Help ID'ing this silver thing

[88Ironduke]

The subject is a 72' Fleet side with 3OTT / I-6. The engine side of the wire harness was pretty beat up so I ordered a new harness from a local vendor on the forum. I got a new harness as advertised but I have to reuse some of the original for charging and headlights...

My dilemma is this little silver relay box and the section of wire it resides with. It bolts up on the drivers side fire wall outboard of the master cylinder. I have tracked it back to the bulkhead head connector via a brown/white strip wire that goes to what I would consider pin 1 of the engine harness next to a large purple wire. I just can't find that particular wire on the schematics. I'm having trouble uploading so any help is appreciated.

I am totally lost as to what this little thing is or does but I assume it is staring me in the face. Ideas and opinions welcomed.

88Ironduke

[chiefcfd]

It sounds like the Transmission controlled spark unit. It was a poor excuse for emissions back in the day. It controlled the vacuum advance with the transmission.

[88Ironduke]

Thanks Chief. I haven't found if anything in the emissions harness is required to operate the truck but I am assuming it hasn't worked since... 1972. 100K miles and I'm thinking delete.

[geezer#99]

Originally Posted by chiefcfd

It sounds like the Transmission controlled spark unit. It was a poor excuse for emissions back in the day. It controlled the vacuum advance with the transmission.

It was called a TVS. Transmission vacuum switch. Had nothing to do with emissions. It helped your power and timing on your motor. Your carb would of had only manifold vacuum ports to supply vac to your distributor. No ported sources. With no vac to the distributor your initial timing could be increased to give more power in the lower gears. In top gear where your motor could handle more timing with the vac advance added the tvs supplied the vac to the vac pot. THe tvs had 12v to it when ignition was in run mode. There was a ground switch on your trans which when you shifted to high gear supplied a ground for the tvs and it would open.

[88Ironduke]

Thanks geezer. My 250 has been given the upgrade of HEI ignition and a slightly more aggressive cam shaft over stock. Would the TVS system be applicable and useful?

The HEI by itself made a huge difference in starting and overall power. Stock initial time was thrown to the curb by my father in favor the modified setup and custom number he conjured. Unless I can come up with a reason to maintain it I am going to nuke the system entirely.

[geezer#99]

It'll run fine without it. THe tvs was mostly for trucks with a heavy load that needed more timing in the lower gears.
The hei is the best move you can make. I ran one on my 67 gmc and the start and run was like night and day. Best bang for the buck.
Set yours up with 12-14 degrees initial. Your motor might even like a little more than that due to the cam change.

[GASoline71]

Originally Posted by geezer#99

It'll run fine without it. THe tvs was mostly for trucks with a heavy load that needed more timing in the lower gears.
The hei is the best move you can make. I ran one on my 67 gmc and the start and run was like night and day. Best bang for the buck.
Set yours up with 12-14 degrees initial. Your motor might even like a little more than that due to the cam change.

This ^^^ 100%.

Gary