What is flaring during shifting in a transmission?
 

I heard the term mentioned many times on this forum. Can someone explain what it is?

Instead of a crisp shift, the transmission misses the shift and hangs between the two gears. The engine revs up higher than desired because it's unloaded........sometimes much higher........before the shift finally occurs.

Sometimes it gets so bad that the driver lifts his foot to get the shift to occur.

Thanks Brian! Great explanation. what causes flaring?

My 83 seems to take longer to shift from 3rd to 4th than from 2nd to 3rd. Also, when coming to a stop going into low gear, the tranny makes a pronounced thunk. Is that normal?

First thing to try, ... nevermind.

Okay, the first things to check would be vacuum lines, check for any cracked/leaking/loose lines. If the transmission is flaring on all gears, could be a vacuum problem, can be adjusted out with the vacuum modulator if the vacuum is not correct. Need to know if it is correct before adjusting though, there is a thread on the diesel forum, title is something like: it is very important to correctly adjust transmission vacuum, some long title. Very useful.

Low vacuum will often cause hard shifts, more common vacuum symptom.

The other thing to check would be transmission fluid level, too high will cause shifting problems as could too low. A clogged filter in the trans pan can also cause problems, if the control pressure is low due to poor flow, often a clogged filter will also cause the pump to sing to you.

There are other internal issues that can cause problems, I think a search for "k-1 spring" might help there.

I'm not a trans guy, I keep changing fluids and filters and adjusting the Bowden Cable and keeping the vacuum lines on, they shift nicely for me (so far).