Transmission slipping at cruise speeds
 

'01 F-150 Lariat SuperCrew 5.4 V8 4R70W Auto. Trans. 145K miles

At highway speeds (55-65) in overdrive engine speed increases (tach shows 100-300 rpm gain) with no increase in mph on speedo (slippage) engine stays at that rpm for 4-6 seconds then drops back to where it was. This occurs intermittently....in the 10 mile trip to town it happens maybe 4-5 times. Sometimes several times in a mile, sometimes I can go 5 miles without it occurring at all. If I accelerate or tap the brakes it stops slipping and returns to normal. Has been occurring off & on for the last 5-6 months. I had the Trans fluid changed and the complete system flushed about a month ago....didn't seem to have any effect on the problem. The Trans. shifts fine and there seems to be no slippage at other than cruise speeds. Doesn't seem to be getting worse but is not going away either.

I live in a remote area and there is only one local mechanic (closer than 100 miles - Boise) who knows anything about Auto. Trans. issues and I took the truck to him and of course it would not slip when he drove it....so he had no suggestions except to "let me know if it gets worse".

Any help, ideas, thoughts, etc. would really be appreciated.

Thanks,

Larry in Idaho :confused:

Several things can cause this. A bad Transmission Range Sensor is hard to diagnose, but is my guess as the cause. A problem in the brake light circuit can do this, too.