Thursday, December 10, 2020

How To Replace C3 Engine & Headlight Wiring Harness

ORIGINAL WIRING, 40 YEARS LATER

It all started with a corroded ignition wire, followed by a falling-apart alternator connector, then a brittle and broken coolant temperature connector, and finally, a complete electrical power failure for a second time!


Fortunately, this time, my car died right in my driveway, so maybe it was my "lucky" day?

In a sense, it was, since towing truck trips are not cheap. And this failure could've happened in the middle of traffic which is dangerous and embarrassing.

Removing the engine and headlight wiring harnesses is not easy, but it is doable provided you can safely lift the front end of your Vette. The higher, the better.

You can use Rhino Ramps, a floor jack and jack stands, or in my case, a QuickJack Portable Lift.

I don't want to sound like a broken record, but by far the best car-related tool purchase I've ever made!


DOES WIRING GO BAD?
Forty-year-old wiring does. And I don't fault Chevrolet for this. I mean, these cars were engineered to run and operate reliably for several years. I am sure they did not envision enthusiasts trying to keep them alive after 40-plus years.

We don't expect tires, shock absorbers, spark plugs, fluids, etc., to last forever, and cars are no different.

According to Lectric Limited
Even if you think your wiring "looks good", it's probably not. Your wiring is made of copper wire with a covering of plastic (mostly PVC) insulation. This insulation is porous. Contaminants, like oxygen, moisture and airborne pollutants, eventually pass through the insulation to attack the wire; a condition called oxidation. There is no practical way to completely seal off the circulation of oxygen and moisture between the individual wire strands. Once the wire strands become oxidized, the internal resistance of the wire increases substantially. The same oxidation/corrosion factor will happen to the wire's metal terminals over time; further increasing the internal resistance of your wiring harness.
 
HOW TO REMOVE
THE ENGINE WIRING HARNESS

The engine wiring harness for my 1976 Corvette Stingray (with a 4-speed manual transmission), connects to the following:

  • Dash harness firewall connection
  • Air conditioning harness connection
  • Alarm system anti-tamper switch
  • Reverse light switch connection
  • Ground (to bell housing bolt)
  • Hood ajar switch
  • Ignition coil (on distributor)
  • Oil pressure sending unit
  • Starter solenoid
  • Starter solenoid extension wire connection
  • Tachometer filter connection
  • Windshield washer pump
  • Windshield wiper motor
  • Windshield wiper motor ground terminal

Even though my car was—in the words of Charles Dickens—dead as a doornail, electrically-speaking, I disconnected the battery anyway.

In hindsight, this job would have been so much easier when the engine was out, but I only think about wiring when it fails.

Since I had to start somewhere, I decided to remove the firewall plug first. That's the big black squarish connector on the firewall, tucked near the brake booster.

Actually, "remove" is an exaggeration since the only thing I was able to accomplish, after removing the 3/8" bolt, was to unplug it from the back of the fuse box.

I then disconnected the alternator wires.

It is important to note that the alternator wires are part of the headlamp wiring harness, but after going through the trouble of replacing the engine wiring harness, I did not want to take any chances and decided that the best course of action was to replace both.

The air cleaner assembly has to be removed along with the distributor cap since you will need room to unplug the windshield wiper motor connectors.


I unplugged all the spark plug wires as well as the distributor connectors to remove the cap and also because those connectors are part of the engine electrical harness.




With the distributor cap out of the way, you can then unplug the windshield wiper motor plug and the ground connector as shown above.

Since I upgraded my QuadraJet to electric choke, I had spliced a wire to the distributor ignition wire, so I clipped the wire and left it there since I plan to splice it into the new wiring harness.

There is another connector near the windshield wiper motor that needs to be unplugged for the engine wiring harness to come off. I think I show that one in the video.

From that point, it was time to crawl under the car to unplug the starter motor wires. I am not going to lie; it's not a fun job, especially since a couple of the starter solenoid connectors are pretty hard to reach.


Once the starter motor wires were disconnected, I moved to the driver's side under the Vette and unplugged the oil-pressure sender terminal and the reverse light switch, even though mine does not have reverse lights since my car has four brake lights.

While I was under the driver's side, I pushed the starter motor wires up from that side. I then reached the firewall plug from above and started pulling it out between the brake booster and the windshield washer bottle which, by the way, has a small electric pump inside and a terminal that is also part of the engine wiring harness.

So I unplugged it and I removed the engine wiring harness.


The photo below shows the inside of the firewall plug. By the way, the firewall plug is actually two pieces. One half is the connector for the engine harness and the other is the front headlamps wiring harness connector.


I may try to see if I can determine what caused a total electrical power failure. But I'm not going to waste too much time doing so, but I will definitely keep the old harnesses in case I need a connector or a correct color-coded piece of wire, for example.



HOW TO REMOVE
THE HEADLAMP WIRING HARNESS

The headlight wiring harness connects to the following systems:

  • Dash harness firewall connection
  • Air conditioning harness connection
  • Anti-theft system on/off switch 
  • Alternator
  • Ground (to the alternator)
  • Coolant temperature sending unit
  • Brake pressure warning switch
  • Ground (to driver side radiator support)
  • Ground (to header support)
  • Headlights
  • Headlight door limit switches
  • Horn
  • Turn signal extensions
  • Side marker lights

I thought removing the headlamp wiring harness was going to be the toughest of the two, but actually, it was not that bad.

Since I had already disconnected the alternator, coolant temperature sender, air conditioning harness, and brake warning plugs (the brake warning plug connects to the proportioning valve), I focused on disconnecting all the headlight, turn signal, side marker lights, and horn connectors, as well as the two ground wires.

I lifted my car and unplugged everything. One of the two grounds is bolted to the bottom left (driver) side of the radiator core support. The other one is bolted to the nose reinforcement crossover piece. The video below shows this in more detail.

The radiator core support grounding bolt is very hard to reach, so I simply clipped the wire and will find a suitable location when I install the new harness.

The following line drawings from the 1976 Corvette Assembly Manual, help understand how the headlamp wiring harness is routed.

With everything unplugged and disconnected, I proceeded to pull the harness out from the engine bay and through the space between the radiator and the inner fender bulkhead. This area also houses the two headlamp mechanism vacuum hoses that run from the firewall all the way to the nose of the Vette.

Since some of the connectors and other wire terminals will inevitably get caught by the many brackets and other items in that area, removal involves a lot of push-and-pull motions until the harness is completely out. But don't force it. Just take your time and figure out the issue. Eventually, the harness will come out.

As I show in the video, the headlamp door limit switches are also part of the headlamp harness, along with the side marker lights. However, the turn signal wires (that must be unplugged from the main harness) can remain with the car unless they are being replaced. I decided to keep the originals.



BUYING NEW WIRING HARNESSES

Most Corvette parts sellers offer wiring harnesses made by different companies. Since I am not an expert, I searched online for manufacturers and customer reviews.

Lectric Limited offers a very nice-looking, factory-correct wiring harness and had a lot of positive reviews, so I ordered one from Full Throttle Corvette, one of their authorized retailers.

Before placing my order, I called Full Throttle Corvette and spoke with Sam who was super helpful and confirmed I was ordering the right engine wiring harness for my 1976 Corvette. 

So I ordered both the engine and headlight wiring harnesses, plus the starter solenoid extension wire.

These are the Full Throttle Corvette part numbers along with current prices as of November 30, 2020. Shipping is additional.

  • Engine Wiring Harness (manual trans.) | Part No. 519231 | $193.00
  • Starter Solenoid Extension Wire | Part No. 519049 | $15.00
  • Forward Lamp (Headlight) Harness | Part No. 519311 | $265.00
So my grand total, including the $25 shipping charge, came to $498.00. Considering I recently spent $100 on a tow truck when the engine wiring harness started to act up, I cannot really complain about the price for brand new components.


WHAT I GOT FOR MY MONEY

According to Lectric Limited, 

Our Original Design Series™ wiring harnesses are manufactured exactly as the wiring was made when your vehicle left the factory. These harnesses are made to the exact O.E.M. (original equipment manufacturer) specifications, using the original factory blueprints, and are factory-correct in every way. These are fully assembled, ready to install, “plug & play” accurate reproduction wiring harnesses. You will NOT have to cut any wires, crimp any terminals, or make any modifications to these harnesses when installing them in your classic car or truck.

I received my new wiring harnesses two days after placing my order, which was awesome, and the first thing I did was compare the harnesses side-by-side with the originals.

They were an exact match!

Above: Can you pick the new harness from this photo?

They also included schematics that explain the function of each plug and connector, which is very helpful. It's nice when companies deliver a great product, and I highly recommend wiring harnesses by Lectric Limited.

For the record, I paid in full for the parts I ordered, so my recommendation of Full Throttle Corvette and Lectric Limited comes from my actual experience as a paying customer and end-user.



HOW TO INSTALL
ENGINE and HEADLAMP WIRING HARNESSES

Taking stuff apart is a LOT easier than putting it back together, and C3 Corvette wiring harnesses are no exception.

And even though many will say installation is simply a matter of doing the removal part in reverse, it's not that simple or that easy, especially when you take into account that your new harness will not be shaped to its intended final form. As a matter of fact, it will come coiled inside a bag.

But all of that aside, you have to be patient and refer to the factory assembly manual as many times as necessary.

Anyway, I started by feeding the engine harness from above, between the brake booster and the fender. Once I had it somewhat situated, I raised the car and, from underneath, continued feeding the harness up through the space between the firewall and the rear of the engine, from the driver's side. One of the videos below shows these steps better. I also took this opportunity to connect the oil pressure sending unit. 

Once that section of the harness was in, I pulled and organized some of the wires from above. In this portion of the harness, you have the brown tach and red ignition wires, as well as the wiper motor plug and ground. You also have the purple starter solenoid extension plus two other wires for the hood-ajar switch and the a/c harness connector. I also plugged the reverse light V-shaped connector.

From there, I pushed the starter solenoid extension wire, the two starter solenoid wires with fusible links, and the ground wire down toward the starter motor. Incidentally, the starter motor ground on my car is bolted to the engine block next to the starter solenoid although the manual shows it attached to the transmission bell housing. Either makes an excellent ground.

With the engine wiring harness in place, I turned my attention to the headlamp wiring harness.

I secured the alternator wires and the coolant temperature sending unit. I then attached both dash harness plugs together and secured them to the firewall fuse box plate with the bolt provided. This took a few tries since the bolt would not thread into the box. Applying pressure while turning the bolt with a socket, extension, and ratchet, did the trick.

With the dash harness properly secured, I connected the brake pressure switch to the proportioning valve followed by the a/c harness connector and the windshield washer pump plug (this connector is part of the engine wiring harness).

Since I deleted the alarm system from my Corvette, I insulated the wires and terminals with shrink tubing and secured them to the harness with cable ties.

The car has several hooks and clips that help route and secure the harnesses in place. They are made out of soft metal so you can bend them to keep the wires in place.


At this point, I was ready to start feeding the wiring and connectors for the headlamps, marker lights, turn signals, horn, headlight door limit switches, plus the two grounds. And because of the small opening where all of these items go through (from the engine bay into the nose of the Vette), easier said than done.

But it can be accomplished. Just take your time and feed one or two connectors at a time. In some instances, it helps if you move and hold the headlamp vacuum hoses out of the way while you push the bigger plugs through. Then you can push and pull the harness through the opening until every single item is inside the front portion of the car. In other words, in the cavity between the bumper and the radiator.

From there you can route the headlamp connectors through a small opening in the headlamp assembly, followed by the door limit switch. Raise the headlamp door and connect the low and high beam connectors. Which is which? The low beam bulb has three prongs, the high beam bulb has two, so there's no way to mix them up. See the diagram below.

You can also connect the side marker light and plug the turn signal harness (this is a separate short-wire piece). With the driver's side done, you can route the harness along the rear top edge of the nose (in front of the hood), down the crossover metal piece while securing the harness with the push routing clips provided. About midway, you can also connect the horn (green wire), and one of the grounds as shown in the inset of the diagram above.

When you get to the passenger side, you can repeat the process for the headlamp, side marker light, and turn signal connections.

There's just one more ground wire that needs to be connected to the bottom portion of the radiator core support and you're done!


I HAD ONE PROBLEM THOUGH...

When I started testing things after the new harnesses were connected, I had an issue with the left side blinker when the headlights were on.

Long story short, the problem was caused by a damaged turn signal harness connector. Either it was bad from the manufacturer or I messed it up as I plugged it in. It was a VERY tight fit and maybe I should've used some sort of lubricant to make installation easier (I did not and simply forced it into the receptacle). Maybe new turn signal harnesses would have averted this problem.


I suppose there is a need for a separate harness for the front turn signals since those big sockets won't fit through the small radiator/fender opening. But since I do not plan to remove the headlight harness anytime soon (or ever again, really), I just clipped the wires and eliminated the connectors, basically making them part of the headlamp wiring harness.

I considered going to the auto parts store and getting a pair of Weatherpack plugs, but frankly did not feel like it, so I just soldered them in place and that resolved the electrical issue I was having.


EVERYTHING WORKS!

After double-checking everything, I took my Corvette for a 30-minute shakedown drive, and tested the headlamps, the turn signals, made sure the coolant temperature, ammeter, and oil pressure gauges were working correctly, and so on and I'm happy to report that everything did.

Replacing these wiring harnesses is a big and involved project, but after four decades, most C3 Corvettes would probably benefit tremendously by having the original wiring replaced.

I have no idea what a qualified shop or mechanic would charge for doing this, and as difficult as it may be, it is a project that could be done by an ambitious do-it-yourselfer, provided he or she has the necessary tools available.

As far as my Vette is concerned, I'd still like to improve the headlight wiring and its performance by adding a relay, but that's a project for another time.

I hope you found this article helpful in case you want or need to replace one or both harnesses.

Thank you for following my '76 Vette Blog!


Product Links... (#sponsored)

1976 Corvette Stingray Owner's Manual


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