Hey all, I am working on a 74 Vette with the 454 and manual trans in it and it's needing a harness replaced. Customer has been wanting to do it anyways because it's kind of hacked up and has some issues- such as the a/c blower motor works in the off position, low and 2nd highest position, but turns off in the middle position and the high position, and the a/c has to be unplugged at the inline sensor (i assume its a pressure switch?) for the compressor to be switched off. Also ran into another issue where there is a short or something in the ignition system- one of the wires on the positive side of the ignition coil plus a wire coming off a solenoid of some kind both get really hot with ignition on, to the point the old wire had some butt connectors that the plastic literally melted off of. It also only supplies 3 volts to the coil, and won't start. I'll include some pics of the solenoid and what not. So long story short, i'm looking for what the best/easiest wiring harness kit is for these cars, and some ideas on how easy they are to install, and how long it takes (not exact just a ball park). Definitely wanting to replace the engine harness and atleast whatever harness runs the a/c system as well if thats not apart of the engine harness so we can hopefully sort out all the electrical issues this car has. Also a solid wiring diagram would be great, I found one online but it has a ton of unlabeled relays on it and what not that's not super helpful. Thanks!



The black wire pictured here is the wire on the ignition coil that gets very hot within 5-10 seconds of the key being switched on.

The red wire that has fresh butt connectors is a section of wire I just added- the previous section had like 3 different butt connectors on it, was practically broken clean off, and the plastic was literally melted off the old butt connectors- the new butt connectors appear burnt too but thats just from me melting the rubber to be weather tight. I replaced the section hoping it was the cause of the issue of the coil only getting 3 volts but it made no difference- this section just takes a little longer to get hot because its a little thicker gauge. Does anyone know what this single pole component is? It resembles a solenoid but it doesn't make sense to me for it to be a solenoid since it just has one stud and has power even with the key off. Was hoping to figure out what this is because I was trying to figure out if the wire that gets hot is apart of the ignition system, or if it shares a common relay or something with the ignition system but I can't find that info anywhere on what it is, what it does etc and the wires being all spliced up doesn't help either. Its mounted to the inside of the driver side fender well.

Edit to add it looks like this car has had a wiring harness previously installed, looks like it was from the zip corvette site as found that exact same harness as whats in the care- not big on it as there is a lot of random plugs and wires not ran to anything, so i'm wondering if its not a great harness, or has options this car doesn't have, or if the harness was poorly installed.

 

I'm about done installing my 1976 American Autowire updated (spade fuses, better alternator wires) harness. Lots of other mods too. You can check out my thread if you want to see some amateur work, lol.

 

I think the AAW harness and instructions are pretty good overall.

 

'Lectric Limited is what I used to re-harness my '72 w/AC. It was a very highly accurate repro to the original deteriorated wiring in the car. The engine main, front lighting, and A/C harnesses were dead simple to install with clear paper maps of what connector goes to what. I don't recall extra connectors but do know for a fact that the factory clipped the occasional wire depending on options - such instances are described in the AIM.

 

When I called Lectric Limited, tech told me they follow the OEM colors and all else so closely they do not make their own wiring diagrams. He said use the factory diagram. So far, the LL harnesses have matched perfectly to the OEM diagrams.

 

How long roughly did it take you to insall? Sounds pretty simple.

 

Last thing I want to figure out is what that single pole electrical component is, if anyone knows, just so i understand what it does lol

 

I have used M & H Wiring products in the past as I restored my 67 Camaro. They were an exact duplicate of the original and installation was a simple process. Then again, my Camaro's dash is a little easier to work on than my 71 Vette. Here is a link to their home page. http://www.wiringharness.com/
Good Luck

 

For all three engine harnesses (main, lighting, A/C) it would probably take about an hour or two but merely swapping the harnesses was not my goal. I ended up cleaning and painting the firewall, hold-down clips, polishing the steel vac line along the firewall, and other refurbishing all sorts of parts that are located in conjunction with the harnesses. The harness routing and connections are pretty easy to access with just the starter requiring crawling underneath - as I recall.

 

I just did a new engine wiring install on a 72' 350/200 4 speed with A/C and it was to the letter of the diagram, every color, length and type of terminal on the end.......nice harness.......car had a dead short I attributed to the turn signal circuit and didn't find it until the I ripped out the rest of the dash....which at that point played mercy on the Owner and cleaned up some of the wiring for him.......
I can tell you this......if you decide to rewire the inside of the car, do yourself a favor and get the kit with modern AGM fuses in it.....the fuse panel is tucked way up in the fenderwell and glass fuses suck to remove when you are upside down on your back......
The engine harness will have wiring for emissions stuff you probably don't have anymore and don't need like the TCS plug.......I opted to cut it all back and cap with shrink for a cleaner look. If you ever want to A/C to work again....get a new plug and relay, the one that mounts to the A/C box in the front.......the engine wiring will not have this stuff in it.......
Off of the horn relay where GM decided to be a good place to split off all of the wiring for the car, I used two AGM fuse holders in lieu of the original fusible links for the Alt sensor wire and the interior power.....make sure you run the Alt. pos. post not only back to the starter lug but to the hot post on the horn relay......
Verify that all of the factory ground straps are there.........this is VERY important.
The A/C switch is easy enough to diagnose if you are not going to replace the interior wiring but you need to verify that it is the wiring or the switch. I recommend buying a Power Probe.....makes life so much easier. The Probe when hooked up can tell you if a wire is hot, if it is a good ground and allows you to shoot 12v to whatever you want to narrow down issues.......one of the best things in my toolbox and can't live without it now.
It really depends on the condition of your wiring right now.........I made the 50 year old wiring in the car work......but this was on the edge of needing a harness......
If you can get your hands on one, the Doc Rebuild wiring schematics are the best there is........they are huge, color coded, and show a pic of every connector in the system that each wire goes to........a must IMHO.......

Jebby

 

My only experience is that I installed a new engine/ignition harness this year from Lectric Limited. i had the engine out which made it very easy but even with the engine in it would have been pretty easy. It fit perfectly. i only did it because i figured what i had was 52 years old and might as well do it since it is probably the easiest harness to replace. after 30 years of starter problems this is the first year i have had zero starter problems so it fixed a problem for me by doing it. i had the lectric limited laminated wiring diagram and the Doc Rebuild diagram and i agree the Doc Rebuild diagram is a must just get it.

Pat

 

Does the lectric diagram match OEM diagrams? I found one online but it shows a white and green wire for the coil +, while it looks like all the aftermarket harnesses including the one in the car has a black and a yellow braided wire. That was part of my issue, Im not sure what wire does what because the car doesn't match wiring diagrams, and something is causing the coil wire and another wire to get really hot and only feeds the coil 3 volts instead of 12, so I also want to be able to make sure i'm not replacing the wiring harness trying to fix a problem with a different component, but I'm not sure what wire does what since it doesn't match the only wiring diagram I can find.

 

The LL diagram isn't a wiring schematic like the GM service illustrations. It's a simple diagram of the assembled harness with the different connectors at their locations along the harness length. Each connector leg tells what it operates. They sell a schematic but the pic below shows the sort of diagram that comes with the harnesses.