1 have a 1999 GMC(Isuzu F series) T8500 with only 10,000 miles on it. Just converted to a flat bed and when loaded it bucks at about 30 mph in 4 and 5 It has a Cat 3126 with an Allison 6 speed (6 is locked out) Any ideas? Cheers Len
Did I read that correctly, 10,000 miles on a 17-18 year old truck? If that is correct have you changed the oil and filter? Is it manual control or electronic? I would start with an oil and filter change and a few road miles first. You possibly have a governor that is sticky, seen it many times on trucks that don't see the blacktop much and get up to a reasonable temperature.
10k Yes, an x fire truck from a quiet community.
Bill The shifter is electronic with digital readout so I would assume the answer is yes. As a fire truck it was very well maintained. However there is no log book so I will change the trans oil and filter right away.
Thanks for the help. Turned out to be chaffed wires.