Breaching the Firewall of an XJ

Recently, I spent a few solid hours tearing apart my dashboard, and a fender or two, for the purpose of finding “the best” way to route auxiliary wiring from the engine compartment to the cab. It wasn’t that I didn’t already know of a few satisfactory spots to pass wires through the firewall, I just wanted to thoroughly research all of my options for an upcoming wiring project.

The goal was to find, or to create, a dedicated pass-through for auxiliary electrical wiring that was easily accessible from both sides of the firewall, and also well-protected.

There is a large, oval, rubber plug that is already being used by a wiring harness on the left side of the brake booster. The common solution is to add a slit to this plug, and to jam your wires through the slit. It works, but it’s awkward to reach on both sides and it’s difficult to feed wires through.

The route that I’ve opted for is located on the right side of the brake booster. On my 2001 Jeep Cherokee, it’s marked by a circular metal plug that has been sealed and painted over. On the outside, it’s not obvious how to remove this plug. However, on the interior, you can very easily gain access by removing the knee panels beneath the steering column.

The large oval plug to the left of the brake booster works, but it's messy. You can do better.

The large oval plug to the left of the brake booster works, but its messy. You can do better.

The 1.5" circular plug to the right of the brake booster is ideal. It's high, dry, and easy to access.

The 1.5″ circular plug to the right of the brake booster is ideal. It’s high, dry, and easy to access.

Once you’ve removed the knee panels (both the plastic one, and its metal backing), you can easily poke out the plug with a pry-bar, revealing a 1.5″ hole; absolutely perfect for our needs.

Remove the knee panels, locate the plug with a pry bar, and you can easily pop it out.

Remove the knee panels, locate the plug with a pry bar, and you can easily pop it out.

Jackpot! If I had known it was that easy, I would've done it a long time ago.

Jackpot! If I had known it was that easy, I would’ve done it a long time ago.

Finding the route was only the first part of the problem. Now I needed to determine how to plug it up with something that I could safely pass wires through, but still maintain some semblance of a seal. Also, I gave myself an extra-credit goal to try to fashion a sort of conduit that would direct the wiring down along the inside of the firewall, to make sure it was still easy to access once the knee panels were reinstalled.

There are multiple ways to skin this cat, so you can be creative here. If you’re an electrician, or just a wiring geek, you’re probably already well-equipped with plugs, grommets, boots, and other appropriate items. However, I found almost everything I needed in the plumbing section at Lowes.

  1. 1.5″ Disposal Discharge Tube
  2. 1.5″ Threaded Fitting
  3. Bicycle Tire Tube from Wal-Mart

The idea was to cut the bicycle tube to form a 1.5″ rubber diaphragm, assemble the discharge tube with the threaded fitting screwed down tightly over the diaphragm, and then cut a slit in the rubber diaphragm for wires to pass through. Once the makeshift conduit was assembled, insert it into our hole through the firewall.

As it turned out, the disposal discharge tube was an exact fit. Some silicone could be used to seal the assembly around the hole for added assurance. The end result met all goals, including creating an interior channel that directed the wires downward. Now I’m ready to run some wiring through the firewall without any of the headaches!

 

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