Auto trans replacement? on 1990 Ford Aerostar

The vehicle was operating fine and traveling downhill on a freeway when the transmisson seems to have failed. The transmission has fluid but we have absolutely no power transmission in any forward or reverse gear. Before we replace the transmission can we determine that the transmission is the problem or is it a torque converter problem? If we decide that the transmission needs to be replaced are there any words of wisdom that may help us along the way?

Thanks, Wayne

Asked by for the 1990 Ford Aerostar
whatever it is, it is internal. you will be replacing the trans. about 1500-1800 job.