35is 2011-Transmission Overheat

[Alanrorr1]

The Z4 is in the shop with the transmission apart for codes 5A53 and 5A54 (Gearbox overheating Red and Black phase). They (not dealer) are recommending a new clutch, flywheel and seals. Originally, they thought the temperature sensor in the transmission. What do you think? If their recommendation sounds like the right direction, any ideas to reduce > $5K with aftermarket reliable parts such as where should I go (for the parts since the shop said they could install them)?

Reasons they gave for the diagnosis:
- There was a fair bit of play in the flywheel (maybe 1/64" to 1/32" guess) when moving from side to side. This could be checked since the transmission was dropped
- They said the clutch was pausing at high speed before shifting. I seem to remember that, but I thought it was due to it not being in sport mode and subsequently causing a delay. In sport mode or normal, I felt no slip at 55 mph in 6th with hefty acceleration but maybe I would not notice it?
- Transmission fluid was close to black but had no metal pieces or shavings

When it initially came into the shop, they tried
- filling the transmission fluid with 1.7 quarts (car has 105K miles)
- looked for a wiring issue to the transmission temperature sensor outside of the transmission
- updated the software in the transmission and ECU
Nothing worked, it still overheated.
I liked their ideas so far. I do not think looking for clogged coolant circulation has been checked though

Notes:
- The clutch oil temperature (using Foxwell tool) would match up to the red and black (lower seemed to happen around 315F and the upper around 345F).
- It could shoot up 90 degrees F quickly (have image going from 220F to 310F in 2 seconds)
- This issue started 3 and 1/2 years ago - happened rarely, now frequently
- The first warning could occur within 3 seconds of putting the car into reverse from a cold start (happened multiple times)
- 2 weeks ago, it went into limp mode for 1 hour in park/engine on after driving for 45 minutes. Temperature would not decrease below 330F during the hour

Let me know if you have any ideas or if you think I missed something. Thanks for looking!

[Alanrorr1]

As an update, due to cost, I went with replacing the sensor and fluid only first. This looks to have resolved the issue.

Detail:
This wss about $1500 versus about $6500. The overheat problem, at time of service, could be seen 20 minutes after starting the car with sudden shifts (i.e., occurring over 2 seconds with about 100F shifts in temp) and fairly quickly reaching the first warning threshold of ~315F. Now, with the repair and 200 plus miles, I have not seen the warning, the clutch oil temp has been staying under 222F and more recently less than 190F. No rapid jumps up or down are seen in the clutch oil temperature. This is even with 15F hotter days than when the car went in for service.

[engine killer]

good to hear that it is only a faulty sensor

[Alanrorr1]

FYI: Thanks and just to complete this thread: At 1 and a half years after the sensor replacement, the transmission is still operating well. That turned out to be it. Much better than the $6500 alternative.