, hit the Mode button, wait for it to change modes. About 10 seconds.
Switching to Normal would then return the car to CD mode, freeing up the existing power being "held" by MM for normal ev use, which was sufficient to provide the acceleration you sought.
Seems that way. Suddenly lots more power.
What happens if you drive in Hold the entire time? ... Kind of sounds like it never 'shifted' into passing mode (all EV) until you selected 'normal'. Passing mode in this case being the car's clutch mode not the driver-selectable mode.
Seems that way to me. Every day driving home, I'm in ICE mode doing 65-70 on freeway, passing cars, etc. Never, ever noticed a problem. I have used hold before, never noticed this.
Steverino, the OP stated he had about 40% charge. I would assume that when he engaged MM with 40% charge would not matter: he had what MM is supposed to provide.
Yep. Engaged MM at home prior to heading out. EV range was depleted in the flats, ICE was running before going uphill.
I think that what we need to know is whether or not MM limits the amount of power released by the battery. Anybody???
That's what I'm wondering. If there's some secondary buffer or limit to keep you from running all the way down to "Reduced Propulsion". If so, I'm unhappy that I can't get full power in the situation where you need it most, trying to pass going uphill.
 
#8 ·
When working properly, and with any buffer left in the battery, the Volt has 150HP on tap. No loss of power due to altitude!!!
Non turbo gassers lose a lot of power at high altitudes.

The OP's car has an issue. That 150HP should be on tap as long as the battery is not totally depleted.
 
#9 · (Edited)
Never went up a mountain, but, both '13 Volt and '14 ELR are slower in MM. IF the generator is both charging the battery and passing power to the motors. If the generator is not charging, then, it's the same as Hold or CS. The generator is o-f-f if that's the case though.

In '14 ELR, CS actually has more power than battery alone.

What happens if you drive in Hold the entire time?

Kind of sounds like it never 'shifted' into passing mode (all EV) until you selected 'normal'. Passing mode in this case being the car's clutch mode not the driver-selectable mode.
 
#11 ·
MM is stingy about releasing any power when it hits the minimum, leaving you with 4 cyl acceleration up a mountain. :p You're supposed to use MM to reserve power for the mountain, then switch to normal when you most need that power.

If you're going to leave it in MM all the way up the mountain, you'll still have 30% battery at the top and then with regen on the way down and eventually have to burn it off... Nothing gained.
 
#12 ·
Interesting, that's the first I've heard that. When you say "MM is stingy about releasing any power when it hits the minimum", what's the minimum? I was a bar or two down from the MM red marker.

So, it sounds I'd be better off either switching to hold at 50% (or use MM to build up charge and then switching to hold) and running that way, rather than using MM.
 
#13 ·
That has nothing to do with Mountain Mode. If your battery is depleted too much, it can't tap into it for additional power to the main motor. That would happen in Hold Mode as well.

What I think you are experiencing is a throttle response issue. I've felt the same thing when mountain climbing in the Volt with MM engaged. My solution is probably not recommended, but I typically drive with cruise control engaged. I will use it to step up my speed, and it seems to respond and accelerate far faster than if I am actively on the pedal. I have no clue why, but it does.
 
#16 ·
The simple answer is found in the manual on page 9-24:

"While driving in Mountain Mode, the vehicle will have less responsive acceleration."

It is working as designed.
 
#17 ·
The simple answer is found in the manual on page 9-24:
"While driving in Mountain Mode, the vehicle will have less responsive acceleration."
It is working as designed.
Hmmm.... I did look in the manual, but must have missed that.

So, the conventional wisdom here is that MM is just Hold, with a different set point. But clearly there is a difference in the way the drivetrain manages power and what it will allow the car to do.

I was driving into work today and put it in MM, and once the battery was run down to where the ICE started and the car went into torque split mode, I tried again (on flat ground). Initially, on full throttle, the car allowed itself to drop out of torque split and run up to 110kW.
I took it out of MM and ran the battery down a little more, then put it back in MM. It once again went into torque split, with the engine running faster to try to put juice back into the battery. When I floored it, it would not leave torque split mode, and power was limited to about 80-84kW.

It seems that the car is engineered to limit the amount of power it will give below a certain internal threshold. Presumably, to prevent you from putting it in MM and blasting uphill, running out the battery, and ending up in 'Reduced Propulsion' mode.

So, from now on, I will use Hold in the hills, instead of MM (unless needing to build power, as opposed to maintaining power).
 
#19 ·
You’ve now demonstrated that placing additional demands on the system while the MM buffer is being recharged leads to the conditions you experienced in the mountains. You’re overlooking one Hold mode scenario that can’t be duplicated on level terrain.

If MM has no programming differences from Hold mode other than establishing a particular soc hold point, then when power is used from the Hold buffer to maintain performance, it, too, will be restored using the ICE generator, and the ICE will be both fueling the motor and recharging the battery. Under such conditions (such as flooring the accelerator to pass at 80 mph or so while driving uphill when the battery buffer is being recharged), one might expect that with Hold as well as with MM, as the manual says, "the vehicle will have less responsive acceleration."
 
#21 ·
The most demanding thing the Volt has to do is go up a long steep mountain in CS mode. It makes no sense to limit power when you need it the most.

In regards to the warning of reduced performance in MM, I think that statement is there because you cannot expect to continuously draw 111 kW from a 55 kW range extender and a dwindling battery, at one point the system has to reduce power to maintain some SOC reasonable for driving in the mountains. So, initially 100% power than some reduction in power in order to stay out of the Reduced Propulsion Power mode.
 
#30 ·
OK, from what I have been reading and my own pre-conceived notions of how MM works, I can envision the following happening if the software allowed the transmission to drop out of torque-split when the battery is being brought back up to full MM reserve of ~40-45%.

Let us say that the car has been climbing for a few miles and the reserve is at 30%. MM requires that the ICE torque be used to generate electricity to replenish the 10-15% while also adding torque to the drive wheels. Conditions are such that there is sufficient power to maintain the desired speed. Now, the go pedal is mashed, demanding maximum torque. Should the system drop the MM requirement to maintain the buffer and allow the battery to drain to the point where Reduced Propulsion Power is reached? That might occur at the worst moment in your life. Or, should MM criteria be followed, limiting available torque to the wheels while replenishing the buffer (and providing the caveat in the Owner’s Manual)?

Given the above considerations, I think that the engineers, again, wisely chose how to have the car behave. Kudos to them.

The Op’s experience highlights how we should be aware of the possible limited power available while in MM. Giving up that extra minute or two means very little in your trip up and down the mountain sides.
 
#33 ·
When I deplete my battery in a normal commute, it is typically in the middle of an interstate hill of about 2 miles length. Whenever you first switch to Normal mode with a fully depleted battery or just hit depletion Charge percentage in Normal, it doesn't run the ICE at full speed as the buffer is calling for until a certain time period, which seems to be about 2 minutes or so for me. After that time period of initial warmup under 40+ kW demand, the ICE will run full speed trying to catch up the buffer. I've never noticed flooring it in that condition whether I've had over 80kW available, although it "feels" like I have less.

Could it be when switching modes you start that "initial warmup" timer and don't get full RPM or maximum ICE charge to battery? If you switched from an under-buffer condition in MM to an over-buffer condition in Normal this would make sense.

Would be easy enough to test by using MM with a depleted battery on such a hill, but I'd expect the same result anytime the SOC was at 3 bars or under - meaning under MM buffer setpoint. I may verify on this evening's commute if I remember!
 
#34 ·
Hi PilotAlan,

I came across your post and reached out to a few of my internal resources to discuss your concerns further. If you allow the vehicle speed to drop to 59 MPH or less prior to full pedal acceleration, you will then be able to perform your passing maneuver with a full 110kW of power. This applies to any mode of operation with the engine running.

If you have any additional questions or concerns, or would like further assistance from my end in any way, please send me a private message. I'd be happy to help as best I can!

Amber G.
Chevrolet Customer Care
 
#35 ·
If you allow the vehicle speed to drop to 59 MPH or less prior to full pedal acceleration, you will then be able to perform your passing maneuver with a full 110kW of power. This applies to any mode of operation with the engine running.

Amber G.
Chevrolet Customer Care
This is a very informative piece of info. Thanks Amber.
 
#46 ·
I guess I have been lucky. I have never experienced the reduced propulsion issue. I have taken several long trips over steep hills in MO and OK driving 75-85 MPH. I always keep a good surplus on the battery though. Maybe that is the difference.

Perhaps the problem is when folks completely deplete the battery and go well below the acceptable SOC that the ICE must stay engaged with the generator. The the reduced propulsion message appears. Don't know for sure. Just saying I have never had it.
 
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