AOD 4th gear finds gear- then neutrals out under load
I have a 91 Mustang GT with an AOD that shifts through all the gears. It has always 'short shifted' 3rd into 4th. When I went to pass a car the other day, it downshifted into 3rd letting out a bark (tires). It shifted back into 4th, then started to freewheel like it was in neutral when it went under load up a hill. It eventually got to the point where it will now neutral anytime under load- but will find 4th gear as soon as you let off. All other gears work fine other than hard shift into 3rd ever since the 3.73 upgrade.
My TV cable is adjusted per the Ford manual. Some modifications that I have made to the car that may affect your input are a 3.73 rear end w/ correct speedo gear, many performance mods and supercharger which is probably putting me at around 350hp or more. The supercharger has only been on the car for about 1,000 miles and the car has 67k.
I have alot of mechanical skill and have worked on c6's about 20 years ago- so lay it on me if you know what I should check into. I have not dropped the valve body yet- but looking for ideas before I jump into it.
Question: what is causing 4th gear to neutral?