'72 Firewall Help

[tjsblazer]

Guys I can't remember all the Firewall holes on the Drivers side of my '72 Blazer. See the picture with my notes on it. What are the 2 holes circled in red.
The lower is 0.75" diameter. I didn't measure the upper one but it is probably around 1.25" diameter.
I think the smaller one may be Tach ??

Maybe these aren't both factory ??
Unfortunately I don't have a good picture of this area of my truck before I tore it all apart.
Thanks for any help.

[dirish]

Top hole is my wiring that runs across top of fire wall.
Bottom hole not sure maybe fuel gauge wire?
Here's a picture.

[ls1nova71]

Small bottom one is the fuel gauge wire. The top one is for the EST junk, goes to the relays under the dash. Judging by the hydroboost, you're not going for a stock resto, so you won't need it.

[Minnz]

As said above, the lower hole is for the gas tank float. The upper went up and along the firewall gutter to the smog stuff. Mine was a California truck with a with a smog pump, that seems to be the connection relating to the upper hole?

I wish that I would have filled that upper hole before final paint as its not necessary, oh well, I guess that's what grommets are for.

[tjsblazer]

Thank you everyone for the help. So to clarify on the upper hole. If I'm not running smog, it serves no purpose. I won't use that branch at all?

[hemi43]

Hey Jeff, my truck is not a CST and had no smog equipment. Actually, it's doesn't have that many options but that hole is used by something. I'll try and find out what it was.
Dan

[ls1nova71]

Originally Posted by tjsblazer

Thank you everyone for the help. So to clarify on the upper hole. If I'm not running smog, it serves no purpose. I won't use that branch at all?

Nope, you won't need it.

Originally Posted by hemi43

Hey Jeff, my truck is not a CST and had no smog equipment. Actually, it's doesn't have that many options but that hole is used by something. I'll try and find out what it was.
Dan

Your white wire looks like a factory tach wire, does your truck have one?. The smog (TCS) stuff started in '71, so yours wouldn't have a hole for that stuff.

[tjsblazer]

My truck is a base grade '72. Not many options. As far as I know it was originally sold in Arizona. No smog on it but could have been removed long ago. The picture I took before I tore it apart was a bad angle and the fender cuts off the line of sight down there. Body guy is shaving a couple of other holes that I know aren't needed. Not sure about that one though.
The lower is the fuel sender for sure. Thanks for jogging my memory.

[hemi43]

I looked through my pictures, and all I can find is where this wire attached to at the fuse panel. I'll go out later and look at my harness to try and find out where it terminated.It's the brown wire at the very top of the fuse panel in the picture. It's plugged into the front of the panel which tells me it's an option.

[Minnz]

It looks like hemi43's upper hole is a smaller diameter than the others?

[ls1nova71]

Originally Posted by hemi43

I looked through my pictures, and all I can find is where this wire attached to at the fuse panel. I'll go out later and look at my harness to try and find out where it terminated.It's the brown wire at the very top of the fuse panel in the picture. It's plugged into the front of the panel which tells me it's an option.

The brown wire is the fuel gauge, they plug into the fuse box on Blazers just like trucks.

[tjsblazer]

Hey Dan. Now that I'm looking at your fuse box pic. What is that relay? just to the right connected with the red and green wire. Right where my clutch master will be sitting. I will have to relocated the electrical there slightly.
I'm realizing now I didn't take near enough pictures considering I tore it apart over a year ago.

[ls1nova71]

Originally Posted by Minnz

It looks like hemi43's upper hole is a smaller diameter than the others?

That's because it's not a '71-'72, his extra wire is for something else.

[ls1nova71]

Originally Posted by tjsblazer

What is that relay? just to the right connected with the red and green wire.

Horn relay. You should also have another one that looks similar in the same general area that was for the TCS and will plug into that harness going through the top hole if it hasn't already been removed.

[tjsblazer]

Originally Posted by ls1nova71

Horn relay. You should also have another one that looks similar in the same general area that was for the TCS and will plug into that harness going through the top hole if it hasn't already been removed.

They say there isn't any dumb questions so i gotta ask what does. TCS stand for?

[ls1nova71]

Originally Posted by tjsblazer

They say there isn't any dumb questions so i gotta ask what does. TCS stand for?

It's not really a smog thing, but kind of an early attempt. it's Transmission Controlled Spark. It hooks to a solenoid on the carb, and the vacuum advance to the distributor, it only allows the advance to work in high gear and with the engine at operating temperature. it was a way of controlling hydrocarbons or something back in the day. I have only seen a few trucks that still have all of it in place. I know the trans sensor has been out of production for decades, and transmission shops would plug the hole with small freeze plug looking things.

[tjsblazer]

So is that the same as the EST you mentioned earlier that the upper hole harness is for or is that a different system.

[ls1nova71]

Originally Posted by tjsblazer

So is that the same as the EST you mentioned earlier that the upper hole harness is for or is that a different system.

TCS is what I meant to put. EST is electric spark timing, I think that came later in the early 80's. Sorry for any confusion.

[tjsblazer]

Well I need to get all my wireharnesses and lay them out and see what all I have. They are like a barrel of monkeys right now.
Debating whether to keep all factory or use an American Autowire upgrade kit.

[ls1nova71]

You can just go ahead and fill that hole. Should look a little something like this when youre done.

[tjsblazer]

Nice. Do you have a closer pic of that area. Thinking I'm not going to need the speedo one either since I will be running electric. Maybe run oil, speedo and tach wires all out of the factory oil pressure tube hole closer to the tunnel.

[ls1nova71]

Originally Posted by tjsblazer

Nice. Do you have a closer pic of that area. Thinking I'm not going to need the speedo one either since I will be running electric. Maybe run oil, speedo and tach wires all out of the factory oil pressure tube hole closer to the tunnel.

I looked and that was the only pic I could find of that part of the firewall. Basically I just filled those two holes. Truck is at a buddy's house right now so I can finish painting a few things or else I would snap you a pic. But yeah, if you're going to be using an electric speedo, I would fill that hole too. Running a few wires through the oil pressure hole would work, I have an extra hole in that area that I ran the engine harness wires through, it's more 'out of sight' there. Edit: Not sure this pic shows much more, but it looks messier.....

[hemi43]

I checked my fuse panel, and LS1 is correct in that the brown wire is the fuel sender. I still have no idea what the upper hole on mine is for, but since my truck is a 70, it will be different than yours.

[Walter White]

This site is awesome. I've been wondering what was in that hole. Mine is duct taped from behind. Does that mean PO had fender off or could it have been accessed from cowl?
Thanks
Edit...72 cst

[tjsblazer]

Originally Posted by Walter White

This site is awesome. I've been wondering what was in that hole. Mine is duct taped from behind. Does that mean PO had fender off or could it have been accessed from cowl?
Thanks
Edit...72 cst

That hole is just up under the dash. If you look up above the fuse panel by your parking brake pedal you should see it.