trans drops out of gear
 

have a 1996 C3500 6.5 turbo. at idle the tach and speedo work together. when you put it in gear it seems ok. back in park, speedo acts like a tach again. now trans (4l80e)drops in and out of gear. when it drops out speedo acts like a tach again. have replaced every thing elec in and on trans. worked 14 yrs as a shop foreman for cummins diesel in memphis. so i'm no dummy but this is killing me. also is there a seal in pump that will let fuel into the oil .