Gas tank sender question

[bigwalleye]

I got a 76 blazer with a 350, th350 tranny, 205 t-case, quadjet carb. I purchased a new tank and sender. I noticed that a manuel tranny has 3 outputs on the sender and a auto has a 4 output senders. I am confused what the 4th output is for.

1-Fuel output tube
2 - filler overfill tube
3 - tank back persure tube
4 - ???

On my current setup the 4th sender output was capped, just trying to figure out what it should be going to.

http://www.carpartswholesale.com/store/index.php?cat=67&year=1976&make=CHEVROLET&model=BLAZER

[/IMG]
SENDING UNIT
Notes: 4 Outlets, 25 Gal., A/T, w/O-Ring & Strainer, (FG12D)
Part Number: G670319
OEM Number: 0
List Price: $98.25
Sale Price: $49.95

 

[mechted]

i believe its a vent line (ie plug it or run a line somewhere up high on the body)

 

[bigwalleye]

Its odd if it is a vent line it is just designated for an auto. A vent line was my first guess also. Could it be a smog thing?

 

[mechted]

i dont think so, my 73 has the same thing... they had very limited smog equipment back then

 

[dyeager535]

Fittings on the sender would be: (if every one ever put on was in place)

Fuel out
Fuel Return
EVAP or Breather
Fill neck vent

Return is not always there, and if they ran a vented cap early enough, a vent on the tank would have been unnecessary as well. Emissions setup was very much based on final sale location, chassis, and drivetrain combo.