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1972 Corvette Scarlett Project Car - Replacing the Factory Wiring Harness

Barbecued Shark - Replacing the factory wiring harness on our ’72 Project Car

Jeremy Clough Dec 23, 2014
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While we’ll go through wiring up the FAST computer and other associated engine components in a separate installment, here we’ll cover replacing the front and rear wiring harnesses with factory correct Lectric Limited ones provided by Zip Corvette. In stock guise, our C3 has two other major harnesses: one for the engine and another for the dash, which we’ll cover when we do the gauge installation. The front lamp harness has the headlights, corner markers, and turn signals, as well as the alternator wiring and other associated circuits. The rear harness has all the rear lighting, as well as the dome light, door switches, and most of the alarm wiring.

The good thing about a “correct” harness, and this is likely to be the only correct part on this car, is that the wire color and size match the factory spec, so it’s easy to compare them with what’s already in the car and with the factory wiring diagram found in the assembly manual. It’s important to know, though, that the wiring harness is just that: just wires, with the connectors at the end of each wire so that it can plug into whatever it mates with, be it a bulb, relay, subharness, whatever. Along the way you’ll find smaller parts that it connects to that are broken or missing, such as our long-gone horns or broken door switches, so you’ll need to order these separately. In our case, we made a list of parts as we put the harness in and ordered new replacements for virtually everything, even ones that weren’t obviously broken.

After seeing the condition of our factory harnesses, rewiring is probably one of the first things I’ll do to any older car I buy. Assuming you’re not modifying it (that comes later), you can probably do these two harnesses in a long day: I worked fairly slowly, and spent a full weekend of 8-hour days on these, and got a running start on the more tedious dash harness. Hopefully we won’t barbecue this shark when we put the juice to her.

1972 Chevy Corvette Coupe 2/24

01. Scarlett, our ’72 coupe project car. With her 600-plus hp stroker LS3 in place, piped, plumbed, and bolted to a Tremec six-speed. It’s time to start getting her wired by replacing the factory harnesses.

Factory Correct Harness 3/24

02. Our factory-correct harnesses, probably the only correct parts in the car, were provided by Zip Corvette. In addition to the complete harnesses made by Lectric Limited, they also provided a laminated wiring diagram that’s an indispensable part of any serious wiring project, and shows each circuit, marked by color and gauge or wire.

Laid Out Harnesses 4/24

03. One of the first parts of wiring the car is to get the harnesses laid out to make sure it’s the same as the one you’re taking out. You certainly can do a wiring project a little at a time, but it helps to have plenty of space and time to lay things out. We laid out many of the ancillary wiring parts along with the harnesses.

Rear Harness 5/24

04. We did the rear harness first and started by removing the Corbeau seats as well as the five-point harness mounting bar and rear trim pieces and peeling back the carpet. This part of the wiring harness contains the seatbelt warning light circuits as well as the glovebox light and the wiring and flasher unit for the alarm, which are found in the rear well that holds the jack.

Factory Correct Harness 6/24

05. If you have the option of not completely removing the old harness, it makes installation much easier to lay the new harness over the old one to make sure you’re making all the correct connections. Using a factory correct harness also makes it easier, as you simply match wire colors to know where each one goes, assuming yours is largely unmolested.

Split Rubber Frommet 7/24

06. The part of the rear harness going into the jack well passes through a split rubber grommet that reduces the risk of abrasion from the sharp edge of the hole in the fiberglass.

Factory Flashers 8/24

07. There are three flashers in the C3: one on the fuse box, one under the passenger side dash, and one in the jack well. Factory flashers work based on the amount of resistance each bulb creates, but since we’ll be changing all the bulbs to LEDs that don’t provide enough resistance for a thermal flasher to work, we ordered LED-specific flashers from Corvette America. In addition to plugging in, they’ll also need to be grounded.

Four Door Switches 9/24

08. There are four door switches in the ’72: one at the front of each door opening, and one at the rear. The fronts run the courtesy lights, while the rears control the alarm. We unscrewed the switches and pulled them out, then used a thin piece of welding rod with the new wires taped to it to get the flexible wires guided through the hole so they could be connected to the switch.

1972 Chevy Corvette Rear 10/24

09. With the door switches and the wiring for the seatbelt warning lights and alarm in place, we moved toward the rear of the car. The harness runs over the rear wheelwell, just under the rubber Astro Ventilation tubes, to the rear of the luggage compartment.

Factory Astro Ventilation 11/24

10. The factory Astro Ventilation tubes are held in place by rivets, but ours had been replaced with stainless screws. Even though the harness is held in place by the bent metal clips, we later capped the threaded part of the screws with rubber caps to avoid abrasion.

Rear Harness Luggage Compartment 12/24

11. The rear harness passes to the underside of the car through a rubber grommet at the rear of the luggage compartment. There are always surprises when you start peeling back the carpet, and one of ours was finding the green speaker wire that someone had used to wire up the rear dome light.

Rear Dome Light 13/24

12. The only thing more surprising than the fact that the rear dome light hadn’t caught on fire is that it actually worked. Only the bold need peel back the duct tape here on the back of the dome light assembly. We ordered a new socket to replace it.

New Harness 14/24

13. The new harness in place. The orange and white wires are the correct ones for the dome light. Before we put the rubber grommet in place, we made a hole in it to run the separate heavy-gauge red wire that goes to our Aeromotive A1000 in-tank fuel pump.

Factory System Horn 15/24

14. Inexplicably, the horn for the factory alarm system was still bolted in place in its position up beneath the car above the driver-side rear wheelwell, and when we put a pair of hot wires to it, shocked us all by working. It got a good cleaning, and went right back in place.

Alarm Switch Wiring 16/24

15. The rear harness ends with the wiring for the alarm switch and the rear lights. The taillights each have a pair of threaded bolts protruding from them: a ground wire, to which we added a larger washer, bolts to the bottom bolt, while the harness has mounting loops, shown here near the bottom of the photo, that are bolted in place on the upper ones.

Front Wiring Harness 17/24

16. The front wiring harness contains the lighting circuits, including headlights, turn signals, and corner markers, as well as the alternator wiring and horn circuit. The horn relay, which is tucked just behind the front fender, serves as a power distribution point for the rest of the car.

Front Lamp Harness 18/24

17. In addition to the headlights, the front lamp harness also feeds power to the corner markers, via the blue-and-white wires, and the turn signals, which are fed by a separate subharness that plugs into it.

Front Turn Signal Harness 19/24

18. The plug for the front turn signal subharness is a female one, and as it turned out, our mismatched subharness itself also had a female plug, which a previous owner had handled by making this taped abomination to go between the two plugs. I remain grateful but mystified at the fact that the resultant mass of black-taped goo neither caught on fire nor ever stopped working.

Corner Marker New Plugs 20/24

19. The harness includes new plugs for the corner markers, which use a 194 bulb. We’ll be replacing the front ones with amber LEDs, and the rears with red ones. Note the blue overspray on the plug. Add one more color to the red and black that Scarlett has been painted at different times in her life.

Front Lamp Harness 21/24

20. The new front lamp harness is on the left: note the thick conduit and the plastic clip that holds it in place under the car’s nose just forward of the hood, compared with the older harness on the right, which is bundled with traditional loom tape and has a steel clip.

Horn Relay 22/24

21. The all-important horn relay, which bolts to the rear of the driver-side fender skirt opposite the alternator, serves as a power distribution point for the rest of the car. The red 10 gauge wire is fed directly by the alternator; since our LS has an alternator on the opposite side of the engine than a standard small-block, we’ll be pulling this wire out of the harness and replacing it with a longer one that’s properly routed to reach the new alternator.

Front Lamp Harness Alternator 23/24

22. Several wires exit the front lamp harness around the alternator: the top three go to the alternator (positive, ground, and voltage regulator), the middle is for the temperature sending unit, and the bottom plug is for the brake warning light sending unit.

Bulkhead Plugs 24/24

23. The bulkhead plugs for the front lamp harness (bottom/left) and the engine harness (top/right). The wiring harness plug slides into place, interlocking with the engine harness, and the assembled plug is bolted into place on the bulkhead connector on the opposite side of the fuse box.

Sources

Zip Corvette
Mechanicsville, VA 23111
800-962-9632
www.zip-corvette.com
Street Shop, Inc.
Athens, AL 35611
256-233-5809
www.streetshopinc.com

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