I apologise , the code 1870 was in my mind since I see that code so often this year. p1860 is a more wide capture code of solenoid problems. Often the PCS is looked at for p1860, but the TCC solenoid can be at fault too, some trouble shooting will point to the pressure manifold.
Depending on mileage of the transmission, you may want to replace the soleniods and pressure manifold on the v-body.
Has the vehicle been turing on the warning lite for over heated transmission?
At the Bonneville Salt Flats, first gear is known as 130 mph.