I searched the forums for 1st and First and had over a thousand hits so I thought I may ask the xperts.
My 2001 Astro has about 160K miles, a stock 4L60 automatic paired with a NP231 TC. The problem is that at times the first gear lacks torque if in high range. For example I tried to drive up a trailer ramp last week and the engine went to 3500 rpm but the vehicle only rolled up to the ramp and stopped. Than it felt like it "clicked" and it drove up normally. I had this before a few times for example in sand except the click, never noticed that before. TC was in 4H and it was trying, engine revving but not going. Obviously I am not doing this for more than a few seconds to avoid damage. I can make it easier for the transmission and go in to 4L and all is well. Also street driving doesn't seem to be impacted but I also drive pretty gentle.
Fluid level is OK. Fluid color is OK. Solenoid? Any suggestions on a test procedure?