1971 B-Body Firewall help needed

[moparmarks]

StratOvent water valve has vacuum lines on the pass side.
I think it is for the tach but I'm a bit confused about the 71-4B tach wiring. I've seen the stand alone tach harness and think that is the hole for it but the cars that I've owned and ones that I've worked on didn't have the separate harness. The tach wire was in the bulkhead as seen here on this 71 V code RR with factory tach. The grey and black wire goes to the coil. I do have a couple of the stand alone harnesses , can't remember where I got them

 

[moparmarks]

Also I checked several non-tach cars and all have a dimple there so that is probably a factory hole.

 

[Nacho-RT74]

Well the hole is factory for sure. I found a pic of an APARENTLY 72 ( due the engine ) with same location hole with a wire, that could be very well the tach wire.

But as you noted Mark, it seems there is a mix of setups with tach wire for 71 on engine harness and on firewall to 71s, and both diagrams posted confirms that. I'm pretty sure 72/73s are on firewall, because with the NSS ground signal running to cab at NSS harness plug, the cavity used on engine harness was used for compressor clutch on AC cars, and the one free on NSS plug was to the Blower ( AC and Heater )

74s got back the tach to bulkhead at NSS plug, and AC harness runs throught the firewall ( both wires ). Can't tell heater 74 cars where the blower wire leaves the cab... maybe NSS harness?, but what about if tach equipped ?

I think my 74 gets the A/C harness leaving the cab throught that same hole!

 

[RJS]

****Update*****

I believe a previous owner put that hole in or factory screw-up.
The booster backing plate fully covers it and that I why I never noticed it before.

I'm going to start another post about the backing plate because not sure if it's correct.
Thanks for all the help!!!
Ron

 

[Montclaire]

Throw a dab of seam sealer on it before you bolt down the plate.

 

[RJS]

Throw a dab of seam sealer on it before you bolt down the plate.

Seam sealer?? no way!! A gasket goes between plate and firewall seam sealer is for just that seams...
Ron

 

[popslavaz]

Tach wire goes through bulkhead for sure. The dash mounted reverse light goes through firewall,if your car is a 4 speed.