In article ,
"Bob May" wrote:

Actually, there are no syncros in the large truck gearboxes. Rather it is a
dog setup which is used to let the gears join to each other. Thus the use
of the clutch actually removes the ability to easily compare the road speed
and the engine speed by the driver for when to engage the gearbox to the
desired gear.

Most truck transmissions have a five-speed front section with dog
clutches, and a separate air-controlled rear section for selecting
ranges. 99% of the time that rear section will have a big-assed
synchronizer in it. It's likely this synchro is what the first poster
was talking about.
One easy thing to check that might fix it is a teensy air filter on
the right rear of the transmission. It'll get clogged up and restrict
the air to the back section's shifter piston, giving the symptoms of a
fried synchro. Of course, if it goes long enough like that it'll really
fry that synchro, but it's a good first thing to check and easy to do.
The friction surface might get a glaze on it, but a quick trip on the
interstate will burn that off and let you determine if you fixed
anything.
If that doesn't fix it, then you have to do real (expensive) repair
work. It's possible to pop that back section off without pulling the
transmission, but not much fun. Besides, whatever took out the
synchronizer probably did damage to the rest of the transmission.

--
B.B. --I am not a goat! thegoat4 at airmail dot net
http://web2.airmail.net/thegoat4/