Goofy tranny or tach?

[PartTimer]

I noticed the tranny "hunting" while travelling about 30-45 mph even with the cruise set. I have changed the fluid 3x now with the same results. I have now noticed under moderate acceleration as the speedo goes by about 50 I can watch the tach drop a few hundred RPMs and then jump right back up. This is consistent each time. Any ideas? This is a 99 XLT with a 5.4 liter motor and 40k on it.

 

[rwinch]

This sounds like the Overdrive kicking in and out. Try turning the overdrive off and see if the tach stabilizes. If it does, then it is the transmission.

Couple of questions:
1) Is this a recent problem, or has this always happened, or maybe steadily increased in frequency?

2) Did you Power Flush the tranny when you had the fluid changed? If not, this really helps remove a lot of crap that a simple fluid drain does not.

By the way, a '99 and only 40k on it? Do you only take the Expy out on Sundays???? Maybe it is getting "lazy Tanny" syndrome and you need to take it out mud bogging to work it out a little more

 

[PartTimer]

I bought it last year from an elderly couple that past away. It had 19k on it then. Its really from my winter driving and k9 transporting. The tack dropping is relatively new but the hunting issue has been around a while. I will try a power flush next!! If I recall it does it even with the OD off but I will play around with it and let you know.

 

[monsta]

I think the problem is that the Torque Converter is not locking or is having problems decided when to.

The TC can lock in all forward gears, not just OD. What you feel as the speedo passes 50mpg is the TC locking then unlocking.

I would take it to a tranny specialist and ask then to diagnose the proper operation of the Torque Converter.

The TC is electronically controlled, so fluid is not the problem with this one.