top gear

Having a problem with the gearchange - in drive on the selector, but seems to stay in 3rd. I can travel for 30 miles like this, so not a warm-up problem. Fluid level appears ok. Car 1s R129 1996

1990 190E 4 cyl 2.0 ltr Auto saloon. Problem: The car does not change up into top gear. The autogearbox has been functioning without problem. The car is normally used locally, but the other day undertook a fairly long motorway trip and was fine for the outward journey. However the next day...