Mustang Electrical Tips – Grounds, Connection, and Shorts 1965-1973

Underhood electrical components are particularly more prone to fail if they aren’t properly maintained. Here, we see the foam gasket that seals the blower motor to the firewall is in excellent shape, as are the various electrical connectors and even the nearby ground wire. Ideally, the wires would not be routed such that they are in contact with the dipstick tube (or at least should be behind it so they’d hang lower, away from the cap). The wires should also be tied farther from the exhaust.

Simple electrical connectors are fine for areas that are not exposed to the elements. After cleaning, any connectors that are exposed to dirt and/or moisture should have their own internal seals, electrical grease, or an equivalent sealant applied to help keep dirt and moisture away from the metal contacts. They are not usually visible, and are acceptable for a show car.

Electrical connectors such as these can often be the cause of open or intermittent circuits. If practical, open and inspect all such connectors to ensure the contacts are clean and corrosion free. Using various electrical greases and sprays can also help keep them that way.

These connections show the approach the factory took to keep dirt and the elements away from some underhood electrical connections. The rubber grommets and/or plastic connectors that go through the firewall can deteriorate over time and should be replaced if they are cracked or otherwise damaged. Otherwise, the wire bundle is nicely wrapped with electrical tape and is also properly secured to the inner fender panel with a plastic clip/loop.

These steering columns provide an example of how common components vary between models. The 19641⁄2 Mustang column (left) shows how these models used individual wires that were wrapped with electrical tape. The 1965 to 1967 column (center) shows how multiple electrical connectors replace the individual wires while a protective sheath replaces the tape. The 1968+ column (right) is the collapsible variety, but it also has a single, flatter electrical connector instead of the multiple connectors.

This view of the top of each column shows some other differences. Two of the steering columns have dual contacts while the plastic turn signal switch assembly on the 19641⁄2 column (left) has a single horn contact. It can also be replaced when worn (shown on the plastic cap in the foreground) The 1968+ column also features tilt capability and an emergency flasher switch the earlier columns lack. The wires exit higher up as well.

High-quality reproduction harnesses are available for vehicles where one or more of the factory harnesses are so severely damaged that they’re not worth repairing. These can either be of the exact-replica/factory-correct type or they can be fully functional direct-fit upgrades that use newer, improved materials. The latter can also add extra circuits and/or features to support other potential upgrades. The appearance of these harnesses may not match that of the original factory harness as closely, but they make upgrades easier.

A somewhat odd yet also common problem is cracking of the steering wheel. How is this related to electrical issues? When the wheel cracks it can cause the horn contacts to short internally, thus causing the horn to stay on constantly. Until the steering wheel can be replaced (usually the only viable option) the electrical connectors can be pulled from the horn(s) and be secured out of the way to silence the horn(s) until repairs are made. A cracked wheel can also cause turn signal malfunction.

The turn signal switch assembly is a common source of trouble because the plastic pegs, which get moved as the steering wheel turns, often break off. The only option in such cases is to remove the switch and replace it. If the original switch is to be reused, it should first be cleaned and properly lubed.

The fuse block can be a common problem in some early Mustangs. The reason is mainly its location within the car rather than complex circuitry. This block from a 19641⁄2 to 1966 car exhibits the corroded terminals often found in these applications due to water leaking from the cowl area on the block. Later cars don’t have this issue since the block was moved to a location above the throttle pedal, where there was almost no chance of contact with water. The terminals can usually be cleaned with a wire brush. Weak/broken terminals require replacing the block. Luckily, there are relatively few circuits to rewire.

Access to the bulbs for the instrument cluster can be difficult when a Rally Pack is present on the steering column; it has to come off to get the instrument cluster out. With the cluster out, changing the bulbs is a snap. The wiring harness should also be checked (as should the plastic connectors) for any defects/ problems that need to be fixed. Note the added-on aftermarket gauge in the center of the cluster.

The fuel tank sending unit is often to blame when the fuel gauge doesn’t work. The gauge is usually fine; the problem is that the float usually fills with fuel, and thus no longer moves to provide a proper electrical signal. The OEM-style brass floats are inferior to the plastic ones used in most reproduction parts. Unlike older mixtures, modern fuels have sufficiently high levels of ethanol in them to cause the brass floats to corrode.

The turn signal flasher is somewhat prone to failure. Replacement is a matter of removing the very inexpensive part and replacing it with a new one found at just about any auto parts store. The flasher on 1967–1968 cars is especially difficult to reach since it is located directly behind the instrument cluster. The cluster must come out to reach it. Other models usually provide access from under the dash without the need to remove any other components.

By far, corroded battery terminals are the single most common cause of starting problems. If the terminals on your project are corroded, use a terminal cleaner to clean the terminal and the battery cable. If the cable is severely corroded, get a new, correct-length cable with the correct gauge conductor. Watch out for cables that look thick but have a thinner wire core. If you must use an older-style vented battery for the sake of authenticity, then put anti-corrosion rings under each battery cable end. The better choice is to use a modern, sealed battery of sufficient capacity.

The starter relay often fails but is an easy and inexpensive part to replace. But before replacing the relay, be sure to check the push-on terminals for internal corrosion and/or looseness. This can cause unreliable starting, which can appear as if the relay is faulty when the problem is just a poor connection. Clean the contacts in the push-on terminals and, if necessary, pinch them slightly so they make better contact. And if the car starts normally when the relay is jumped with a remote start switch (or a properly placed screwdriver) the problem may be the start switch or the wiring and/or connections between it and the starter relay.

The alternator harness terminals frequently become corroded to the point the alternator does not work properly. Clean the terminals with a wire brush to restore proper function, in most cases. If the terminals are too weak, replace them. The harness should be properly taped and there should be no signs of worn tape or wire insulation. If the alternator makes any significant noise when the engine is running, it probably needs to be replaced.

One very effective way to prevent some common charging system problems is to upgrade the alternator to a single wire type with an internal voltage regulator. This eliminates numerous connections that can corrode or otherwise cause problems. These newer alternators also have much higher idle and peak output, so high-powered audio systems, electric cooling fans, and higher-output headlamps don’t discharge the battery as often. They also generally run cooler and last longer.

Adapting the factory wiring harness is usually just a matter of splicing some wire into the harness provided with the new alternator. Basically, the wires that would have gone to the external factory voltage regulator (no longer used) are re-routed to the side connector of the new alternator. These alternators bolt right in and only require relatively minor wiring modifications such as these to work just fine. Some, such as this one, even allow you to manually adjust the control/output voltage to suit unique needs.

On cars with an external voltage regulator, it is critical the regulator is properly grounded and the connector terminals corrosion free. These problems can also prevent the battery from properly charging even though the alternator may be fine. Being an electrotechnical part, external regulators are more prone to have problems than internal units. They’re also getting harder to find in parts stores, especially if you need a less common type.

On cars with an external voltage regulator, it is critical the regulator is properly grounded and the connector terminals corrosion free. These problems can also prevent the battery from properly charging even though the alternator may be fine. Being an electrotechnical part, external regulators are more prone to have problems than internal units. They’re also getting harder to find in parts stores, especially if you need a less common type.

A far better way to trigger the ignition is with electronic modules, such as this Pertronix Ignitor II unit. It fits inside the distributor so it is invisible except for the two external wires instead of only one. This is a good tradeoff for far better accuracy, higher spark energy, and eliminating constant wear and adjustment. It’s not original equipment, but far superior for cars that get driven.

If the coil case is damaged or leaking, replace it. If you use a breakerless ignition upgrade, also upgrade the coil to a lowresistance unit to get max performance from the new ignition. This coil is only 0.6 ohm. This figure would be far too low if points were still being used because this would cause them to arc and burn up prematurely. Higher-output coils are an effective upgrade as long as other parts of the system are compatible.