Drilling The Firewall

[Gryphon]

I am going to be installing several electronic accessories in the cab that will hog a bit of power. I also need to put several multi-conductor cables through the firewall to the engine compartment. I've tried penetrating the BCM harness boot but that thing is just too damn thick.

 

I know there is a specific spot just above that harness that is intended for drilling the firewall. I found the cover on the inside that pops out and have removed it. Is there anything I should do specifically or watch out for when doing this? The plan is as follows:

 

1. Drill a pilot hole

2. Enlarge the hole with a Unibit

3. Coat the edges of the firewall with primer and paint

4. Add a grommet to fit the hole

5. Run wires and cables, in loom if space allows

6. Silicone the crap out of the hole

 

It's in a really awkward spot though so I'm not sure how I'll do the Unibit thing. I think my old man has a drill bit extension somewhere...

[08SGMSierra]

I am going to be installing several electronic accessories in the cab that will hog a bit of power. I also need to put several multi-conductor cables through the firewall to the engine compartment. I've tried penetrating the BCM harness boot but that thing is just too damn thick.

 

I know there is a specific spot just above that harness that is intended for drilling the firewall. I found the cover on the inside that pops out and have removed it. Is there anything I should do specifically or watch out for when doing this? The plan is as follows:

 

1. Drill a pilot hole

2. Enlarge the hole with a Unibit

3. Coat the edges of the firewall with primer and paint

4. Add a grommet to fit the hole

5. Run wires and cables, in loom if space allows

6. Silicone the crap out of the hole

 

It's in a really awkward spot though so I'm not sure how I'll do the Unibit thing. I think my old man has a drill bit extension somewhere...

 

I used to do this every year when I built our new squad cars. Its a little hairy the first time, and it wasn't even my car!

 

One thing we started to do, was to put a PVC fitting inside the hole we drilled out. Then we used a PVC "nut" (sorry, don't know the correct name for this part) to tighten it down, and added silicone both around the pvc to firewall, and inside where we ran the wires through the fitting. This would ensure no metal to wire contact, and when we were done with the car we screwed a PVC cap on it and called it a day. Our holes were pretty big, but it worked nice. Just a thought.

[bill190]

Get the grommet first!

 

Things like this can be impossible to find. Might go to a wrecking yard and pull one or two off another vehicle - any vehicle. Take wire cutters so you can cut away the wire and then remove the grommet(s).

 

Then once you get the grommet, you will know what size hole to drill.

 

Then just be careful that where you are drilling, there is nothing on the other side you are going to drill into. Also that you will be able to drill there without something getting in the way of the drill (and not being able to drill straight).

 

I like to apply silicone sealant around these as well sometimes. Just like sealing pipes going through walls in a house. Keep the weather out!

 

Then a good idea to tape the wires with electrical tape and use that black plastic tubing. This keeps the wires from shorting should they rub against any sharp metal parts along the wiring route. I like to tie wrap the wiring along its entire run as well so it will stay put and not bounce and rub against anything.

[Draenor]

You could always use a bulkhead connector. Just remember to run an inline fuse. Put it as close to the power source as possible.

[Griptruk]

Go to this pdf and check out pg. SP-18

http://www.gmupfitter.com/publicat/2005_20...Sp_Apps_SPP.pdf

 

This shows where the factory hole is for the snow plow prep package. It seems only snow plow prep trucks may have it but the body should still have the same spot to drill even if the hole isn't there. If you can get that grommet and the correct size bit (you should come close with a uni-bit) then you'll be just like factory.

 

I would add that if you can get your hands on a right angle drill you might have better luck.

 

-Jer

[Gryphon]

Thanks for the help guys! Probably gonna give it a try next week.

[Zembonez]

It's no big deal to drill the firewall. Check, double check, and then check again to make sure that your path is clear on both the inside and outside. Take a center punch and make a dent where you are going to drill, then check again from the outside to make sure your are going to clear everything... then drill your ass off. Follow up with a rubber grommet with liberal amounts of black silicon (it holds up better). Don't use permatex gasket maker for this.

 

Like others have said. Once you do it, it won't seem like any big deal.

[gave20]

I used the BCM grommet to run several wires for my bed and backup lights. I used a 10 gauge piece of wire to feed through the light wires through.

[sonjaab]

On the gmt-800 even with the VYU plow prep GM didn't always pre-drill the hole

for you! (Ask me how I know!)

 

Best bet is to look under/next/a couple of inches down from the bottom of the master

cylinder at about the 7 o'clock position. There SHOULD be a dimple in the firewall !

This is where the factory VYU plow prep hole usually is.

Pull back carpet/insulation to verify nothing is behind it.

A straight regular drill can be used from the engine compartment side as there is

just enuff room to work it in there.

 

FYI-----My 01 had the hole predrilled and plugged from the factory. My 04 didn't.

My gmt-400s had a flat spot above the gas pedal to drill a access hole.

[Gryphon]

Did it! I'd like to thank Jeff for the suggestion of using a piece of PVC pipe as that's exactly what we did. I was going to do the grommet but realized the biggest I could find easily was 3/4" ID, which left precious little room to run six strobe cables, power and ground, control harness, speaker wires, etc. At Rona they had the PVC fittings in the same aisle as the grommets, and Jeff's post stuck out in my mind. I looked at a 1" PVC threaded bushing and realized it would be absolutely perfect for this. Grabbed a couple for spares, a pack of 1" locknuts, and it totaled out to like four bucks. Cheaper than a pack of grommets!

 

I removed the plastic insert in the driver's dash area to expose the firewall. We used a Makita angle drill to get up inside the dash area to drill out (thank GOD for this tool, would have been impossible otherwise). Drilling from the engine compartment would have been preferable but it was impossible to get the bit that far in with all the stuff in the way. We started with an automatic center punch and a 1/8" bit for a pilot hole. The first one was about in the middle of the cutout, but a bit too far over in the engine compartment. Drilled another further over, liked the position, then followed up with a 1 3/8" Unibit liberally juiced with cutting fluid. This combination let it tear through the firewall like a hot knife through butter. We had to stop and undo the chuck, and clamp the bit higher on the shank a couple of times because the drill body was bottoming out against the plastic firewall cover. Even with the angle drill we couldn't get the bit perfectly perpendicular to the firewall but eventually the 1 1/4" step made it, and the bushing was able to fit.

 

For the bushing we used a standard 1" PVC electrical conduit threaded bushing. We chucked it up on the milling machine and took the non-threaded side down about an inch as it didn't need to be that thick. Cleaned it off with some brake cleaner, applied exterior-rated silicone, and threaded it into the firewall forming a perfect seal. A locknut went on the inside and we had a perfect channel for which to run all manner of cables and wires. The full 1" inner diameter and the total lack of resistance when pulling bundles of cables through is just awesome.

 

Enough blab, some pics!

 

The spot under the dash where the pre-cut plastic cover was removed, prior to drilling any holes.

 

 

Hole drilled, bushing and nut installed.

 

 

Engine compartment side, silicone just visible around the edges.

 

 

Zoomed out shot showing where the hole ended up being in relation to the brake booster.

 

 

Oh one thing I learned, see that unplugged connector with the green around it? That's the Accellerator Pedal Position harness. If you don't remember to plug that in before you drive off, expect codes to be thrown.

 

The new hole/conduit allowed us to pull through this bundle of four strobe cables with no problems. Kickass!

 

[trplxl2]

Holy S$!!!!

 

That has got to be one of the coolest ideas that I have ever seen, great job with the PVC pipe through the firewall. Thanks for the pictures of the finished product, and I can see why you needed that big enough of a hole. I guess that is the bad part about strobes, there are so many thick wires that have to be ran.

[Gryphon]

Thanks! Those four cables were only the beginning though. That conduit now has SIX strobe cables plus two speaker wires, an 8 ga power and 8 ga ground, plus an 8-conductor 18 ga control harness that I ran to the engine compartment for future projects (onboard air system, air horns, extra lighting, etc). I doubt I'll even need silicone now, it's pretty tight in there and the loom in the engine compartment blocks the hole almost completely from view.

 

Now that the install is pretty much done I should have a few more pics to share shortly!

[NS Sierra Guy]

Has anyone attempted this on the GMT900? Have any pictures to share? I am about to attempt the same on my 09 as I will need to run power cables as well as lights and antenna's through to my installed center console and any prior experiences would be a great help.

[Z71kris]

I remember the first time I drilled my fire wall! Oh boy what a mess!!!! I spent hours separating a wire harness in my second car, a Chevy Corsica, and re-splicing the wires back together. I don't remember what the harness went to any more... but I remember doing that though and it sucked!!!!

[sc4808]

For future reference it is much easier and makes a cleaner hole if you use an Knock Out tool like us Electricians use, that way you only need a 3/8" hole to start with and then you can put the drill away.

 

You have way less metal shavings and you get a very smooth hole.