what is the problem when the transmission will not go into reverse. on 1991 Toyota Corolla

all forward gears work fine, But when I go in reverse you can hear the engine idle drop like it is in reverse car will not back up. I let the car sit for two days after I changed the filter and fluid thinking that would solve the problem it didn't. I checked the car again ( after the two days) transmission goes in reverse strongly no problems. Drove the car in reverse for about a 1 block down the street worked fine. backed the car in the yard shut it down. 30 min. later no reverse. is this a computer issue or a valve body issue. please help

by in Los Angeles, CA on March 08, 2013
1 answer
ANSWER by on March 08, 2013
seek help to diagnose at aamco trans