What Is This? (Engine bay, 1970 C10.)

[dflarsen77]

Came across this old thread while looking at wiring pictures. I have that thing on my firewall too. And it had me stumped too According to the 1970 factory service manual, it's the relay for the transmission controlled spark system. Mine actually had wires attached and running down to the trans area, although it wasn't plugged in. My truck has factory air too and I had thought it might be something for it, but the manual is pretty clear. Cheers!