Transmission gears are selecting but the van is not moving?

Please I am making an addition to my earlier question posted some hours ago.
After my car stopped and the gears were selecting without the car moving at all. I got an auto electrician who checked it using a computer scanner and told me its the transmission range sensor that is bad.
I bought another TRS sensor and replaced it but still my van doesn't move even when all the gears are selecting so softly as if its in neutral.
You throttle it and it and the engine runs normally but the van doesn't move when you select all the gears.
Do you think its a gear problem?
A mechanic told me it's a gear problem.

Sorry for the delay. Yes. This does sound like an internal issue the transmission.

More then likely the pump is not operating or you have cut seals that is allowing the pressure to bleed off rather then build to the proper level to allow the transmission to engage.

Basically the clutch packs inside need to be applied with a force of fluid pressure and if the pressure is not high enough then it will not engage the gears.

The best way to check this is to check the fluid pressure with a gauge and then do what is called an air check which is basically applying a low pressure of compressed air to listen for the clutches to clunk which shows they are sealing. If there is no clunk then it is a clutch pack issue.

As you will see in the info below that a no drive situation can still occur even with correct pressure so we need to make sure the air check is done to confirm this.

Let me know if you have questions. Thanks

Thank you very much for the response and detailed explanation. I will get my mechanic to check all the components you mentioned in your reply to me and let's see how it goes.
I will give you feedback after we carry out all the checks and test.
Once again, thank you very much. I appreciate.

You are welcome. That would be great if you let us know what happens. I am curious about this one and it would be great info for others that may have the same issue in the future. Thanks

The mechanic did the check on the pump and discovered that the pump is not pumping enough oil into the gear.
Another discovery was that we saw oil inside the gear control switch. This means that there is a possible oil bleeding due to the seals been cut.
So the mechanic advised that the gear box be brought down for him to change the damaged seals and pump .
I will give you further feedback as the gear will be loosed down today and inspected properly before any repair is done.
Many thanks .

That is a great update and great information. Please let us know how this turns out. Thanks for the update.