I have a 1997 F250 7.3 Diesel automatic with trans cooler. I took a trip with it pulling a camper trailer. At steady speed of 70mph on highway the transmission kept down shifting every 5 minutes or so. Wasn't increasing speed or going up hill? On way home it didn't do it near as much at first. Even climbed some small upgrade hills without it downshifting. As day went on it seamed to downshift more often. Can anyone give me some direction on how to slow or stop this? It really killed my mileage...went down to 8.5mpg. I had to be real careful not to push on the gas at all or it would downshift. Any info is helpful. At 70mph my tac was showing around 1800 rpm. Issue decreased dramatically if I dropped speed to 60-65mph.

 

You might have been fighting a head wind?

 

Replace the Manual Lever Position Sensor (MLPS.) In these years there was a problem where they would start to fail and indicate that you moved the shifter to the 2 position. That causes a downshift.

You can diagnose this if you can watch a live data stream. Watch the PRNDL data to see if it moves from drive when you have a downshift. It may only indicate it for a second or two, but that is enough to make a downshift happen.

 

1800 rpm at 70 mph?? Are you using a gearvendor?

 

1800 rpm at 70 mph is what my automatic does with factory 23585R16s and 3.55 differentials in overdrive.

 

That's about right with an auto, 3.55 and pizza cutters.

 

My truck doesn't have the digital gear indicator.. it is pre that system.

 

That is what my truck is doing also...but for no good reason at all it suddenly decides to downshift. If I try to kick the downshift out I will loose almost 5mph doing it. If I speed up and allow the normal shift... it will be ok for a while until I get back to the 70mph area then downshift again. about every 5 miles or so. Wondering if I'm loosing vacuum that's causing it to downshift?

 

DOn't even know what a gearvendor is so I'm guessing I'm not using one.

 

Mark K meant you should hook up a computer with AutoEnginuity or some other scanner that will read trans info to see if the PRNDL signal changes when your trans drops a gear.

 

This doesn't have anything to do with a digital gear indicator. That's the sensor that tells the PCM where you have the shift lever.

 

Vacuum isn't an input for shifting. The problem is a failing Manual Lever Position Sensor. See my post above.

 

I stand corrected, I'm just so used to my 4:10's I'm not thinking outside the box, lol.

 

what causes it to downshift for passing gear? I thought that was vacuum?

 

The C6's were vacuum controlled, the E40D's are electronic.