I've Had this car about 6 months and it has 108k miles on it. Today I was Driving down the interstate when The engine started reving high and I realized I didn't have any high gear left. luckly I was almost at my off ramp and got it to the side. I can put it in 2 or 3 and goes just fine, but if it's in drive when it tries to make the last shift is just spins. On the way home I intentionally reved the engine and it seemed to try to kick into high gear hard. I have check for vacum leads (none), checked the vacum line going to the modulator and didn't see any xmission fluid in it. I did have to put in a quart of xmission fluid, even though the shop told me everything was okay about 2 months ago. I need help ! can the modulator do this? and exactly where is it located on the transmission. It's a 1990 190e 2.6 Auto

Thanks