MY 97 F150 with the 4.2 v6 is downshifting erratcially on a constant load anywhere from 45-60 mph it suddenly downshift to second and revs higher. I had the transmission checked and the local garage could not find codes although it allowed them to clear the codes. they then suggested it might be the voltage with the original factory battery still in place. Even now with a new battery it still does it and now I don't know what to check next. The tranny was serviced at 75k and again at 130k, any ideas?

 

If you have an auto, check the modulator valve on the tranny. I'm not sure whether the newer Ford's have them or not, but my old 83 f-150 had one. Haven't checked my 01'. The modulator valve looks like a small fuel filter in appearance, and is located either on the back or sides of the tranny. Can't remember exactly where it's located. But just look around the tranny for something that looks like a fuel filter with a rubber hose attached to it. These things work off of vacuum. Also check the hose for leaks to see if your losing vacuum pressure to the modulator valve. I Hope I helped.