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Discussion Starter #1
Hi guys,
My dad has a 2012 Hyundai Sonata hybrid limited edition. He had been complaining of transmission problems so I switched cars with him to see what the issue was. He took it to Hyundai, who said replace the transmission for $4000 without even testing it or reproducing the problem. He took it to a mechanic, who said change the fluid first, then changed their mind and told him to replace the transmission. Then he took it to a transmission shop who said rebuild it for $3000.

My concern is none of them really know what the problem is because I've had to drive from the suburbs, into the city (30 minutes) before the problem occured. I know none of them did that.

So here is what happens: after driving for a while the shifting became erratic. Then, while in second or third gear I felt like the transmission disengaged (like when you disengage a clutch in a manual transmission) and I heard the engine revving up while I was trying to slowly speed up.

Soon I realized that this was only happening while the car was in electric mode, but never in gas mode and I think what I heard was the electric motor revving up. It was hard to keep the car in gas mode in the city, but if gunned it from a light to keep it in gas mode, no problem. As soon as I had to slow down, it switches to electric and that's when the transmission began to act up. So I messed around with the car while taking the highway home.

The transmission kept "slipping" in the city, until I got on the highway. Then I drove the car on gas mode about 70% off the way by keeping the speed above 70mph: no problems. Then I tested it by slowing down to 60ish and letting it coast to electric mode and the problem came back. Then I sped back up and again no issues.

I was trying to do some research, but couldn't find anything. I'm wondering, does anyone know whether there is a clutch that might be the issue after the transmission? Is there separate clutches for gas engine vs electric motor? Isn't the electric motor direct drive so maybe there is some sort of wear issue between the the electric motor and whatever the motor connects to to spin the wheels (not sure what it would be called).

There aren't any good explainers out there on how the electric motor is actually connected that I found, but if anyone is well versed in this, please let me know. My theory is that it's not a transmission issue, but whatever is connecting the electric motor to the wheels is worn out or an issue with that part heating up after about 30 minutes of driving.

Also, the car has 192,000 miles. Apparently, my dad never changed the transmission fluid and now I'm afraid to do it. He had a new engine put in a few months ago because it seized (he changed every other fluid on time). The "transmission" issue was there prior. Prior to the engine seizing, Hyundai had told him it was some sensor behind the front bumper and charged him $1000 to replace it, but that didn't fix the problem. I don't know what that sensor is.
 

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Discussion Starter #2
Just to clarify, oil changes were done on time. Only fluid he didn't change was the transmission fluid.
 

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This is a good basic video from youtube showing the Sonata hybrid technology workings... It shows graphically the mechanical coupling / layout of the ICE / Traction Motor / transmission assy. which may aid in your understanding of where your problem lies.

From your symptoms, it sounds like the ICE and traction motor are not "in sync" with each other (rpm's) when the coupling clutch energizes to allow both to power the transmission. Can you watch on the large Hybrid display screen to view exactly what "mode" the vehicle is operating in when you feel this slippage occur? Is it in EV only or Hybrid mode when it happens?

 

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Discussion Starter #4
Just saw the video. There is a point in the video that mentions that the "engine clutch becomes disconnected" I'm wondering now if that clutch is the problem. Maybe the revving up I hear is the gas engine, not the electric motor (would an electric motor be loud enough?). So what happens is I hear the noise, lay off the gas pedal, and then feel the connection being made (power going to the wheels).

I think the issue occurs when the vehicle switches from electric mode to hybrid mode (where you see power going to the wheels from both engines at the same time on the screen). So it's engaging the the clutch, but maybe the clutch is slipping? Maybe the clutch has teeth that are worn out?

I wish there was a way to deactivate the electric motor so that I'm only driving on the gas engine. Then I would know for sure.

Any thoughts?
 

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I am having the same problem. 2012 Hybrid Sonata 120k miles. I changed the transmission fluid in my car a couple of months ago and the problem went away. This week it happened again so I changed the fluid again and it seems to help. I think it is related to the trans losing pressure and not being able to shift. Do you have any updates?
 

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Perhaps if changing the fluid is helping and you don't want to take in to a dealer to relearn the trans, you could disconnect the battery long enough to have the computers reset and relearn (due to the wear going on from 120K miles, and then new fluid, etc.)
 

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At this point in the life of the car, an option is to discontinue the hybrid option, and keep it totally gas.
I wonder if there is a software available that would do it. You might have to ask the Atkinson people if
it is possible. Sell off the batteries, then maybe the car, with a new engine. lol
 

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My stepdaughter's car had somewhat similar problems (I think it was a '12 or '11, can't remember, car was t-boned and totaled). In electric mode, it wouldn't engage but would 'slam' into gear once the gas engine came into play. I ended up getting a used transmission and doing the swap myself. Even the swapped transmission needed a new electric oil pump to deal with slippage. That might be something worth trying yourself. The oil pump is on the front of the motor, runs about $400 on Amazon and is a really easy swap.
 
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