2008GT

I recently purchased a 2008 Mustang GT. The previous owner told me that the automatic transmission was slipping at times. Now that I have driven the car for a while, there are two issues with the transmission. 1) Sometimes, when shifting from park to reverse when it is cold, it will hesitate and then slam into reverse. 2) Often, after about 10 minutes of driving after a cold start, it seems like the transmission shifts into neutral and the engine revs up. If I slow down and let the transmission down shift, then the problem goes away. After it has happened, the transmission will work fine the rest of the trip. Any thought on what is wrong?

Dino Dino Bambino

How many miles has the car done, and when did it last have the transmission fluid/filter changed?

JimC

That sounds like it could be low fluid. If the fluid was changed they many not have properly filled it.

2008GT

Thanks for you quick replies. The car currently has 118k miles. I checked with the previous owner. He bought it four years ago with 85k miles. He never changed the transmission oil or filter. It sounds like changing the oil and filter may solve the issue. When I change it, should I change it, drive the car a little, then change it again to get all the old oil out of it?

Gravydog316

That's common with these cars.
Search around here & other forums

or Google

i have read that stuff before

JimC

Changing the transmission fluid and filter is not easy. You have to get the transmission up to operating temperature in order to fill it to the proper level. The recommended procedure involves filling while the car is running and in gear - tough to do for most of us. The fluid is added through the drain plug, you pull the center part out of the plug and pump the fluid up into the transmission. Then put it in gear and get it to operating temperature and fill it more to get it completely full.

Dino Dino Bambino

Why did Ford have to make refilling the tranny so simple when it could instead have been complicated?

outceltj

This is exactly why I installed a different transmission pan. I have the ability to check it and change it. I also have a transmission catch can.

Here is what happens. If you get on the car the fluid heats up and then expands. The expansion is then let out through a tube. So you are losing fluid by design. After awhile it starts to run a bit low thus causing issues. Probably the dumbest thing i have seen. Many car manufacturers are doing this.

Derf00

Having done everything to this transmission except for rebuild it myself here are my thoughts from my own experience and in-depth studying on problems and resolutions for the 5r55s. The 5R55S uses an electronic brain (Solenoid Pack) to control fluid gateways within a valve body. The combination of these two components use the pressure of the transmission fluid to control how and when the transmission shifts. If any of the three variables fails or is failing, it will affect how and when the transmission shifts.

1) Slamming into Reverse - Known issue as the transmission gets older and/or fluid needs changing. In the worst case, you will need a new revers servo piston. More in-depth info here.
https://www.sonnax.com/tech_resource...elayed-reverse
and here:
https://mustangforums.com/forum/gt-s...ot-work-2.html

2) After a slight warm up (10 minutes) transmission seems go into neutral then slam into the next gear. This is called a flare shift or flare shifting. This can and will cause transmission damage if not addressed sooner vs later.. The specific gear(s) that exhibit this is very crucial in understanding what the potential problem and solution is. For Example, does it happen when the trans goes from 1st to 2nd, 2nd to 3rd, or what?
At Worst (unless damage has already occurred), it's servo bore wear and you need to replace the intermediate and Overdrive servos..

More info here
https://www.fordservoboretransmissionfixsolution.com/

If this slip happens in ALL gear, then it could actually be your converter or the Valve body TCC circuit (you would need a new Valve body).

Always start with basics - Do a Fluid and Filter change first. See if anything gets better or is resolved.. Neglected fluid or low fluid can cause problems since it affects the pressure the system has. It's not that hard. You just have to be a tad creative to make the job easier.

Post 4 in the thread below has the method I use for refilling the trans. And yes, you can use the Differential Pumpkin for lifting the entire rear at once. There's a flat spot under the Diff that is perfect for a hockey puck spacer between the diff and the floor jack to spread the force out and not damage anything.
https://mustangforums.com/forum/4-0l...r-ideas-2.html

This is the pretty inexpensive tool I built to refill the transmission using a garden sprayer, 3/8 ID clear vinyl tubing , brass fittings and the all important fitting to the pan itself. Don't bother using the $5 hand pump to try and pump each bottle or transmission fluid into the trans independently. It's a real hassle and takes FOREVER vs about 5 minutes with the tool.

BTW you can cut the cost of the tool in half if you skip the brass ball valve. I put the valve in there so I could manually control the fluid once I pressurized the tank.
https://mustangforums.com/forum/v6-s...ut-to-die.html

2008GT

Thank you so much derf00 for all the information and all the links. I will start with the basics. Filter and fluid. I am considering fixing the reverse servo, since I will take the pan off during the fluid change.

About the forward issie, I don’t know in what gear it happens, but typically after about 10 minutes of driving and usually at about 30 mph. It does not slam when it engages again. Usually I just slow way down, which seems to make it down shift smoothly and then it will **** up again smoothly and no issues after that. I have not had this happen twice in a row on the same trip. Only once per trip.