I have a 1998 4.6 f150 lariat and when the transmission gets warm it doesn't want to go into reverse. If you put it in drive and get it moving and shift into reverse really quick it usually goes in. I had my transmission rebuilt, including torque converter, because of the same problem and it was good for about 20 000, which was what the warrenty was good for. That makes me think that it is something external. The fluid looks good and the problem seems to be worse in the summer and better in the winter. It NEVER does it when the truck is cold. I bought another truck and am looking to sell this one but would like to get it working as this is the only thing wrong with this truck.

SOURCE: I have a 1998 F150

it pos that the transfer box may be the prob but take it to your dealyer just to be on the safe side

SOURCE: transmission/ transfer case

As funny as it may sound in alot of instances it must be reset. try turning the key on and off about 60 -70 times. seems rough on the starter but has worked for me in the past. not sure if it works on fords but works well on all mopars.

SOURCE: AOD trans wont shift when cold

Ok I have no Idea who told you not to change the fluid? However there is a reason you have to drop the pan to drain it, or change filter and the reason for this is. So you cant just change fuild and leave dirty filter in, or change filter and leave dirty oil in. Your wouldnt just change one or another on your motor would you? lol Anywho your problem is either burnt fuild. Yes the fuild can be come burnt and lose it vascoity, and weight. (basicly your runing gravey through it.) lol Or the fuild is termendously dirty strained or not. FYI there are clutches in your transmission.... Have you ever heard of clutch dust? Well in an automatic because of the fuild its not clutch mud..... explaining why when its cold and the oil is super thick it has problems, but when it thins up when its warm its fine.

CHANGE YOUR FLUID & FLITER

SOURCE: Transmission shifts hard

The first thing to do on thoes Automatic Overdrives is to reset the Throttle Valve cable. At the throttle valve assy. find the cable at the rear (that goes down to the transmission). You don't have to take the cover off, but only if you want to. Place the transmission in neutral and set the parking brake. At the cable take a pocket screwdriver and pry the locking mechanism up. Its a little knurled so it dosen't slip when you push it back down with your thumb. With that up and the cable feels extremly loose grab an assistant. You and them go under the truck and at the shift shaft you both will find the Throttle Valve lever located outside of the shift shaft. Have your assistant pull that lever to the rear of the transmission with 8 lbs of force. Then you go back to the locking mechanism you just pulled up. While your assistant is holding that lever back take your thumb and push that locking mechanism down. Next, have your assistant let go and leave from under the truck. Put your throttle valve assy. dust cover back on (if you decided to remove it). Place your transmission back in park, then release the parking brake. Shut the hood. After that you will not be able to feel one single shift take place.

SOURCE: Hi,

The weld on the manual lever is known to break loose. First, with the wheels off the ground, with the lever in the park position, does the drive shaft lock in park? If yes or no sometimes, drop the trans pan. I bet the nut on the lever camd loose.