From what I can make out in what I have...which MIGHT not be the right info for your particular truck but worth checking........there are three single wire switches at the control handle for shifting,these switches are the grounds for the Solenoids...the brown/black wire on the harness side for FWD switch2 changes to red/white at the switch,when the handle is moved for second speed it opens the fwd one switch and closes the fwd2 switch....it looks like fwd 2 switch is the left most switch under the control handle,fwd1 is the middle and the right switch is reverse,check this switch to see if it is faulty....Hope this wasn't to confusing and helps??