gbeaird, if I could find enough room under the dash and the Holley harness were long enough, I wouldn't have to spend $1250 or more on a custom length harness. I'll have to get my HVAC box back into the car sometime and confirm how much room there really is. Won't be anytime soon, though. I hurt my back really bad this past Wednesday. Car work is pretty much out of the question right now.

 

I'm gonna leave these here with the Dominator/HP guys. It also works for the Terminator X/Max guys too. Safeties and 4l80e torque management. Spreadsheets are included with each as well. If y'all have any questions reach out to me any time

https://drive.google.com/file/d/17xR...w?usp=drivesdk

https://drive.google.com/file/d/17pO...w?usp=drivesdk

Edit: I updated things on my end I didn't realize I had to give access to this. Should be good for anyone now

 

Today, I test fitted my HVAC box back into the car. There is no room for the Holley Dominator to fit horizontally with that big HVAC box in place.

I'm now thinking of using the hole that is pictured in post #13. I think the hole will have to be enlarged to fit the Holley wiring harness through as well as the stock wires that go through this hole.

 

Thanks for the update. Still thinking there's room somewhere inside the dash, or is that out?

 

There's no room for a Dominator under the dash with HVAC box in there. I've got mine mounted under the dash on a bracket I made and there's no way I could ever see the creature comforts being there and the huge ECU

 

i needed a hole at least 2" in diameter (might have been a little more) to fit the Terminator X harness through the firewall. there isn't a lot of room between the LS engine and firewall at that existing hole below the heater hose nipples. i think your wiring will be smushed in that area and need to make sharp 90 degree bends.
i ended up making a hole below the wiper motor on my build. you can see a picture in this thread:
https://www.thirdgen.org/forums/ltx-...ml#post6565551

 

It's out. Not a chance the Dominator will fit with the HVAC box in place.

 

It didn't occur to me that the engine would be real close to the firewall at this OEM hole:


Freshly painted engine bay on 9 Aug 2025 in the 95 degree Houston heat.


I've got my Dominator wire harness on the dining room table. I measured the length. I looked at the diameter of the harness. With the HVAC box out of the way and maybe some wire harness disassembly (the relay,) I'm pretty sure the harness will fit through there. However, if it's going to get crushed by the engine, that's not going to work.

battmann, any chance that you could take another look at your installed engine and see what kind of room there is there?

I might just have to cut the additional hole like you did on the left side.

 

Here is a picture of the OEM AC hole from when I first removed the TPI 350 this past spring.




The 700R4 transmission mounting surface is easily visible. Granted, the transmission is tilted down in this picture. It's normal position would probably be a little higher and closer to the AC wiring hole. The heater core hose and pipe are probably going to be different for my LS7. I don't plan to use the OEM, 350 coolant pipe so none of that should be in the way of the AC wire hole.

I just looked at the back of my LS7 and the transmission mounting surface is pretty much lined up with the back of the passenger side head. Minimal room between the head and that AC wire hole.


It's a bummer. This head is pretty much right in line with the transmission mounting surface resulting in minimal clearance with wiring coming out of the OEM AC wiring hole. The drive's side head is 1" forward. I could use that extra room on this side.

 

my car is currently at the upholstery shop but it *should* (finger crossed) be done early next week. i can take some pics and measurements for you once i get it back home.
in the meantime i'll look to see if i have any pictures in my build thread.

 

battmann, that would be great. Thank you.

I'm now thinking that I'm going to mount my Holley Dominator ECU under the front seat. If I can use the AC wiring hole, it'll go under the passenger side seat. If I have to cut a hole slightly to the left on the driver's side, I'll probably just mount the ECU under the driver's side seat. My seats are currently up in the attic and the interior of my car is gutted. With the seats in the car, is there some room under the seats to where the ECU would fit? Most cars have a decent amount of room under the front seats but I don't want to assume this without asking,

 

I'm reviving this thread from its 4+ month hibernation. Just to summarize this lengthy thread, I'm wanting to mount my Holley Dominator ECU under the passenger seat as I have it in my Foxbody Mustang coupe. This requires a longer harness than what came with my Dominator kit if I want to use the OEM, C207 hole in the passenger side, foot well area.


With Holley's Christmas sale going on right now the price on their Unterminated 15' Flying Lead Main Harness is now (21 Dec 2025) $382.95 with free shipping.

Pocket, I took a look at your 15' Flying Lead Harness build from post #46 back on 28 June 2025.

From what I can see in the pictures from Holley and Pocket's thread is that the J1A and J1B, ECU ends are already terminated by Holley. It ends up being a matter of determining which wires I would want to use for my build and remove the rest. Then, on the engine end of the harness, I'd have to source the required plastic connectors and metal terminals.

 

If you're gonna be buying harnesses I'd give this a hard look from HCR Innovations

https://hcrinnovations.com/shop/plug-n-play-harnesses/

 

Sort of.... it has no shieled cables (O2 sensor, crank or cam). And none of the wires are spliced like what would be in a finished harness. It's barely more than starting from scratch with spools of wire.

 

Crank, cam and WBO2 are shielded cables in the flying lead harness. The CAN pair is also fully twisted, even though they're basic wire
CRK/CAM leads are also pre-spliced near the ECU in a LS 24x configuration

While it is a starting point, it's better than just some wire and two connectors

The dominator J2A/J2B, J3, J4 connector kits are more like you're imagining as basic wire and a connector with a hefty price tag

My personal preference is to start with a flying lead and buy the other ECU connectors loose

 

Pocket, is there any splicing that needs to be done in the J1A or J1B? From what I understand, the pros splice multiple wires together using an ultrasonic splicer, whatever that is.

 

There are several splices near the connectors, mainly for the crk/cam cables which are mostly already done. I like to also split off IGN and low ref for the CAN connector there too

Sonic welding requires a large piece of machinery which you aren't likely to have. You'll need to make splices with a crimp and/or solder joint with the appropriate covering. There's a pretty in-depth thread by qwiktrip regarding proper splices and their many forms

In my opinion, planning where to make a splice is far more important than what type of splice you make

 

Thank you for the explanaion, Pocket.

 

That's good to hear. The flying lead harness I bought many years ago just had every wire pinned individually with no splices between anything. Joining commons to cam/crank is one of the trickier tasks. If that's already done for you then that's great!

 

If I don't have to do any crazy multiple splices, then this 15' flying lead harness could be just what I need. The fact that it's on sale right now makes it very tempting.

 

How long ago did you buy the flying lead? I go through a bunch every year and have never seen one without shielded cables. That's the holley main harness J1A/J2A
The dominator specific J2A/J3A, J3 and J4 kits have all been connectors and generic coils of wire which isn't a good investment to me

If a handful of splices changes your opinion on a flying lead, then it probably isn't the route for you. The rest of the harness still needs to be assembled, which is no small job

One of the pre-fab harnesses may be a better route like the generic 58x LS harness. You won't have the option to custom tailor them nearly to the extent that you could a flying lead, but it's a decision you'll have to make

Lots of research and labor for exactly what you want with the perk of experience OR an easier job that hits most of your objectives so you can hit the road sooner

 

I just ordered the Unterminated 15' Flying Lead Main Harness for $382.95 plus tax.

 

I started working on the 15' flying lead main harness yesterday. Once all the electrical tape was removed, I installed a removable nylon cable tie about every two feet to hold it all together. I then removed 4 wires from the harness that are not used with LS engines. I still need to remove the wires for the TPS as well as the IAC since these are not needed on a DBW engine.

I ordered all of my connectors, terminals, seals, TPAs/CPAs from Custom Connector Kits and they should arrive at my house tomorrow.

 

As pocket confirmed about a month ago, the flying lead main harness does come with a 7-wire shielded cable for WBO2 sensor #1 as well as two, 3-wire shielded cables for the cam and crank sensors. These are already connected and the shields are grounded at the J1A plug.

I have my original, too short Holley main harness that came with my Dominator EFI 550-651 kit. I noticed that Holley covered the crank and both knock sensors with an aluminum foil type of shielding over the wires. Here is a picture:


I've tried to find this stuff on the internet and it might be EMI shielding tape. Maybe, but I'm not sure.

My Mustang in my signature has a Dominator in it. It is the 550-611 kit that is no longer available. I just looked under the hood at the Holley harness that came with the 550-611 kit and it has the foil shielding on the knock sensors' wires but not on the crank sensor's wires. Does anyone know what this foil shielding is and where I can buy it?

 

BTW, I found out on the Holley forum yesterday that Holley actually makes a main harness that is 22" longer than standard: LS2/3/7+ (58x/4x) Engine Main Harness - Extended Length

For anyone looking for a slightly longer harness, this is a good option. I wish I would've known about it before I purchased the 15' Unterminated Flying Lead Harness, all the connectors/terminals as well as all of the hours of research and studying schematics. The extended harness would've been plug and play.

 

The foil wrap is heat shielding. Holley's routing takes those connectors behind the exhaust. Clever planning would make those unnecessary

 

Yes. I did find out last night after texting with my tuner that the foil is heat shielding. Thank you for confirming that.

I did look around on the internet and heat shielding tape is readily available. It does look like Holley used a heat shielding, foil tape. Both of my Mustang's knock sensor wires are somewhat near the headers and I think that the foil shielding is probably a good idea.

 

Sorry, just saw this. WFO is something from Kelly in Phoenix who is a NOS freak and has a 959 cu in Musi motor in his car. .

 

I've found that there is NOT enough vertical room under the passenger seat to mount the Dominator. There IS plenty of vertical room under the driver's side seat. This kinda changes things.

If I use the C207 hole in the passenger side foot well, the wire harness ends up being real long. In fact, I've got the 15' flying lead harness in the car and I'll end up using at least 11-12 feet of it. Take a look at these pictures:


Here is the Dominator under the driver's side seat and the 15' flying lead harness going over the transmission hump, under the passenger side seat and then along the doorsill to the C207 opening. It's a long way and uses a lot of the 15'.



This shows the ends of the wires in the engine bay. All of a sudden, most of that 15' is used up!



In addition, the shielded cables (WBO2, cam and crank) will be long. To me, that means that there is extra wire that could potentially pick up more EMI. BTW, the Dominator supports two wide band O2 sensors and like my Mustang, I'll be using two. That means that I'll be running 4 shielded cables.

 

Take a look at this picture and the hole that I'm point to.



That's an OEM hole that is about 2' in diameter. I've looked under the car and there is plenty of room between the T56 and the tunnel to run 4 shielded wires out of this hole. Two of the cables would be nice and short since they go to the O2 sensors. The crank sensor cable will be pretty short as well. The longest would be the cam sensor cable but it would only be half the length vs this cable existing the C207 hole.

If I'm going to use this hole, why couldn't I run the entire Dominator harness out of this hole? If I were to use this hole for the entire harness, I could actually use the finished harness that came with my Dominator kit. Yes, I'd have to enlarge the hole but that is easy to do. The harness routing really wouldn't be near the headers. Does anyone see or know a reason why this wouldn't be a good idea?

 

I know it's early in the wire job, but you should really work on planning the breakouts. All I see is the two CAN wires near the ECM. CAN also needs IGN and GRD which would be easier to do now
I'd also suggest more slack at the ECM. Binding and looming have a tendency to shrink the length some and those connectors will be a battle to get plugged in without excess tension

With two WBO2s and DBW, you should have two 7 cable bundles for the two WBs, one for the TB, two 3 cables for CRK/CAM and one 4 cable for the pedal
Dominators also require either LSU 4.2 or NTK sensors, both from holley exclusively. If going NTK, leave them unplugged until the startup tune is set and the key has been cycled multiple times. If not, they're super easy to fry and expensive to replace

 

I assume your car was originally an automatic because that's the shifter cable hole

You absolutely could use that hole for that purpose. I highly recommend a grommet/pass thru to protect the new wiring

Here's a few inexpensive versions I keep on hand for that purpose


RFI isn't a huge issue for LS engines running a holley ECU. I had the most issues with carb to sniper conversions where input cables were in close proximity to the distributor/plug wires
If you think about it, a LS crank sensor (the most important sensor on the engine) is directly behind the larget electro-magnet on the car (starter). Not affected. All the main data wires run in close proximity to the coils and injectors. Those have pretty big EM fields themselves
IMHO, it's not a big deal

 

Once I determined that the Dominator was going to reside under the driver's seat, I started routing the 15' fly lead harness. The only wires broken out at this point are the twisted, CAN wires. It was at about this point that I noticed the automatic shifter hole (1 1/2" diameter) just to the left and forward of my Tremec shifter. This is when I started wondering if I could route the two, 7-wire and two, 3-wire shielded cables through that hole. Three of the four would be much shorter with only the cam's, 3-wire cable being a few feet longer.

As I was thinking about it, I wondered if maybe I could just route the entire Dominator harness through that hole. I know that 1 1/2" is too small but I can enlarge the hole. If I'm going to run the entire harness through this hole, I can use the original, Holley, pre-made harness supplied with my Dominator kit. I wouldn't need to build the 15' flying lead harness.

I have a DBW harness but it is not currently in the car. If I try to route it through the C207 hole, it'll need to be lengthened. Going through the shifter hole should work out fine.

I currently only have one LSU 4.2 WBO2 sensor. I need to buy a second so that I can run/monitor both engine banks. Yes. They are very spendy. The NTKs must be made of gold.

 

Yes. It was a 350TPI with the required, 700R4 automatic.

Thank you for those Amazon links. I didn't even know that those items existed. They are great options for the harness hole.

That's good to know that RFI isn't all that big of an issue with LS engines and Holley ECUs.

 

I bought a couple NTK wideband o2 sensors a few years ago when this company was having a going out of business sale. I got a 2 for 1 deal. I also have my sensor 8" from the turbo in the downpipe on a raised bung. If you're just staying NA there's no reason to bother with an NTK wideband the Bosch 4.2 sensors work perfectly fine.

Having a sensor on each bank doesn't mean much at all. You can only select in the software Averaging, left only, right only, leanest or richest

 

Awesome deal!

Right before Cruisin' the Coast 2024 (on the Gulf Coast of Mississippi,) my Mustang started running badly. It was easy to look down at my Holley 12.3" ProDash and see that the right bank was reading lean. The left bank was reading normal. I had a misfiring cylinder. Turned out to be a burned plug wire. Being able to see both AFRs on my ProDash was helpful even if it doesn't do all that much for the Dominator.

 

Yea that is pretty slick. The CL comps would be another huge tell

 

I'd cram the whole harness through that hole (enlarged)
I don't recommend splitting engine functions along multiple routes

 

I plan to send all wires through that hole after I enlarge it.

Just to confirm; you don't think that there will be any problem with potentially more water getting to the wire harness routing up inside the transmission tunnel vs going out of the C207 hole and through the passenger fender?

 

No