This is mounted on the top of the passenger side firewall on my '76 40. It looks to be hooked up to the harness, but has other hook-ups that are not connected (I'm wondering if it's even functioning). As much as I've been able to research, it is a maintenance interval detector. I am disassembling the whole truck for frame off. If I rebuild but leave this off, will it matter?
Leave it off. It doesn't matter. It is sometimes referred to as a "speed marker". The speedo cable originally went through it and it has an odometer inside. It changes the state of a switch inside every 25,000 miles, to light the EGR indicator under or on the dash. After servicing the EGR, the dealer would just operate the switch to the other state. They are interesting devices and I'd be curious if the odo in your "speed marker" matches the odo in your gauge cluster. If you have a one-piece speedo cable (I don't think the two-piece is available any longer), the speed marker is just something extraneous hanging from the firewall.
Thinking about a previous statement, your speed marker odo most likely doesn't agree with your instrument cluster odo, because I don't see a speedo cable to the speed marker and it was probably replaced with a one-piece cable at some point in the past.
