I need to run a few wires from the inside of the cab to the engine bay.
Is it OK to punch a hole in this grommet and run my wire through here?

 

Yes that's fine, just be gentle. Try not to cut a slit, more so a hole for the wire so it seals well around it. Doesn't take much, a pointy Phillips screwdriver will do it without much risk of damaging other wires.

 

cool.. thanks!

 

Why not use one of the factory pass thru wires?

 

I'm not Frank, but I'll chime in. I've run several wires through into the cab from the engine bay and not once used the pass through wires. Very inconvenient that they are located on the passenger side of the bay, plus they are on the opposite side from the upfitter switch wires. Easier to poke through right on the drivers side.

 

Yeah, it’s a pain. From the upfitter relays all the way across the engine bay and then two splices and then back to the driver’s side for whatever you’re trying to power. I did the first couple that way and then resorted to an awl thru the big grommet for the next ones.

 

I would sooner Greenlee punch a specified diameter hole in the sheet metal firewall (refer to unused holes already located in insulation) and use a new OEM pass through grommet spec'd for that location, rather than ruin the internally multilayered sealing membranes of an existing grommet by poking an awl through it.

 

This is certainly THE preferential way, no doubt. But I think the majority of the population isn't equipped for that. I am, and I have the punches, but still opted for the existing pass through grommet on my last two trucks and have had no issues, fortunately. Just my experience! But yes, if you have the means to do a fresh punch and grommet, then YES!!

 

Agreed, I think Im pretty annoying about well done, clean wiring but I would have a hard time justifying trying to access the footwell / firewall of a fully built truck with a Greenlee punch.
If the chassis was empty and I could stand in the engine compartment yes 100%, but access to that area will suck.
The combination of the pass through wires being in a ridiculous location and too short to work with, combined with the load sides being on the opposite side of the truck would probably have me doing the same operation with an awl and the large grommet. I hate to admit it but even though I have all the right stuff, I'll probably do the same thing.

 

If you look at the large grommet, there is a nipple ~3/4 in diameter on the engine side. It’s relatively easy to reach from the engine bay. Take a razor blade and cut the tip. Cut it in the recessed band close to the firewall. This opens a ~1/4 in hole through to the passenger side. I have a CB coax and two power wires running through it just fine. No awl needed, looks clean, maintains the rubber grommet.
also, during install you can reach up from the passenger compartment and pull the entire grommet inside the truck. Just grab a corner and work it around and it’ll pull in and give you 4” or so of play. Then you can easy pass the wires through the large gap from the engine side and work then through the grommet where you can reach much better (upside down in the drivers footwell). Then work the grommet back into place.

 

Great post. Thank You.

 

Internally multilayered sealing membranes? Here in Arkansas it’s just a piece of rubber!

 

Yup....I just punched a hole through the rubber grommet. Just be warned, but it is 2 or 3 layers thick. Run an extra wire through, so you can use it to pull new wires in the future.

 

Physics works the same in Arkansas as the rest of planet Earth.

Water wicks along wires, which is why OEMs spend the extra dollar for firewall grommets that have multiple internal membranes designed to reject water, since engine packaging constraints under the hood don't always allow the space for drip loops. Ford has improved over the years in raising the height of firewall wire penetrations, but any failure in the cowl to hood seal can cause water to cascade down the firewall, finding wires that serve as wicking conduits which pass through penetrations inadequately defensed against the probability.

I lost the floor pan in my 1979 truck, and found wet carpet and ruined jute padding in my 1989 Lincoln... from water wicking through inadequately sealed firewall penetrations. The damage isn't noticed until too late, but the procedures to avoid the event are found in various OEM Body Builder Best Practice guides for managing wire penetrations through the firewall. The issue is a big enough deal for OEMs to go ahead and spend the extra money to offer pass through circuits, as well as offer upfitters and end users suggestions for how and where to drill new holes through the firewall, along with part numbers for OEM approved grommets.

At no time, with any domestic OEM manufacture, have I ever encounterd instructions that recommend sticking an awl through an existing grommet and ruining the layers of sealing membranes within said grommet in order to pass through additional wiring.

 

You clearly haven’t spent much time in Arkansas! I think it’s the Bermuda Triangle of the continental US!

Yeah, I got lazy and went looking for a shortcut. I reached into the drawer looking for a wrench and got poked in the finger by the awl so I figured if it could draw blood from me, it’d make quick work of the rubber grommet!