Car does not move when transmission is in Drive or Low 1/2, But moves when in Reverse?

Thanks for covering the basics first so just to confirm, did you drain the fluid into a pan and check for metal in the pan? I know you said it didn't have any, but we just want to make sure this is how you checked it.

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

I suspect we have a loss of pressure in forward gears or an issue with the pressure regulating solenoid. I am attaching some info on this below, but I suspect we are going to have to dig into the unit in order to figure this out.

It would be helpful if we can confirm pressures when you are in gear and then in park.

So, let's start with this info and we can go from there.

Thanks

Hi Kenny, thanks for the response and the information provided.

On your question about metal finding, yes that's how I drained and replaced the transmission oil and found no metal shavings in the pan.

Can you provide more details on how I should get the pressure information you are looking for? Do I need to get my hand on an advanced diagnostic scan tool? If yes, is there any budget scan tool for DIYers you can recommend, so I don't spend a fortune on it?

Also, if you can elaborate more on this:
"I suspect we are going to have to dig into the unit in order to figure this out."

This is what I did: car jacked up and on a cold start, I measured the pressure (using an Engine and Transmission Oil Pressure Test Kit) through the UD tap located next to the drain plug.
- In Park and Rear - No Pressure
- In Drive or Low 1/2 - Pressure spikes to 150+ PSI with a jolt, then drops and maintains to 45 PSI on idle (750 RPMs). TCS and ABS light get on, now only one wheel moving. At RPMs 1,250, it goes to 195 PSI, still one wheel moving. And then further increase in RPMs say 1,500+ the pressure needle moves swiftly between 210 - 240 PSI, still one wheel moving.
Note: Surprisingly, seconds later suddenly both wheels come to full rotation, pressure drops to 0 PSI, and even when I take the foot off the gas pedal the RPMs maintains at 1,500. That's weird never seen that before but maybe since the car is jacked up, which triggered TCS and ABS, and thus it's acting up.

Hope this data helps anyway.

It's hard to replicate the issue of no motion in forwarding gear when the car is jacked up since it has to be a hot start. But jolting is always a replicable cold or hot start. However, if you for sure want the pressure at the tap point when the car doesn't come in motion, then I will try hard to replicate it. Let me know.

What do you suggest as the next step?

Also, with a lot of google research, thinking if a bad Transmission Fluid Temperature sensor is causing all this? Seems like that's an obvious failure point with Hyundai and Kia's.

This does help. I doubt that a temperature sensor is causing this, but we need to check the codes just to make sure we don't have something to take us in another direction.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

As for the pressure, that is exactly what you were supposed to do.

Basically, those pressures are showing that you most likely have a cut seal which is why the pressure is low and then jolts when you put it in gear, and you get the pressure spike.

As for the wheels spinning when on a lift, that is normal for these vehicles. Basically, when there is no load on the transmission the torque goes to one wheel and then it will switch sides easily and it causes it to act crazy.

Lastly, what I meant by digging into the unit means we are looking at a rebuild in order to see what is happening. So, if I am correct with a cut seal then we will find that when the unit is rebuilt.

However, let's check the codes and go from there.

Hi Kenny, thanks again for your prompt response. As I stated in the original post, there is no check engine light and no DTC stored in the system.

Sorry. I forgot about that. This means that the issue that is causing the jerking is not a monitored system. That means the vehicle does not see an issue with what is happening which normally always points to an internal mechanical issue.

So, if it was me then I would be pulling the unit apart for a rebuild.

Clearly, we could be missing something on this, but no codes and these pressures are telling me we have an internal issue.

Hi Kenny, I really appreciate your help so far here. I was able to get hold of the advanced diagnostic tool, and I replicated the issue to monitor some TCM params. Here are some more observations or data points for you:

- Issue of "Car not moving forward in Drive gear" is easily replicable at Transmission Oil temperature reading at 85 Celsius. Monitored the temperature reading from cold start to hot, it was going up gradually which means the temperature sensor is good.
- In Drive gear when the Brakes are on and Input Speed reads 0, the car will move forward when Brake Switch turns off. However, when the Input Speed is not 0, the car won't move forward. Attached also found a picture of diagnostic steps from Hyundai on another forum, stating this condition as Internal Transmission Failure and suggesting replacement.
- When in Drive gear, foot off the brake, and the car doesn't move, the physical pressure reading at UD tap point is 10-15 PSI. Then if I rev up the engine to 1,500 RPMs, the pressure suddenly spikes to 75-90 PSI, the car will jolt forward and come back to normal forward motion.
- Attaching both, when the car moves forward and when it doesn't, snapshots of the diagnostic record.

I understand at this point I definitely have to look inside the transmission, but just wondering if you can pinpoint me specifics on what I should be looking for damage and repairing. Maybe some video or guide links will help. Thanks again.

Okay. This is good.

I am suspecting you are getting more confirmation that the internal issue is the direction we need to go.

So, from what I am seeing and mostly because of when you accelerate and raise the speed at which the pump is turning and this increases pressure in the unit that it only then engages, and it jerks points to either a torque converter issue or a leak in the under drive is leaking causing pressure to escape and then once it is high enough it sells and bangs into gear.

So, if it were me, I would get a complete rebuild kit and go through the entire transmission and replace the converter and pump while you are there.

I attached the info below, but the last page is a power flow showing in D the pressure goes through the underdrive and the one-way clutch.

Hi Kenny, I am thinking of replacing it with used transmission instead. I think it will be quicker compared to trying to rebuild. And the cost difference is marginal, so it is worth just replacing it. My Hyundai is US-built but I am only finding Korea-built transmissions online or local. US builts are out of stock everywhere. I was wondering if it doesn't matter and both built are interchangeable? Here is my VIN# 5NPDH4AE5EH501830, the transmission used is A6GF1.

Also, will it require to do Transmission Relearn through Scan tool after replacing it with a used one?

I think you are exactly on the right path. I would just replace it as well.

However, I am not finding anything that explains if they are interchangeable, but I can't imagine they are different. Clearly, I would like to have a document that says they are the same, but I don't so I am not going to be able to confirm that for you.

As for the relearning, you technically do not need to do one as has already been learned to the previous vehicle, but it would be a good idea.

Hope this info helps.

Thanks