'72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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?

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

Nope, you won't need it.

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.

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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. :lol:

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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..... :lol:

Re: '72 Firewall Help
 

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.

Re: '72 Firewall Help
 

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

Re: '72 Firewall Help
 

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.