4L80E help?

[adz55]

Just picked up a 92 suburban with trans issues. Allegedly the previous owner did a fluid change and then parked it for three months on a steep hill and now the forward gears slip and feel weak but reverse is perfect. The truck will move and shift manually (I didn't try it any other way come to think if it) but when starting off it slips pretty bad will make a fan clutch slipping noise until about 35 mph and then it goes away. The thing backs up like it's perfect. Anyone else deal with this?

[gmtech4]

forward clutch MAY be farked ,it needs to come out and apart for an estimate or replace it (trans),,,dont forget to flush the cooler