PART 2: THE SCARY PART (Electrical)
Alright, here’s Part 2: the scary part! This post will deal with how I wrangled the billions of feet of wires that Holley gives you into my vehicle in a way that was completely invisible. Or, at least, no more visible than my wiring was before this install. This part will be long, because I think that the wiring process is very important and I’m pretty meticulous about it. I do not like wires all over the place so I think very carefully about how things should be routed/mounted.
To start off with, here’s a photo of the wiring harness that comes with the Holley unit. This does not include the auxiliary harnesses for whatever fancy ignition, accessories, fans, whatever. It’s just the base harness and it’s all that I needed to use.
It looks like a lot. But really it kind of isn’t, at least in terms of the connections that you need to make. You end up with a lot of extra harness.
This is the wiring diagram that comes with the instructions:
For me, the easiest thing to do when installing a bunch of wiring is to first determine exactly where everything needs to go. So I just annotated my wiring diagram – red X’s indicate parts of the harness I don’t need to use, blue boxes indicate parts that go to the engine, green boxes indicate parts that go to the battery, and yellow boxes indicate parts that go underdash.
So, as this works out, these are the connections that need to be made:
A. Underdash
1. Fuel pump (I have a relay for the fuel pump installed under the dash, so connect to that)
2. Ignition switched 12V (connect to stock red-green wire)
B. Engine
1. Coolant temp sensor
2. Coil (-)
3. Main throttle body harness
4. O2 sensor harness
C. Battery
1. Main ECU power (heavy red wire, must have full 12V)
2. Aux power (lighter red wire, must have full 12V)
3. Main ECU ground (heavy black wire, must be grounded to engine/battery)
4. Black chassis ground, which can actually go to any chassis ground
With that in mind, I needed kind of wrangled the wires around on the garage floor until I had the branches generally configured the way I wanted, and found that I would need to modify Holley’s harness a little bit. Some of the wires just came out in kind of bizarre places for my application – for example, the fuel pump wire was wrapped in with all of the ignition wires for some reason, but I wanted it to stay under the dash instead of going through the firewall with the rest of the underhood wiring. The switched ignition wire was also on a weird end of the harness, at least as far as I was concerned.
Really, the problem was that Holley wants you to feed one entire end of the harness through to the engine bay. That end of the harness includes the throttle body wiring (which I needed), the coolant temp sensor (which I needed), the coil (-) wire (which I needed), another loose ignition wire (do not need), another entire ignition harness for aftermarket MSD/whatever ignitions (do not need), a fuel pressure sensor harness (do not need), and an auxiliary input/output harness (do not need). First of all, to install that harness, you are supposed to drill a 2” hole in your firewall, which I was uninterested in doing. And second of all, I would have then had a whole bunch of useless wires just clogging up the bay, and I would much rather them stay under the dash and clog that space up instead where I can’t see them.
So I made my life hard and I pulled apart the entire harness to do some restructuring / rerouting. This sounds easy, because the whole harness is covered in very nice Techflex split-loom sleeving! But it is not, because when you pull the Techflex off, you find that Holley is, in fact, one of the biggest supporters of the electrical tape industry. Seriously, I am willing to bet that these guys used more than a roll of electrical tape just in the one harness. Insane! Of course this made things very difficult when I was just trying to pull wires through, so I had to unwrap the whole thing, pull out the wires I needed, rewrap the whole thing, yada yada. It was annoying. But, I guess in the end it’s gratifying to know that they were very interested in putting together a bombproof high-quality product. It’s just that I wanted it a different way, that’s all.
With the restructuring done, I had a subset of the wiring harness that needed to go into the engine bay that was exactly the right length to reach everything without looping around in crazy places and looking messy. The other end of the harness was supposed to attach to the ECU, and then whatever was in between those two points was supposed to go underdash. Bueno. The next order of business was to figure out where to put the dang ECU.
This ECU is not small, and it is not conveniently shaped. I think I measured and it is something like 6x8”. It’s fairly thin but it takes up a lot of real estate. It’s not so much of a problem to find a flat surface in the later year cars when big dashes became popular. But for a ’65-66, this monstrosity can be difficult to hide. Our dashes are just not very large. Additionally, the ECU must be mounted such that it is electrically isolated (touching no metal). It comes with plastic isolating ears on the mounting tabs, so that’s fine, but man, try to find somewhere for a 6x8” computer that can’t touch anything else that’s metal… under the very small dash that is full of components entirely made of metal. It’s not easy.
I tried a few different spots wedged up under the cowl, near the heater box, above the kick panels, and nothing really fit right. But then I had a stroke of genius! I suddenly recalled the glovebox. This computer will not fit in the glovebox (nor do I want it in there, because I keep maps and insurance and stuff in there), but I figured that it would fit either on top or beneath it since they are roughly the same size/depth. Initially I wedged the ECU up on top of the glovebox but I ultimately got concerned about whatever unseen metal could be touching it during driving vibrations, since I can’t see in that area with the glovebox installed. Underneath the glovebox, however, is a PERFECT spot exactly the size of the ECU.
Now, it’s important to note that I may have the world’s strongest cardboard glovebox. This thing has resisted anything I try to do to it for as long as I have had the car. The only time I’ve been able to get it out of the car, in fact, was when the heater box was out and I could just drop the whole intact glovebox out the bottom. I know, I know, people say “oh but there’s fold lines” and “yes but have you tried folding on the lines” and “did you know that those things fold?” but it is patently untrue. This is the unfoldable glovebox. So I figured it could hold up 5 lbs of ECU, no problem.
I stuffed the ECU in the gap between the dash and the glovebox and then marked two mounting points on the bottom of the glovebox. It should be noted that I asked the Holley reps and the ECU does not technically need to be mounted to anything – so hitting only 2 out of 4 mounting points is not an issue. I drilled the holes in the glovebox, which took about 6 hours because it is cardboard made of titanium (just kidding, it took like 30 seconds).
[Pictured: hardware (2 bolts) installed in the Unfoldable Glovebox]
The next step was to ensure that the ECU would be isolated from the metal of the dash. Since it was hanging from the bottom of the glovebox, there was a potential for it to contact the lip of the dash, especially towards the front of the glovebox area. I found out that a rubber bushing from my old rear sway bar, when placed in the dash runner/floor/whatever you would call that, made a perfect pad/spacer for the ECU to sit on so that it could never touch the dash. But I also wanted to go one step further, so I acquired some adhesive-backed rubber to stick to the ECU in key places where there was potential for contact. I spoke to a Holley rep and they said that as long as the ECU was not fully covered on any of the sides, this would be fine (I guess covering it all the way would make it overheat).
With that taken care of, I installed the ECU in its new home and hooked up its three connectors. They fit in right next to the space occupied by the heater box connectors. In the first photo below, you are looking straight up at the ECU with the heater box in the top of the photo. You can just barely see the red rubber spacer that the ECU is partially supported by (though I don’t think that it actually needs it). In the second photo, I really just wanted to show clearance between the wiring harnesses and the heater box.
Ok, so the ECU is mounted and harnesses are attached. Of the three connectors, two connect to the main harness, and one connects to the power harness (which is just 2 wires, a power and a ground). I wanted to deal with the routing of the power harness first.
Holley specifically states that the ECU harness MUST be attached directly to the battery, no secondary sources. Well, as far as I’m concerned, as long as they touch the battery cables, that counts as being attached directly to the battery. So really what I needed to do was connect the red 12V power wire to the battery side of the solenoid, and the black ground wire to the end of the ground cable. In my case, the ground cable is attached to the engine block below the alternator. Additionally, I had looped the red 12V auxiliary power wire into this harness as well since it also stated that it needed a constant 12V signal. So there’s another wire that needs to end on the battery side of the starter solenoid.
With all three wires going to roughly the same place, it made sense to bundle them together. I didn’t want to run them all the way over to the driver’s side, out the hole there, and then all the way back over to the passenger side to the battery. First of all, I’m too lazy for that, and second of all, those wires are kind of heavy gauge and I didn’t want to beef up my main wiring harness in the engine bay because that would require me to buy a larger size of Techflex (ugh!). Instead, I got clever, and routed the power harness over to the kick panel and through the radio antenna hole.
Then it was a simple matter to remove the splash guard and route the harness up to the top of the fender and around the shock tower, then back down to another convenient grommeted hole just beneath the solenoid. I attached the harness with a combination of Mag Daddy magnetic cable clips and standard rubber insulated cable clamps, taking advantage of preexisting bolts and holes so I didn’t have to drill new ones. Yes, I have now become one of those people who is so anal about wiring harness looks that I am willing to hide wires under fenders. But looking on the bright side, really I just see this as saving me from myself. None of these three wires can ever be tapped into, for any reason, so it is better for me to keep them somewhere where I can’t get at them 
Looks nice, at any rate.
Once through the grommeted hole, it was a simple matter to route the two power cables up to the solenoid, and the ground cable over to the engine block with the battery ground. It should be noted that I did not connect them at this time, as the instructions specifically said to connect these cables last after all other wiring installation was complete. But I did go ahead and put terminals on the end and get them ready to go.
Next was the really hard part: getting the other end of the main wiring harness from the inside of the car into the engine bay.
I have really developed a pretty significant aversion to drilling holes in Jane’s firewall these days. Mostly it’s because I live in Texas and the engine produces a ton of heat and when it’s 120*F outside, and the engine is running high on the highway, any hole in the firewall becomes a conduit for the fires of hell to reach my flip-flopped foot. So when the Holley instructions said “drill a 2” hole in the firewall for the harness” I laughed really hard and threw the instructions into a far corner of my shop.
Then I set about disassembling all of the connectors carefully, so that I could poke the wires through a half-inch hole in my firewall that already exists. I had to disassemble the main throttle body harness connector (which is the big one that wants a 2” hole to route through) as well as the O2 sensor harness. Fortunately, I have the tools to diassemble these connectors and am well-familiar with them. Also fortunately, I have a camera to document where things go, and I’m smart enough to use it.
After taking the connectors apart, I fished a coathook through the firewall hole, attached the snugly-bundled wires to it, and pulled it back through. This sounds easy but was actually a real bear because the Techflex kept snagging, but I did get it done with a lot of swearing. From there, I just had to reinstall the two connectors and I was in business! I made sure everything was properly connected, then wrapped the bundles up and secured them all nice and pretty. Really all I had to do was connect the coolant temp sensor, the coil (-) wire, the main throttle body harness, and the O2 harness, all of which were plug-n-play. I also had to connect the ground, which I placed on the backside of the driver’s side head (identical to the spot where the stock engine-chassis strap goes, but on the other head).
The final wiring required for this installation was the two underdash wires. I ran the ignition 12V wire and the fuel pump wire in a piece of techflex from the harness where it was exiting the firewall straight up to the ignition cylinder. The 12V wire got spliced into the ignition wire there, while the green fuel pump wire continued along the dash to the relay. Easy peasy. The hardest part about all this was getting the heat gun in the dash to shrink the heatshrink.
At this point everything was connected but the harness under the dash was still kind of unwrangled. The main harness from the ECU to the firewall exit was about a foot and a half too long, and the unused wiring for the engine bay was kind of just floating around. I bundled everything up together, then folded the main harness in a big loop that fits between the AC and the defroster ducts, and called it good enough. Nothing is in the way and there was nowhere else better to put it, so it’s just going to have to live there. Fortunately it’s not visible.
So that should, roughly, conclude the installation! All that’s left now is for me to hook back up some stuff I removed to get access, and to do a final run-through of the fuel lines and wiring to check for anything loose. Hoping to get her started up this weekend!