What can you do about flickering lights, a dead sound system, psychotic instruments, undependable turn signals, and horns that don't always work? Our classic Mustang electrical systems are notorious for grief and frustration, mainly because most are now reaching four decades of service.
After 40 years, insulation becomes brittle, exposed copper corrodes, terminals become pitted, and electrical tape begins to roll off. With all of these elements in play, there's potential for problems, even disaster. Exposed copper wiring can ground out against the body or chassis, smoking the harness and starting a fire. Brittle wiring can break, thrusting you into darkness on the roadside. And imagine a bad wiper connection in a rainstorm. We visited JMC Motorsports in San Diego, California, while they were installing a new stock electrical system from Mustangs Plus.
Replacing a vintage Ford's factory electrical system is easy because everything plugs together one way. Firewall connections are easy to pop out and in, and switches plug in quickly. Almost anyone can do it. When we arrived at JMC Motorsports, the old electrical system had already been removed. Removal of the old system isn't that difficult. It begins with disconnecting the battery. We suggest shooting photographs of your Ford's old electrical system before removal. Zero in on the instrument wiring and everything behind the dashboard. Because Ford designed this stuff for rapid assembly on an assembly line, plugs mate only one way. Grounds speak for themselves, especially when you photograph them before removal. Firewall plugs mate at the firewall to tie the underdash main wiring loom to the headlamp and engine looms. Plugs that tie the main wiring loom to the taillight harness are also behind the dashboard on the left side. The turn-signal switch plugs into the main wiring loom at the base of the steering column. Always examine potential problem areas before they can arise. For example, we've seen fuse-box screws accidentally run through wiring, causing a short circuit. We have also witnessed wire chafing, which caused the wiring to ground out. Though reproduction wiring looms are wrapped and protected exactly like the originals, we can make them even safer. Plastic antichafe can be used to minimize contact, and you can also wrap the wiring loom with electrical tape for greater protection. Examine how the wiring is routed, which can also be a factor in protection. Sharp metal edges can chew right through wiring, especially when you consider road shock and vibration.
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 1 You can begin the new wiring...  1 You can begin the new wiring harness installation virtually anywherein your Mustang. Dave Toth of JMC Motorsports begins at the radiatorsupport and works aft. Dave uses an aftermarket rubber grommet where theheadlamp/parking lamp wire looms penetrate the left and right innerfender aprons. Once the leads are completely through to the headlampdoor, Dave connects the parking lamp and ground. The heavy rubbergrommet is not stock, but it protects. |
 2 Dave does the same thing...  2 Dave does the same thing on the righthand side, connecting the parkinglamp and ground. |
 3 Horn leads are routed through...  3 Horn leads are routed through the radiator support and connected. |
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 4 At the radiator support,...  4 At the radiator support, there are two grounds. Connect both, usingthe proper hex-head sheetmetal screw with a star washer for solidcontinuity. Bad grounds, often caused by bad connections or no ground atall, create all sorts of electrical-system gremlins. |