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Good Morning All,

I bought my Clarity a couple of months ago and twice have had situations where the ICE starts revving really high. Like ridiculously high for a couple of minutes. It goes away but it doesn't seem normal at all and I'm a little concerned about it.
I saw an older post about it but there were no comments on the possible cause of it or any fixes.

Any ideas?
THIS!
I own a 2020 Clarity Plug-In and just started experiencing this issue. I found this post through a search engine and immediately joined the forum. (nice to know there's a tribe of knowledgeable folks with my car!)
The engine revved to ridiculous levels and lost 90% of power. I was on a hill and was able to crawl to the top and pull off. I turned it off and back on and it still did this for the remaining 1 mile to the house.

It's happened I think when the battery ran out and it switched to ICE only. When I charged and ran on electric, all was fine again.
 

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2018 PHEV Touring Atlanta, GA
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The engine revved to ridiculous levels and lost 90% of power.
You seem to be describing a different problem, because the OP on this thread only reported very high revs, basically redlining, but not a near total loss of power as you described which would be a much more serious situation.

Do you think it's possible that you did not try pressing the accelerator pedal farther? It's understandable because if the engine sounds like it is racing at very high RPM there would be a natural hesitation to press the accelerator pedal any further, figuring that the engine is already at its maximum. However in a hybrid the RPM's are not always tied to accelerator position, it's possible that if you had pressed the pedal farther that even though the high RPM's continued there would have been more power available from the electric motor.

If you are not sure how far you pressed the pedal that is understandable because I can imagine that it is a very disconcerting situation. But if it happens again, try pressing the pedal all the way to the floor while watching the power needle on the instrument panel to see how the car responds.

The redlining problem seemed more common in 2018 models, seems very rare for a 2020. For most people the problem seems to eventually go away, at least I assume so because I don't remember hearing someone say, "I have had this problem off and on for two years", it is usually much more recent. Which means at some point it apparently goes away.

There is a theory that the very high rev problem is solved by what we call a 12V reset, which is pretty simple you just disconnect the negative terminal on the 12V battery for a few minutes then reconnect it. This clears out data history and a few other things. When you first drive the car afterwards several seemingly ominous warning messages appears about ABS, VSC etc. believe it or not this is quite normal, happens for example anytime someone replaces their 12V because obviously the old battery has to be disconnected first. But is not harmful and the messages go away within the first mile of driving. The 12V reset has been known to cure a variety of odd situations related to software, including we think the engine very high RPM. It's possible that someone who had the problem didn't know that and when the 12V battery got disconnected for some other reason that this fixed their redlining issue, although of course they wouldn't know that. Just a theory anyway and something that is easy to do just in case it helps.
 

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2021 PHEV Touring HB, CA
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THIS!
I own a 2020 Clarity Plug-In and just started experiencing this issue. I found this post through a search engine and immediately joined the forum. (nice to know there's a tribe of knowledgeable folks with my car!)
The engine revved to ridiculous levels and lost 90% of power. I was on a hill and was able to crawl to the top and pull off. I turned it off and back on and it still did this for the remaining 1 mile to the house.

It's happened I think when the battery ran out and it switched to ICE only. When I charged and ran on electric, all was fine again.
It appears that climbing your hill required more power than the ICE could provide, given that the battery was depleted. Using HV or HV+ (depending on battery SOC) when approaching any hill climbs should solve this problem. In high load situations like hill climbing, the traction drive system may requires both inputs, from the battery and ICE, to provide the necessary instantaneous power for the climb.

This was certainly the case with my 2011 Volt. In my Volt, there was "Mountain Mode", which uncorked the ICE to run at maximum RPM, until an additional ~10 miles of range was added back to the battery bottom-end reserve. (Similar to invoking HV+ in the Clarity.) Then it would throttle back down and rev up as needed to supplement what the battery could provide. (Similar to dropping back to HV in the Clarity.) I forgot to invoke Mountain Mode a couple of times climbing over mountain passes at 75 mi/hr. Having your vehicle drop to 50 mi/hr from 75 mi/hr ("reduced performance") when you're in the number one lane on a freeway is a bit disconcerting, to say the least.

It took me a while to learn the quirks of a PHEV, but after several road trips over passes, doing the necessary management became second nature. My normal driving mode is in Sport, with max regen selected, and HV set at about 50% SOC. If I happen to forget to turn HV back on at the last gas station (not uncommon), I'll just put my Clarity in HV+ about ten miles out from any mountain pass, and let it manage things. If I forget to do all of these things, it's me, ACC and a convenient truck cruising along at 55 mi/hr slogging over the pass. There are worst things in life.
 
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