99 S320 will not upshift , w/s switch question

[BamaMB]

My 1999 S320 transmission has started to mis behave. On occasion, it will not upshift from 4th to 5th. The most common is a failure to go into 5th during gradual acceleration. Letting off the gas at 70 to 80 mph or moving the selector from D to 4 and back has no effect. The trans remains in 4th. I found that if I cycle the W/S switch from S to W and back, the upshift will occur the moment I switch to W. The car has 105K and the trans fluid was replaced about 20K ago. What type of switch is the W/S? Is it a normally open or normally closed switch and all you are doing is adding a power signal to the controller to tell it which mode you want. Or is there more involved? Why would that switch cause an upshift to occur? I live in Alabama and I have never taken the W/S switch out of S.