Transmission not working properly

More than likely this is fluid related. There really isn't anything that cares about the temperature other than the fluid. If things get too hot then it can cause slipping but complete disengagement just sounds like the fluid is not able to keep the pressure needed. Then when the fluid cools down a little, it is able to work again until it gets too hot.

There is a chance that this is also a cut seal that only acts up when the unit it hot.

However, let's start changing the fluid as that is the easiest and most common issue. Then we dig deeper if needed.

Here is a guide that will help with this.

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

Below you will find the process from the manual. If you can get a picture of what the fluid looks like when you checked it. I understand that it is not low but what does it look and smell like? If it smells burnt then that is the first sign that this is at least related to the issue.

Please see the info below that will help with this as well.

Thanks

Hi Kenny, thanks for the reply.

however, I do not think that is the cause.
When this problem started up a few years ago and some 10,000 miles ago, I had the oil changed out.
It still looks clean, that pink color of ATF and smells the same as out of the bottle.
the attached photo is a few drops on white paper.
I do not understand what is a "cut seal" in your reply. Are you talking about a shaft seal that is broken and leaks oil? if so where?
I had the car into a gearbox specialist some time back and they plugged the diagnostic instrument in to see what was on the memory as defect code but it came out clear. Presumably the electrics will store the defect code so that after a time it would have still been there.
I thought of the brake band not clamping to effect the change, but then why only sometimes?
In a sense its like a slipping clutch like maybe the torque converter is empty.
Or the oil is all froth and the pump is just pumping air and not oil, or not enough oil.
Are there not sensors or solenoids that are perhaps become defective when hot and do not function to change gears?
When it comes out of gear, it will only re-engage when the engine revs are at idle level and then it correctly defaults into first.
by using the gearshift as a like a manual car from fist to second to third etc., does not make it change up it stays stuck in first.
Changing the selector switch to "Power" from "AT" mode does not help either.
Is it possible that the "shift lock" comes into operation. I do not really know when or why a shift lock is used and have never used it.
it has got me stymied!

So if you have this issue that the vehicle just revs up until you return to an idle then that points to this pressure issue that I am talking about.

If it is not the fluid then that means most likely there is a seal internal to the transmission that is cut and not holding pressure when it is supposed to.

This is not an external leak just a seal on what is called a drum or piston on the inside of the unit.

Basically there are about 4-5 clutch packs and drums with pistons and they all have seals on them. So when you need first gear you apply one combination of clutches and then second gear is a different combination. Each combination will result in a different gear ratio which is how the transmission shifts through the gears.

Take a look below at a cut out of a transmission and you will see the clutch packs. Then the next picture is of one of these pistons where they are showing a seal.

If one of these seals is cut then when it is supposed to be holding pressure it won't and that prevents the gear from engaging.

Imagine trying to water a garden with a hose that has holes in it. You have no pressure at the nozzle because you are loosing it all out of the holes in the hose.

If there are no codes and it just neutrals out, then unfortunately if it were me, I would remove it and go through the unit inspecting this and the valve body for issues. Especially if this has been happening for this long and it progressed to this point. It sounds like it has finally failed.

Thanks, Kenny, for the detailed reply.
Not knowing the innards of an auto box could not problem solve the issue.
your probable cause makes sense. I suppose that since it "trips" out of top gear it is likely to be the top gear cluster pack that has a defective oil seal.
I shall go back to the Gear Box shop to have it repaired or a reconditioned unit swopped out, just hope they replace all the seals when they swop-out.
Thanks again, great chatting!

Agreed. That is good info as well. I didn't know (maybe I missed it) that this was recently repaired. Since that is the case, I would do exactly that if it were me. They should look at it again and make sure it wasn't there mistake and get it repaired.

Please let us know how it turns out. Thanks again.