Put a line pressure gauge ($55 shipped on Ebay, or visit a local trans
shop) on the pressure test port and go for a drive to verify proper line
pressure. Base pressure at idle in P,N,OD should be around 70psi and
around 140psi in R. Max line pressure should be at least 190psi at WOT
in 1st or 2nd, 225psi is optimum. If base pressure is low, trans or pump
is worn out. If max pressure is low, check EPC solenoid. If reverse is
slow to engage, and line pressure does not come up to at least 130psi in
reverse, replace the boost valve with a Transgo 0.490" boost valve
(easy fix in pan).
Slow, slipping or no reverse: "lo-reverse" clutches are worn out,
fluid leak in the reverse apply circuit, or broken sunshell. It is
possible to remedy a fluid problem by removing the checkball from its
cage in the case in the rear of the trans may help (must remove
valvebody), or adding a high-viscosity additive such as Lucas
Transmission additive or other seal restorer product. May also have worn
boost valve (can replace in the pan)
From what you have described, it may just be a broken Sunshell which is the weak point to the 4L60E transmission. Gm had a upgraded program to lower the transmission line pressure when revers is engaged to keep the Sunshell from breaking when put in to revers.
Thank you for using Fixya and keep us posted
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