New to this, so hopefully this is in the right area to ask a question about the UD diesels.

I bought a 98 UD 1400 with the Jatco automatic. I was told it had no reverse, but it actually has only 1st and 2nd. If you start off and then let off the accellerator it shifts fine into 2nd, and every once in awhile seems to want to shift into 3rd but never can. Is there anything on these automatic transmissions that could be anything other than shift solenoids? Could there be any connection to something with the exhaust, or vacuum? I assume this has no type of vacuum pump anyway. It acts like a bad modulator to me ---with the way it shifts when letting up on the pedal. If you don't let up, it winds out pretty high before it shifts. Some of the numbers are 31020 10DF0, and AD 005 JATCO. It looks to have a removable plate for a possible pto on the let side. Any help is appreciated. Also, are the solenoids internal? Feel free to email mjohns63114@gmail.com or call or text 314-397-7278. Thanks!