My trans has both billit servos, vac. modulater, transgo shift kit, 5gear planetaries, new seals. 2200 convertor.I had my tranny gone through whe i built my 355, since then it has been pulled out and gone through again and air checked, installed , line psi's checked, wiring harness to comp ohmd out good, put a snapon scanner while driving and the computer is calling for all gears and the lockup is working. When driving the car in drive it takes off on 2nd gear goes to 3rd and and convertor locks up after awhile of driving tis way i get 1870 slip code . When puting the car in 3rd at start 2nd gear take off to 3rd and you will never get the 1870 code, if i am driving 60mph, floor it it will take off like a rocket until 6000rpm and then whaaaaaam slips hits rev limit and wont shift to3rd. Here is the kicker if I manually start in first ,floor itshift 6600,2nd shift 6600 with no slips at all,then 3rd and i let off , who has my magic answer? The tranny worked fine before i ever took it to this guy.

 

Unfortunately, this is a near impossible one to diagnose on the computer. I'd have to say that whoever built your transmission has something messed up. This sounds like a valvebody problem.
My next question...WHY are you people still letting anyone put a vacuum modulator in a 4L60E?

Frank
CPT