I should start this out by admitting I know very little about engines. I have a 1993 F150, it has a 351 in it.

The other day I was going fishing. I always tow with the overdrive off, well about a block from my house the "off" light on the shifter started flashing, and a little later it went off. I parked, shut off the truck and tried it again and the light would not come on. I decided to not to try to turn the overdrive off and it ran just fine. What could be the possible symptoms or remedies? I'll take it to a mechanic Monday, but I wanted to get some ideas first.

Thank you.

 

I checked the transmission fluid this morning, I put it on a white shirt and it looked red, there was no bad smell. It runs fine when it's in overdrive but I have noticed a slight zzzzzzzzzz when I accelerate, it only lasts for a couple seconds. When I take it out of overdrive it seems to run a little rougher, but that may just be in my head.