Hi, my 1996 xjr will shift in and out of overdrive without reason. This happens when cruising in hilly/undulating territory on moderate throttle between 80-100k in normal mode with D selected. This will happen when its clearly not needed. If i shift to 3 there are no problems.
There are no faults either on the dash or shown on the OBD2 reports with my adapter and phone app. The gearbox has been serviced and the fluid changed very often.
Has anybody had the same experience?

 

On the GM 4L80E transmission there are 2 electric shift solenoids giving 4 shifting possibilities as that based on percentages is probably what it is . You can read them from the Transmission ECU as pin 53 to 39 ( Solenoid A ) and pin 53 to 43 ( Solenoid B ) . But may not see them as a failed circuit until they are heated up as this is a solenoids primary mode of failure ( if it is that ) . But if you can see them cold it would be worth a try as a least intrusive try . I don't have the GM L80E transmission so it is an outside observation from the same issue I had on the GM 4T65E series I had .Shift Solenoid Pattern :

1-2 Solenoid ( A ) 2-3 Solenoid ( B )

1st Gear On Off
2nd Gear Off Off
3rd Gear Off On
4th Gear On On

Look for CEL code P0755 through P0759

See link below and the post # 6 link for shop manual :

https://www.jaguarforums.com/forum/x...operly-192847/

See page 67 with respect to the # 10 ball check valve as is plays a role which may be resolved by a fluid replacement that cleans the passages . An expensive ( fluid cost and DIY labor ) but easy try but if you are to retain the original transmission better off replacing the fluid anyway . But you did say fluids are changed often . Look at the dipstick color as the fluid should be pink and not burnt brown .

and :

http://etereman.com/blog/general-mot...c-transmission

and : you will see other GM L80E solenoid videos in the area , this video is for a similar GM 60 series