Mystery holes in firewall. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

D-n-Tyke · 03-06-2023, 01:04 AM

Was cleaning up the firewall of random wiring and stuff today. Can someone help me understand what these 4 holes are in my firewall. They are on the driverside above the fuse block and between the brake booster and the fender and look to be factory stamped.

**pjmoreland** · 03-06-2023, 02:09 AM

One of the bigger holes is for the speedometer cable. I'm not sure what the other big hole is for. My firewall has an extra hole like that, and it just has a plug in it.

The two smaller holes may be for a factory tachometer (brown) and a TH400 kickdown wire (pink).

**HO455** · 03-06-2023, 02:56 PM

What year? I think the 71-72s have more holes than the early trucks. Being a California truck may make a difference too.

**Thealien** · 03-06-2023, 03:21 PM

I believe the two smaller ones on the right are the fuel gauge and possibly the tachometer wire. One of the top holes I believe was for the TCS relay wiring.

D-n-Tyke · 03-06-2023, 06:00 PM

Quote:

Originally Posted by HO455

What year? I think the 71-72s have more holes than the early trucks. Being a California truck may make a difference too.

1969 C20. No Factory Tach in the dash

Keith Seymore · 03-07-2023, 08:47 AM

Quote:

Originally Posted by D-n-Tyke

1969 C20. No Factory Tach in the dash

It could still be for the factory tach even if your truck was not so equipped.

Front of dash and frames have to accommodate every possible option combination they will ever see and so there will be holes that are not used for a specific build.

I had design/release responsibility for a short time for the front of dash/underbody and pickup box on the GMT900 program.

I guarantee nobody knows what every hole is for, even the guy that put them there!

K

**HO455** · 03-07-2023, 04:43 PM

It can even differ from plant to plant.

Back in the early 90's I helped a friend move and as part of that adventure he had several 1970 GTO's that were completely disassembled we had to move. After the work was done and we were enjoying some barley pops we noticed that the 2 firewalls on the cars were different.

One had every conceivable hole punched and the other one had only the minimum holes. Both cars were bare bones cars, with no options other than disc brakes. Likely dealer loss leader orders. But they came from two different manufacturing plants.

**RenoKeene** · 03-08-2023, 02:34 PM

70 gmc 1/2 ton

**RenoKeene** · 03-08-2023, 02:39 PM

72 GMC 3/4 ton

**RenoKeene** · 03-08-2023, 02:48 PM

72 Chevy 1/2 ton

D-n-Tyke · 03-08-2023, 07:02 PM

Reno, thanks for the pictures. Looks like they are holes I am not going to be really reusing moving forward (LS Swap) and can just make sure they get plugged up.

03-06-2023, 01:04 AM	#1
D-n-Tyke Registered User Join Date: Aug 2016 Location: El Cajon, CA Posts: 284	Mystery holes in firewall. Was cleaning up the firewall of random wiring and stuff today. Can someone help me understand what these 4 holes are in my firewall. They are on the driverside above the fuse block and between the brake booster and the fender and look to be factory stamped. Attached Images

03-06-2023, 02:09 AM	#2
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,978	Re: Mystery holes in firewall. One of the bigger holes is for the speedometer cable. I'm not sure what the other big hole is for. My firewall has an extra hole like that, and it just has a plug in it. The two smaller holes may be for a factory tachometer (brown) and a TH400 kickdown wire (pink).

03-06-2023, 02:56 PM	#3
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 11,350	Re: Mystery holes in firewall. What year? I think the 71-72s have more holes than the early trucks. Being a California truck may make a difference too. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

03-06-2023, 03:21 PM	#4
Thealien Senior Member Join Date: Jun 2007 Location: Pine Ridge Florida Posts: 4,134	Re: Mystery holes in firewall. I believe the two smaller ones on the right are the fuel gauge and possibly the tachometer wire. One of the top holes I believe was for the TCS relay wiring. __________________ Click on my Not really a build thread, yet NOT SO RUST FREE RUST FREE BLAZER LS ENGINE SWAP INFORMATION THREAD BOLT RESTORATIONS 4l60 to an NP205

03-07-2023, 04:43 PM	#7
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 11,350	Re: Mystery holes in firewall. It can even differ from plant to plant. Back in the early 90's I helped a friend move and as part of that adventure he had several 1970 GTO's that were completely disassembled we had to move. After the work was done and we were enjoying some barley pops we noticed that the 2 firewalls on the cars were different. One had every conceivable hole punched and the other one had only the minimum holes. Both cars were bare bones cars, with no options other than disc brakes. Likely dealer loss leader orders. But they came from two different manufacturing plants. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

03-08-2023, 02:34 PM	#8
RenoKeene Florida Edition Join Date: Oct 2010 Location: Fruitland Park, FL Posts: 4,028	Re: Mystery holes in firewall. 70 gmc 1/2 ton Attached Images __________________ 70 GMC Short Stepside "Rose"-An American Beauty: Factory 402/TH400, AM/FM, AC, Tilt, Tach, Buckets, Posi, PS, PB, 3-5 Drop, Complete Resto-Rod 67 C-20 Slant Back Wrecker "Mad Max" 67 C-10 Ratrod "Step-N-Wolf" 71 Serro Scotty Sportsman camper "Scotty" 97 LT1 Z28 "The Hornet" Link to more pics of "Rose" http://67-72chevytrucks.com/vboard/a...p?albumid=1684 Check out my "Cheap Tricks" thread and add to it if you can, lots of good info there. http://67-72chevytrucks.com/vboard/s...d.php?t=489394

03-08-2023, 02:39 PM	#9
RenoKeene Florida Edition Join Date: Oct 2010 Location: Fruitland Park, FL Posts: 4,028	Re: Mystery holes in firewall. 72 GMC 3/4 ton Attached Images __________________ 70 GMC Short Stepside "Rose"-An American Beauty: Factory 402/TH400, AM/FM, AC, Tilt, Tach, Buckets, Posi, PS, PB, 3-5 Drop, Complete Resto-Rod 67 C-20 Slant Back Wrecker "Mad Max" 67 C-10 Ratrod "Step-N-Wolf" 71 Serro Scotty Sportsman camper "Scotty" 97 LT1 Z28 "The Hornet" Link to more pics of "Rose" http://67-72chevytrucks.com/vboard/a...p?albumid=1684 Check out my "Cheap Tricks" thread and add to it if you can, lots of good info there. http://67-72chevytrucks.com/vboard/s...d.php?t=489394

03-08-2023, 02:48 PM	#10
RenoKeene Florida Edition Join Date: Oct 2010 Location: Fruitland Park, FL Posts: 4,028	Re: Mystery holes in firewall. 72 Chevy 1/2 ton Attached Images __________________ 70 GMC Short Stepside "Rose"-An American Beauty: Factory 402/TH400, AM/FM, AC, Tilt, Tach, Buckets, Posi, PS, PB, 3-5 Drop, Complete Resto-Rod 67 C-20 Slant Back Wrecker "Mad Max" 67 C-10 Ratrod "Step-N-Wolf" 71 Serro Scotty Sportsman camper "Scotty" 97 LT1 Z28 "The Hornet" Link to more pics of "Rose" http://67-72chevytrucks.com/vboard/a...p?albumid=1684 Check out my "Cheap Tricks" thread and add to it if you can, lots of good info there. http://67-72chevytrucks.com/vboard/s...d.php?t=489394

03-08-2023, 07:02 PM	#11
D-n-Tyke Registered User Join Date: Aug 2016 Location: El Cajon, CA Posts: 284	Re: Mystery holes in firewall. Reno, thanks for the pictures. Looks like they are holes I am not going to be really reusing moving forward (LS Swap) and can just make sure they get plugged up.



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