Friday, August 13, 2010

Pontiac GTO Wiring Harness

Let There Be Light!

Here’s our Judge waiting for a little electrical system upgrade. Being pro-active instead of re-active can literally save your car from electrical fire damage. We’ve all seen it, so don’t be a victim of it. Disconnect the battery before starting the project.

Improve your Pontiac’s lighting system and avoid potential fire damage by replacing its harness.

Story and photography by Jim McGowan

Any Pontiac enthusiast who owns a 40-year-old Poncho is familiar with the many and varied problems that come with that age.

No, we’re not talking about a larger belt size, but rather things like dry, cracking and peeling engine compartment wiring.

Heat, chemicals and other underhood factors can cause the once supple wires to deteriorate to the point of being dangerous. Many an engine compartment fire has been the result of worn wiring shorting out and flaring up, or old wires becoming hot and causing the shielding to melt or burn, leading to a fire. The original factory wiring was still powering the engine and front lighting of our resident ’70 Judge project car, and we were no longer willing to take a chance on having a catastrophic failure. We have already replaced the engine harness in a previous article (PE, January 2010), and now will tackle the lighting harness.

In the old days, replacing the harness would have required either finding a factory replacement, or recreating your own, wire by wire. While the wire-by-wire method might be good for learning electronics and wiring, it’s not a fun or easy task.

The forward lamp harness (#13635) that we installed fit precisely, requiring no cutting or soldering. It’s literally a “Plug and Play” installation. Simply remove the old harness and install the new one. But, of course, it’s not quite that simple, so we’ll show you what is required for the replacement in this 1970 model GTO Judge, but most other Ponchos will be similar.

No special tools are required, but you’ll be doing some parts disassembly to access the required work areas. It’s a good idea to take digital photos of the disassembly so you have a reference for putting things back together. Pay particular attention to which plugs go to which lights, as they are different, and you’ll be using a small spade connector from the original turn signal socket, which needs to be transferred to the new harness. That is illustrated in the photo sequence. If possible, have a factory assembly manual available for reference.

Since you’ll be taking apart some of the front end bright work, it’s a great time to do a little detailing of these parts and the areas behind the front grilles, too.

1 Here’s the new harness ready for installation. The harness contains connections to all four headlights, the turn signals, two ground wires, the dual horns, the side marker lights and the bulkhead connector. There is also a connector to the brake fluid distribution block on the frame rail and another to the horn relay.

2 We begin by removing the front cowl air dam from the core support. We’ll use this top access to get at the grille mounting points and the wiring across the front of the car.

3 Continue disassembly by removing the four bolts securing each of the front grilles and removing them. You’re now reaching down into the area previously covered by the air dam.

4 Here’s the original factory lighting harness. The car was disassembled years ago for paint, and the body shop guys never returned the harness to its proper location in the clips under the core support.

5 In the top center of the photo is one of several harness clips that should be used to secure the new harness. We will use all of these for the reinstallation.

6 Now remove the bezels surrounding all four headlights and then the bulb retaining rings, which are secured with three small screws – don’t lose them.

7 The headlight bulb and wiring can now be pulled forward. Check out which harness plug is used on each light and the location and number of the bulb. If you reinstall them wrong, the main headlight bulbs could end up in the inside position. Take pictures or make a diagram and notes as needed.

8 This side view of the crossmember support shows the original harness as it passes through a curved area in the support; you must use the same routing as the factory due to the length of the harness wiring.

9 Under the front of the driver’s side fender, at the core support, is this small plastic baffle. It is pressure fit into the core support and covers the access hole through which you will pass the wiring harness and all connectors. A slit in the top allows you to remove it and replace it on the new harness. The original harness and ground wire are visible in the rear.

10 After unplugging all the lamp connectors, and disconnecting the ground wire, the original harness is now being removed from the passenger side through the access area in the center core support. We’re working from the passenger side to the driver’s side.

11 Here’s the original harness removed from the front, but still connected at the bulkhead. Do not throw this harness away. There are two connectors you’ll need still in it, plus if you ever need factory connectors in the future, it will come in handy. Leave it connected at the bulkhead for now.

12 Snake all the connectors and new wiring through the access hole in the core support and route them to the correct areas using the same path you used to remove the old harness. Let the plugs hang out of the correct bulb sockets and check your photos to make sure they are right.

13 This is the bulkhead connector. It is to the right of the master cylinder. If your car has a power booster, accessing the connector is more difficult due to space limitations, but you can do it. We marked the wires to the horn relay with tape to make sure everything was properly connected. The tan wire goes to the brake distribution block.

14 The ground wires are fairly easy to disconnect, but trying to route the new wire back in place is tough. Use an alligator clip style wire, snaked through the hole under the fender and down to the headlight bucket. Connect it to the ground wire and pull up through the access hole. You can then connect it.

15 Here’s the alligator connector wire, still attached to the ground wire, which made this almost impossible routing a success. These are available from hardware stores, Harbor Freight, etc.

16 While the underside of the core support is a little grungy, all the factory harness clips were still in place and useable. Our new harness is now suspended as original.

17 Since we started installing all the wiring at the front, the bulkhead plug is not yet wangled through an access area in the driver’s side core support bracket, and then up to the access hole at the top of the panel. This takes a little patience, but will work.

18 The complete harness is now hanging in the support brackets under the core support with all the various plugs connected. It is important to have the wiring up high to prevent damage from road junk or minor collision damage. Make sure to connect the horn wiring!

19 The bulkhead connector has been passed through the access opening and now can be routed across the fender and to the connector area on the firewall.

20 Here you see the pressure fit panel back in place with the harness passing through it. You can also see the ground wire connected to the core support. This connection location is the same for both ground wires. Make sure you have cleaned the metal for a good ground.

21 Here’s the bulkhead connector removed from the firewall. It is secured with one bolt and contains the engine compartment connections on one side and the lighting harness connections on the other. The two plugs slide together to form the complete unit. Install the lighting harness connector as the original and carefully replace it on the firewall.

22 Here you can see the harness in the fender well clips, the bulkhead connector in place and the horn relay wiring connected and under the factory cover. But we’re not done!

23 It’s a perfect time to detail and polish any bright work items you have removed. These parts take a beating during normal driving, but will last for years if cared for.

24 Here’s a tip for reinstalling the headlight retaining rings: Those three little screws are difficult to install by hand. Insert the screwdriver into the screw head and wrap a small piece of masking tape around the screw head and screwdriver shaft. Then you can carefully insert the screw attached to the screwdriver in the hole and start the threads.

25 Before putting everything back together, connect the battery and turn the lights on. When we turned on our headlights, the turn signal bulbs didn’t light.

26 We removed the lenses and checked the bulbs and compared the old and new sockets. The small spade connector shown here must be removed from the old socket and installed into the corresponding slot in the new harness socket. If you don’t do this, your turn signal lights won’t work, either. Ours do now! n


  1. (#13635) does not come up at Year One. Where can I find it and how much does it cost?

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