Vacuum switch for the backup lights?

[76MaBronco]

I'm patching a very lightly hacked harness on my '72. I follow the wire from my 3 speed up the firewall and it terminates at a vacuum thingy on the front drivers side of the engine, has another terminal that goes into the firewall and ends up being a brown wire (backup lights?) The wire grounds when the trans is put into reverse...

What the heck is the vacuum thingy???

 

[DirtDonk]

Typical switch arrangement on your '72 would have been a small plunger-type switch mounted to the bottom end of the steering column.
Power comes to it from the dash, and then straight out to the backup lamps from the switch. No grounding at all, until it gets past the bulbs.
Wire color should be Black w/red strip on yours I think.

Anything else under there that looks somewhat like that?

The '72 model year trucks had some oddball smog-related devices that most other years did not have. Where exactly is that vacuum thingy mounted? Is it screwed into the intake manifold?

Paul

 

[76MaBronco]

There's no switch on the column that I can see, and there doesn't seem to be any hanging wires off of it either. Would that switch be inside the cab or external?

It would make sense it's emissions. It's to the right of the dizzy, bolted on a bracket, has a little cap (protective air cap, it spins) on the end, and a couple vacuum lines that go under the air cleaner and to the left of the engine manifold (didn't look futher than that)

 

[DirtDonk]

Yeah, that fits one of the '72 emissions device descriptions.

Hmm, you have an automatic swapped in, by any chance? In that case, the switch would most likely be down on the side of the trans.
Or a manual shifter on to the floor perhaps? In that case the PO might have yanked the wires and put it down near the trans, on the shifter.

When you said you followed the wires up from your 3-speed, were they coming from any kind of a fitting on the trans case itself? Or were you just finding wires that looked like they were coming from the area? The only Brown wires I can think of off the top of my head are for the ignition bypass and, oddly enough, some of the circuits controlling things like vacuum switches, high-idle solenoids, and (if I remember) something called an Ambient Switch. Not sure, but that might be the thing they were discussing a couple of weeks ago that's bolted into the driver's side door hinge pillar.
You have some odd looking plate thingy bolted to the door hinge post?

I know you already have, but do a little more digging for some Black w/red stripe wires somewhere.

Good luck.

Paul