Overdrive common problem.

[KazuyaIB]

Im having a problem with my 91 suburban not kicking in the overdrive. The Wire labeled " A" (also purple) on the square looking plug that goes to the transmission is not receiving any power... I checked on my dad's 92 truck and it has a strong power. On his 92 that wire leads to a small compartment box on the passenger side with a small looking piece of motherboard type of stuff. Was wondering if the 91 has that same thing as the 92... If not, can someone help me identify where this wire labeled A leads to and why it might not have power...