Trans random shifting

Transmission shifts back and forth from overdrive to lower gear at 55mph to 60 mph. At 70mph runs fine. What could cause this.

I don't see what year the car is?

Marvin Potter wrote:

Transmission shifts back and forth from overdrive to lower gear at 55mph to 60 mph. At 70mph runs fine. What could cause this.

My guess is you have a '96. Is your Service Engine Soon light on? If so, what is the code?

The 4T60E transmission can do this (enter/exit overdrive frequently) when certain parts (and/or the fluid) are worn.

Do a search on this site for discussion about this problem, see also https://www.cadillacforums.com/threads/4t60-e-losing-overdrive.160172/ and https://buickforums.com/forums/threads/possible-solution-for-4t60e-transmission-issues.33373/

...for perspective.

One real solution is to rebuild the transmission using the Sonnax and TransGo improvement kits, but when you look at the other posts about this on this site you will see other, less expensive strategies. Sometimes even just a drain and fill of transmission fluid, to get the viscosity back where it should be. Other times, something else. Have a look at this subject on these pages, as well as the links I posted.

It’s a 95. Has new trans synthetic fluid and filter. It had 2 codes. One for cruise control. CC servo switch. The other code was egr which I replaced and made engine run smoother but little effect on trans problem. I just recently bought the car. I suspect CC problem is in multifunction switch.

What is the actual cruise control code?

Reason I ask -- hunch is your brake switch may be worn out. The 4T60E is electronic and will go into or out of lockup depending on the state of the brake light switch(es). A flaky switch can cause the "transmission" problem you cite.

Brake switch malfunction will also cause problems with the CC.

Brake switches are cheap, and it would not be a waste (given the age of the car) to just replace it.

It could be that simple.

Actual code was P0565

Marvin Potter wrote:

Actual code was P0565

Isn't that a 'cruise control on malfunction' code?

That could be the turn signal switch (unlikely), the brake switch (age/mileage/flooding/accident), but in any event a malfunction in that switch will make the electronic transmission control erratic.  Could be something else. My wild guess is the brake switch. I forget exactly when mine went but I remember what an absolute pain in the butt it was when it malfunctioned - in a way that it still *kinda* worked.

Fix all your codes then troubleshoot your transmission if it is still acting up.

Thanks for your help. Will do that.